Chloromethanes,' in: Ullmann's Encyclopedia of Industrial …tep028/pqi/descargas/Industria quimica organica/tema_1... · compound by the action of chlorine bleach on both ethanol

� 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Article No : a06_233

Chloromethanes

MANFRED ROSSBERG, Hoechst Aktiengesellschaft, Frankfurt/Main, Germany

WILHELM LENDLE, Hoechst Aktiengesellschaft, Frankfurt/Main, Germany

GERHARD PFLEIDERER, Hoechst Aktiengesellschaft, Frankfurt/Main, Germany

ADOLF TOGEL, Hoechst Aktiengesellschaft, Frankfurt/Main, Germany

THEODORE R. TORKELSON, Dow Chemical, Midland, Michigan, United States 48674

KLAUS K. BEUTEL, Dow Chemical Europe, Horgen, Switzerland

1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . 15

2. Physical Properties . . . . . . . . . . . . . . . . . . . . 16

3. Chemical Properties . . . . . . . . . . . . . . . . . . . 19

4. Production . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

4.1. Theoretical Bases . . . . . . . . . . . . . . . . . . . . . . 20

4.2. Production of Monochloromethane . . . . . . . . 23

4.3. Production of Dichloromethane and

Trichloromethane . . . . . . . . . . . . . . . . . . . . . 25

4.4. Production of Tetrachloromethane . . . . . . . . 29

5. Quality Specifications. . . . . . . . . . . . . . . . . . . 33

5.1. Purity of the Commercial Products and their

Stabilization . . . . . . . . . . . . . . . . . . . . . . . . . . 33

5.2. Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

6. Storage, Transport, and Handling . . . . . . . . . 34

7. Behavior of Chloromethanes

in the Environment . . . . . . . . . . . . . . . . . . . . 35

7.1. Presence in the Atmosphere. . . . . . . . . . . . . . 35

7.2. Presence in Water Sources . . . . . . . . . . . . . . 36

8. Uses and Economic Aspects . . . . . . . . . . . . . . 36

9. Toxicology . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

References . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

1. Introduction

Among the halogenated hydrocarbons, the chlo-rine derivatives of methane monochloromethane(methyl chloride) [74-87-3], dichloromethane(methylene chloride) [75-09-2], trichloro-methane (chloroform) [67-66-3], and tetrachlor-omethane (carbon tetrachloride) [56-23-5] playan important role from both industrial and eco-nomic standpoints. These products find broadapplication not only as important chemical inter-mediates, but also as solvents.

Historical Development. Monochloro-methane was produced for the first time in1835 by J. DUMAS and E. PELIGOT by the reactionof sodium chloride withmethanol in the presenceof sulfuric acid. M. BERTHELOT isolated it in 1858from the chlorination of marsh gas (methane), asdid C. GROVES in 1874 from the reaction ofhydrogen chloride with methanol in the presenceof zinc chloride. For a time, monochloromethanewas produced commercially from betaine hydro-chloride obtained in the course of beet sugarmanufacture. The earliest attempts to produce

methyl chloride by the chlorination of methaneoccurred before World War I, with the intent ofhydrolyzing it to methanol. A commercial meth-ane chlorination facility was first put into opera-tion by the former FarbwerkeHoechst in 1923. Inthe meantime, however, a high-pressure metha-nol synthesis based on carbon monoxide andhydrogen had been developed, as a result ofwhich the opposite process became practical –synthesis of methyl chloride from methanol.

Dichloromethane was prepared for the firsttime in 1840 by V. REGNAULT, who successfullychlorinated methyl chloride. It was for a timeproduced by the reduction of trichloromethane(chloroform) with zinc and hydrochloric acid inalcohol, but the compound first acquired signifi-cance as a solvent after it was successfully pre-pared commercially by chlorination of methaneand monochloromethane (Hoechst AG, DowChemical Co., and Stauffer Chemical Co.).

Trichloromethane was synthesized indepen-dently by two groups in 1831: J. VON LIEBIG

successfully carried out the alkaline cleavage ofchloral, whereas M. E. SOUBEIRAIN obtained the

DOI: 10.1002/14356007.a06_233.pub3

compound by the action of chlorine bleach onboth ethanol and acetone. In 1835, J. DUMAS

showed that trichloromethane contained only asingle hydrogen atom and prepared the substanceby the alkaline cleavage of trichloroacetic acidand other compounds containing a terminal CCl3group, such as b-trichloroacetoacrylic acid. Inanalogy to the synthetic method of M. E. SOU-BEIRAIN, the use of hypochlorites was extended toinclude other compounds containing acetylgroups, particularly acetaldehyde. V. REGNAULT

prepared trichloromethane by chlorination ofmonochloromethane. Already by the middle ofthe last century, chloroform was being producedon a commercial basis by using the J. VON LIEBIG

procedure, a method which retained its impor-tance until ca. the 1960s in places where thepreferred starting materials methane and mono-chloromethane were in short supply. Today, tri-chloromethane – alongwith dichloromethane – isprepared exclusively and on a massive scale bythe chlorination of methane and/or monochlor-omethane. Trichloromethane was introduced in-to the field of medicine in 1847 by J. Y. SIMPSON,who employed it as an inhaled anaesthetic. As aresult of its toxicologic properties, however, ithas since been totally replaced by other com-pounds (e.g., Halothane).

Tetrachloromethane was first prepared in1839 by V. REGNAULT by the chlorination oftrichloromethane. Shortly thereafter, J. DUMAS

succeeded in synthesizing it by the chlorinationof marsh gas. H. KOLBE isolated tetrachloro-methane in 1843when he treated carbon disulfidewith chlorine in the gas phase. The correspondingliquid phase reaction in the presence of a catalyst,giving CCl4 and S2Cl2, was developed a shorttime later. The key to economical practicality ofthis approach was the discovery in 1893 byM€uLLER and DUBOIS of the reaction of S2Cl2 withCS2 to give sulfur and tetrachloromethane, there-by avoiding the production of S2Cl2.

Tetrachloromethane is produced on an indus-trial scale by one of two general approaches. Thefirst is the methane chlorination process, usingmethane or mono-chloromethane as starting ma-terials. The other involves either perchlorinationor chlorinolysis. Starting materials in this caseinclude C1 to C3 hydrocarbons and their chlori-nated derivatives as well as Cl-containing resi-dues obtained in other chlorination processes(vinyl chloride, propylene oxide, etc.).

Originally, tetrachloromethane played a roleonly in the dry cleaning industry and as a fireextinguishing agent. Its production increaseddramatically, however, with the introduction ofchlorofluoromethane compounds 50 years ago,these finding wide application as non-toxicrefrigerants, as propellants for aerosols, asfoam-blowing agents, and as specialty solvents.

2. Physical Properties

The most important physical properties of thefour chloro derivatives of methane are presentedin Table 1; Figure 1 illustrates the vapor pressurecurves of the four chlorinated methanes.

The following sections summarize additionalimportant physical properties of the individualcompounds making up the chloromethane series.

Monochloromethane is a colorless, flam-mable gas with a faintly sweet odor. Its solubilityin water follows Henry’s law; the temperaturedependence of the solubility at 0.1MPa (1 bar) is:

t, �C 15 30 45 60

g of CH3Cl/kg of H2O 9.0 6.52 4.36 2.64

Monochloromethane at 20 �C and 0.1 MPa (1bar) is soluble to the extent of 4.723 cm3 in 100cm3 of benzene, 3.756 cm3 in 100 cm3 of tetra-chloromethane, 3.679 cm3 in 100 cm3 of aceticacid, and 3.740 cm3 in 100 cm3 of ethanol. Itforms azeotropic mixtures with dimethyl ether,2-methylpropane, and dichlorodifluoromethane(CFC 12).

Dichloromethane is a colorless, highly vol-atile, neutral liquid with a slightly sweet smell,similar to that of trichloromethane. The solubilityof water in dichloromethane is:

t, �C � 30 0 þ 25

g of H2O/kg of CH2Cl2 0.16 0.8 1.98

The solubility of dichloromethane in water andin aqueous hydrochloric acid is presented inTable 2.

Dichloromethane forms azeotropic mixtureswith a number of substances (Table 3).

16 Chloromethanes Vol. 9

Dichloromethane is virtually nonflammablein air, as shown in Figure 2, which illustrates therange of flammable mixtures with oxygen –nitrogen combinations [10, 11]. Dichloromethanethereby constitutes the only nonflammable com-mercial solvent with a low boiling point. Thesubstance possesses no flash point according to

the definitions established in DIN 51 755 andASTM56–70 aswell asDIN51758 andASTMD93–73. Thus, it is not subject to the regulationsgoverning flammable liquids. As a result of theexisting limits of flammability (CH2Cl2 vapor/air), it is assigned to explosion categoryG 1 (VDE0165). The addition of small amounts of dichlor-omethane to flammable liquids (e.g., gasoline,esters, benzene, etc.) raises their flash points;addition of 10 – 30%dichloromethane can rendersuch mixtures nonflammable.

Trichloromethane is a colorless, highlyvolatile, neutral liquidwith a characteristic sweetodor. Trichloromethane vapors form no explo-sive mixtures with air [11]. Trichloromethanehas excellent solvent properties formany organic

Table 1. Physical properties of chloromethanes

Unit Monochloromethane Dichloromethane Trichloromethane Tetrachloromethane

Formula CH3Cl CH2Cl2 CHCl3 CCl4Mr 50.49 84.94 119.39 153.84

Melting point �C � 97.7 � 96.7 � 63.8 � 22.8

Boiling point at 0.1 MPa �C � 23.9 40.2 61.3 76.7

Vapor pressure at 20 �C kPa 489 47.3 21.27 11.94

Density of liquid at 20 �C kg/m3 920 1328.3 1489 1594.7

(0.5 MPa)

Density of vapor at bp kg/m3 2.558 3.406 4.372 5.508

Enthalpy of formation DH0298 kJ/mol � 86.0 � 124.7 � 132.0 � 138.1

Specific heat capacity of

liquid at 20 �CkJ kg�1 K�1 1.595 1.156 0.980 0.867

Enthalpy of vaporization at bp kJ/mol 21.65 28.06 29.7 30.0

Critical temperature K 416.3 510.1 535.6 556.4

Critical pressure MPa 6.68 6.17 5.45 4.55

Cubic expansion coeff. of

liquid (0 – 40 �C)K�1 0.0022 0.00137 0.001399 0.00116

Thermal conductivity at 20 �C W K�1 m�1 0.1570 0.159 0.1454 0.1070

Surface tension at 20 �C N/m 16.2� 10�3 28.76� 10�3 27.14� 10�3 26.7� 10�3

Viscosity of liquid at 20 �C Pa � s 2.7� 10�4 4.37� 10�4 5.7� 10�4 13.5� 10�4

(0.5 MPa)

Refractive index n20D 1.4244 1.4467 1.4604

Ignition temperature �C 618 605 – –

Limits of ignition in air, lower vol% 8.1 12 – –

Limits of ignition in air, upper vol% 17.2 22 – –

Partition coefficient air/water

at 20 �C

mg=LðairÞmg=LðwaterÞ 0.3 0.12 0.12 0.91

Figure 1. Vapor pressure curves of chloromethanes

Table 2. Solubility of dichloromethane in water and aqueous hydro-

chloric acid (in wt%)

Solvent Temperature, �C

15 30 45 60

Water 2.50 1.56 0.88 0.53

10% HCl 2.94 1.85 1.25 0.60

20% HCl – 2.45 1.20 0.65

Vol. 9 Chloromethanes 17

materials, including alkaloids, fats, oils, resins,waxes, gums, rubber, paraffins, etc. As a result ofits toxicity, it is increasingly being replaced as asolvent by dichloromethane, whose properties inthis general context are otherwise similar. Inaddition, trichloromethane is a good solvent foriodine and sulfur, and it is completely misciblewith many organic solvents. The solubilityof trichloromethane in water at 25 �C is3.81 g/kg ofH2O,whereas 0.8 g ofH2O is solublein 1 kg of CHCl3.

Important azeotropic mixtures of chloroformwith other compounds are listed in Table 4.

Ternary azeotropes also exist between tri-chloromethane and ethanol –water (boiling point

55.5 �C, 4 mol% ethanol þ 3.5 mol% H2O),methanol – acetone, and methanol – hexane.

Tetrachloromethane is a colorless neutralliquid with a high refractive index and a strong,bitter odor. It possesses good solubility proper-ties for many organic substances, but due to itshigh toxicity it is no longer employed (e.g., as aspot remover or in the dry cleaning of textiles). Itshould be noted that it does continue to findapplication as a solvent for chlorine in certainindustrial processes.

Tetrachloromethane is soluble in water at 25�C to the extent of 0.8 g of CCl4/kg of H2O, thesolubility of water in tetrachloromethane being0.13 g of H2O/kg of CCl4.

Tetrachloromethane forms constant-boilingazeotropic mixtures with a variety of substances;corresponding data are given in Table 5.

Table 3. Azeotropic mixtures of dichloromethane

Azeotropic boiling

point, in �C, atwt% Compound 101.3 kPa

30.0 acetone 57.6

11.5 ethanol 54.6

94.8 1,3-butadiene �5.0

6.0 tert-butanol 57.1

30.0 cyclopentane 38.0

55.0 diethylamine 52.0

30.0 diethyl ether 40.80

8.0 2-propanol 56.6

7.3 methanol 37.8

51.0 pentane 35.5

23.0 propylene oxide 40.6

39.0 carbon disulfide 37.0

1.5 water 38.1

Table 4. Azeotropic mixtures of trichloromethane

Azeotropic boiling


15.0 formic acid 59.2

20.5 acetone 64.5

6.8 ethanol 59.3

13.0 ethyl formate 62.7

96.0 2-butanone 79.7

2.8 n-hexane 60.0

4.5 2-propanol 60.8

12.5 methanol 53.4

23.0 methyl acetate 64.8

2.8 water 56.1

Figure 2. Range of flammability of mixtures of CH2Cl2 withO2 and N2 [10]

Table 5. Azeotropic mixtures of tetrachloromethane

Azeotropic boiling


88.5 acetone 56.4

17.0 acetonitrile 71.0

11.5 allyl alcohol 72.3

81.5 formic acid 66.65

43.0 ethyl acetate 74.8

15.85 ethanol 61.1

71.0 2-butanone 73.8

2.5 butanol 76.6

21.0 1,2-dichloroethane 75.6

12.0 2-propanol 69.0

20.56 methanol 55.7

11.5 propanol 73.1

4.1 water 66.0


3. Chemical Properties

Monochloromethane as compared to otheraliphatic chlorine compounds, is thermally quitestable. Thermal decomposition is observed onlyat temperatures in excess of 400 �C, even in thepresence of metals (excluding the alkali andalkaline-earth metals). The principal products ofphotooxidation of monochloromethane are car-bon dioxide and phosgene.

Monochloromethane forms with water orwater vapor a snowlike gas hydrate with thecomposition CH3Cl � 6 H2O, the latter decom-posing into its components at þ 7.5 �C and 0.1MPa (1 bar). To the extent that monochloro-methane still finds application in therefrigeration industry, its water content must bekept below 50 ppm. This specification is neces-sary to prevent potential failure of refrigerationequipment pressure release valves caused byhydrate formation.

Monochloromethane is hydrolyzed by waterat an elevated temperature. The hydrolysis (tomethanol and the corresponding chloride) isgreatly accelerated by the presence of alkali.Mineral acids show no influence on thecompound’s hydrolytic tendencies.

Monochloromethane is converted in thepresenceof alkali or alkaline-earthmetals, aswellas by zinc and aluminum, into the correspondingorganometallic compounds (e.g., CH3MgCl, Al(CH3)3 �AlCl3). These have come to play a roleboth in preparative organic chemistry and ascatalysts in the production of plastics.

Reaction of monochloromethane with a sodi-um – lead amalgam leads to tetramethyllead, anantiknocking additive to gasoline intended foruse in internal combustion engines. The use ofthe compound is declining, however, as a resultof ecological considerations.

A very significant reaction is that betweenmonochloromethane and silicon to produce thecorresponding methylchlorosilanes (the Rochowsynthesis), e.g.:

2CH3ClþSi!SiCl2ðCH3Þ2

The latter, through their subsequent conver-sion to siloxanes, serve as important startingpoints for the production of silicones.

Monochloromethane is employed as a com-ponent in the Wurtz–Fittig reaction; it is also

used in Friedel–Crafts reactions for the produc-tion of alkylbenzenes.

Monochloromethane has acquired particular-ly great significance as a methylating agent:examples include its reaction with hydroxylgroups to give the corresponding ethers (meth-ylcellulose from cellulose, various methyl ethersfrom phenolates), and its use in the preparation ofmethyl-substituted amino compounds (quaterna-ry methylammonium compounds for tensides).All of the various methylamines result from itsreaction with ammonia. Treatment of CH3Clwith sodium hydrogensulfide under pressure andat elevated temperature gives methyl mercaptan.

Dichloromethane is thermally stable totemperatures above 140 �C and stable in thepresence of oxygen to 120 �C. Its photooxidationproduces carbon dioxide, hydrogen chloride, anda small amount of phosgene [12]. Thermal reac-tion with nitrogen dioxide gives carbon monox-ide, nitrogen monoxide, and hydrogen chloride[13]. In respect to most industrial metals (e.g.,iron, copper, tin), dichloromethane is stable, ex-ceptions being aluminum, magnesium, and theiralloys; traces of phosgene first arise above 80 �C.

Dichloromethane forms a hydrate with water,CH2Cl2 � 17 H2O, which decomposes at 1.6 �Cand 21.3 kPa (213 mbar).

No detectable hydrolysis occurs during theevaporation of dichloromethane from extracts orextraction residues. Only on prolonged action ofsteam at 140 – 170 �C under pressure are formal-dehyde and hydrogen chloride produced.

Dichloromethane can be further chlorinatedeither thermally or photochemically. Halogenexchange leading to chlorobromomethane ordibromomethane can be carried out by usingbromine and aluminum or aluminum bromide.In the presence of aluminum at 220 �C and 90MPa (900 bar), it reacts with carbonmonoxide togive chloroacetyl chloride [14]. Warming to125 �Cwith alcoholic ammonia solution produceshexamethylenetetramine. Reaction with pheno-lates leads to the same products as are obtained inthe reaction of formaldehyde and phenols.

Trichloromethane is nonflammable, al-though it does decompose in a flame or in contactwith hot surfaces to produce phosgene. In thepresence of oxygen, it is cleaved photochemical-ly byway of peroxides to phosgene and hydrogen


chloride [15, 16]. The oxidation is catalyzed inthe dark by iron [17]. The autoxidation and acidgeneration can be slowed or prevented by stabi-lizers such as methanol, ethanol, or amylene.Trichloromethane forms a hydrate, CHCl3 � 17H2O, whose critical decomposition point isþ 1.6 �C and 8.0 kPa (80 mbar).

Upon heating with aqueous alkali, trichloro-methane is hydrolyzed to formic acid, orthofor-mate esters being formed with alcoholates. Withprimary amines in an alkaline medium the iso-nitrile reaction occurs, a result which also findsuse in analytical determinations. The interactionof trichloromethane with phenolates to givesalicylaldehydes is well-known as the Reimer-Thiemann reaction. Treatment with benzeneunder Friedel-Crafts conditions results intriphenylmethane.

The most important reaction of trichloro-methane is that with hydrogen fluoride in thepresence of antimony pentahalides to givemono-chlorodifluoromethane (CFC 22), a precursor inthe production of polytetrafluoroethylene (Tef-lon, Hostaflon, PTFE).

When treated with salicylic anhydride, tri-chloromethane produces a crystalline additioncompound containing 2mol of trichloromethane.This result finds application in the preparation oftrichloromethane of the highest purity. Undercertain conditions, explosive and shocksensitiveproducts can result from the combination oftrichloromethane with alkali metals and certainother light metals [18].

Tetrachloromethane is nonflammable andrelatively stable even in the presence of light andair at room temperature.When heated in air in thepresence of metals (iron), phosgene is producedin large quantities, the reaction starting at ca. 300�C [19]. Photochemical oxidation also leads tophosgene. Hydrolysis to carbon dioxide and hy-drogen chloride is the principal result in a moistatmosphere [20]. Liquid tetrachloromethane hasonly a very minimal tendency to hydrolyze inwater at room temperature (half-life ca. 70 000years) [21].

Thermal decomposition of dry tetrachloro-methane occurs relatively slowly at 400 �C evenin the presence of the common industrial metals(with the exception of aluminum and other lightmetals). Above 500 – 600 �C an equilibriumreaction sets in

2CCl4�C2Cl4þ2Cl2 ð1Þ

which is shifted significantly to the right above700 �C and 0.1 MPa (1 bar) pressure. At 900 �Cand 0.1 MPa (1 bar), the equilibrium conversionof CCl4 is > 70% (see ! Chlorethanes andChloroethylenes, Section 2.5.).

Tetrachloromethane forms shock-sensitive,explosive mixtures with the alkali and alka-line-earth metals. With water it forms a hydrate-like addition compound which decomposes atþ 1.45 �C.

The telomerization of ethylene and vinylderivatives with tetrachloromethane under pres-sureand inthepresenceofperoxideshasacquiredacertain preparative significance [22–24]:

CH2 ¼ CH2þCCl4!CCl3�CH2�CH2Cl

The most important industrial reactions oftetrachloromethane are its liquid-phase conver-sion with anhydrous hydrogen fluoride in thepresence of antimony (III/V) fluorides or itsgas-phase reaction over aluminum or chromiumfluoride catalysts, both of which give the widelyused and important compounds trichloromono-fluoromethane (CFC 11), dichlorodifluorometh-ane (CFC 12), and monochlorotrifluoromethane(CFC 13).

4. Production

4.1. Theoretical Bases

The industrial preparation of chloromethane de-rivatives is based almost exclusively on thetreatment of methane and/or monochloro-methane with chlorine, whereby the chlorinationproducts are obtained as a mixture of the indi-vidual stages of chlorination:

CH4þCl2!CH3ClþHClDH ¼ �103:5kJ=mol ð2Þ

CH3ClþCl2!CH2Cl2þHClDH ¼ �102:5kJ=mol ð3Þ

CH2Cl2þCl2!CHCl3þHClDH ¼ �99:2kJ=mol ð4Þ

CHCl3þCl2!CCl4þHClDH ¼ �94:8kJ=mol ð5Þ

Thermodynamic equilibrium lies entirely onthe side of the chlorination products, so that the


distribution of the individual products is essen-tially determined by kinetic parameters.

Monochloromethane can be used in place ofmethane as the starting material, where this inturn can be prepared from methanol by usinghydrogen chloride generated in the previousprocesses. The corresponding reaction is:

CH3OHþHCl!CH3ClþH2ODH ¼ �33kJ=mol ð6Þ

In this way, the unavoidable accumulation ofhydrogen chloride (hydrochloric acid) can besubstantially reduced and the overall process canbe flexibly tailored to favor the production ofindividual chlorination products. Moreover, giv-en the ease with which it can be transported andstored, methanol is a better starting material forthe chloro derivatives than methane, a substancewhose availability is tied to natural gas resourcesor appropriate petrochemical facilities. There hasbeen a distinct trend in recent years towardreplacing methane as a carbon base withmethanol.

MethaneChlorination. The chlorination ofmethane and monochloromethane is carried outindustrially by using thermal, photochemical, orcatalytic methods [25]. The thermal chlorinationmethod is preferred, and it is also the one onwhich the most theoretical and scientific inves-tigations have been carried out.

Thermal chlorination of methane and its chlo-rine derivatives is a radical chain reaction initi-ated by chlorine atoms. These result from thermaldissociation at 300 – 350 �C, and they lead tosuccessive substitution of the four hydrogenatoms of methane:

The conversion to the higher stages of chlori-nation follows the same scheme [26–30]. Thethermal reaction of methane and its chlorinationproducts has been determined to be a second-order process:

dnðCl2Þ=dt ¼ k�pðCl2Þ�pðCH4Þ

It has further been shown that traces of oxygenstrongly inhibit the reaction. Controlling the highheat of reaction in the gas phase (which averagesca. 4200kJperm3of convertedchlorine) atSTP isa decisive factor in successfully carrying out theprocess. In industrial reactors, chlorine conver-sion first becomes apparent above 250 to 270 �C,but it increases exponentially with increasingtemperature [31], and in the region of commercialinterest – 350 to 550 �C – the reaction proceedsvery rapidly. As a result, it is necessary to initiatethe process at a temperature which permits thereaction to proceed by itself, but also to maintainthe reaction under adiabatic conditions at therequisite temperature level of 320 – 550 �C dic-tated by both chemical and technical considera-tions. If a certain critical temperature is exceededin the reaction mixture (ca. 550 – 700 �C, depen-dent both on the residence time in thehot zone andon the materials making up the reactor), decom-position of the metastable methane chlorinationproducts occurs. In that event, the chlorinationleads to formation of undesirable byproducts,including highly chlorinated or high molecularmasscompounds (tetrachloroethene,hexachloro-ethane, etc.). Alternatively, the reaction withchlorinecangetcompletelyoutofcontrol, leadingto the separation of soot and evolution of HCl(thermodynamically themoststableendproduct).Once such carbon formation begins it acts auto-catalytically, resulting in a progressively heavierbuildup of soot, which can only be halted byimmediate shutdown of the reaction.

Proper temperature control of this virtuallyadiabatic chlorination is achieved by workingwith a high methane : chlorine ratio in the rangeof 6 – 4 : 1. Thus, a recycling system is employedin which a certain percentage of inert gas ismaintained (nitrogen, recycled HCl, or evenmaterials such as monochloromethane or tetra-chloromethane derived from methane chlorina-tion). In this way, the explosive limits ofmethaneand chlorine are moved into a more favorableregion and it becomes possible to prepare themore highly substituted chloromethanes withlower CH4 : Cl2 ratios.

Figure 3 shows theexplosionrangeofmethaneand chlorine and how it can be limited through theuse of diluents, using the examples of nitrogen,hydrogen chloride, and tetrachloromethane.

The composition and distribution of the pro-ducts resulting from chlorination is a definite


function of the starting ratio of chlorine to meth-ane, as can be seen from Figure 4 and Figure 5.

These relationships have been investigatedfrequently [32, 33]. The composition of thereaction product has been shown to be in excel-lent agreement with that predicted by calcula- tions employing experimental relative reaction

rate constants [34–37]. The products arising fromthermal chlorination of monochloromethane andfrom the pyrolysis of primary products can alsobe predicted quantitatively [38]. The relation-ships among the rate constants are nearly inde-pendent of temperature in the region of technicalinterest. If one designates as k1 through k4 thesuccessive rate constants in the chlorinationprocess, then the following values can be as-signed to the relative constants for the individualstages:

k1 ¼ 1 (methane)k2 ¼ 2:91 (monochloromethane)k3 ¼ 2:0 (dichloromethane)k4 ¼ 0:72 (trichloromethane)

With this set of values, the selectivity of thechlorination can be effectively established withrespect to optimal product distribution for reac-tors of various residence time (stream type ormixing type, cf. Fig. 4 and Fig. 5). Additionalrecycling into the reaction of partially chlorinat-ed products (e.g., monochloromethane) permitsfurther control over the ratios of the individualcomponents [39, 40].

Figure 3. Explosive range of CH4 –Cl2 mixtures containingN2, HCl, and CCl4 Test conditions: pressure 100 kPa; tem-perature 50 �C; ignition by 1-mm spark

Figure 4. Product distribution in methane chlorination, plugstreamreactora) Methane; b) Monochloromethane; c) Dichloromethane;d) Trichloromethane; e) Tetrachloromethane

Figure 5. Product distribution in methane chlorination, idealmixing reactora) Methane; b) Monochloromethane; c) Dichloromethane;d) Trichloromethane; e) Tetrachloromethane


It has been recognized that the yield of par-tially chlorinated products (e.g., dichloro-methane and trichloromethane) is diminished byrecycling. This factor has to be taken into accountin the design of reactors for those methane chlor-inationswhich are intended to lead exclusively tothese products. If the emphasis is to lie more onthe side of trichloro- and tetrachloromethane,then mixing within the reactor plays virtually norole, particularly since less-chlorinated materialscan always be partially or wholly recycled. De-tails of reactor construction will be discussedbelow in the context of each of the variousprocesses.

Chlorinolysis. The technique for the pro-duction of tetrachloromethane is based on whatis knownasperchlorination, amethod inwhich anexcess of chlorine is used and C1- to C3-hydro-carbons and their chlorinated derivatives areemployed as carbon sources. In this process,tetrachloroethene is generated along withtetrachloromethane, the relationship between thetwo being consistent with Eq. 1 in page 13 anddependent on pressure and temperature (cf. alsoFig. 6).

It will be noted that at low pressure (0.1 to 1MPa, 1 to 10 bar) and temperatures above 700 �C,conditions under which the reaction takes placeat an acceptable rate, a significant amount oftetrachloroethene arises. For additional detailssee ! Chlorethanes and Chloroethylenes, Sec-tion 2.5.. Under conditions of high pressure –greater than 10 MPa (100 bar) – the reaction

occurs at a temperature as low as 600 �C. As aresult of the influence of pressure and by the useof a larger excess of chlorine, the equilibrium canbe shifted essentially 100% to the side of tetra-chloromethane. These circumstances are utilizedin the Hoechst high-pressure chlorinolysis pro-cedure (see below) [41, 42].

Methanol Hydrochlorination. Studieshave been conducted for purposes of reactordesign [43] on the kinetics of the gas-phasereaction of hydrogen chloride with methanol inthe presence of aluminum oxide as catalyst togive monochloromethane. Aging of the catalysthas also been investigated. The reaction is firstorder in respect to hydrogen chloride, but nearlyindependent of the partial pressure of methanol.The rate constant is proportional to the specificsurface of the catalyst, whereby at higher tem-peratures (350 – 400 �C) an inhibition due to porediffusion becomes apparent.

4.2. Production ofMonochloromethane

Monochloromethane is produced commerciallyby two methods: by the hydrochlorination (es-terification) of methanol using hydrogen chlo-ride, and by chlorination of methane. Methanolhydrochlorination has become increasinglyimportant in recent years, whereasmethane chlo-rination as the route to monochloromethane asfinal product has declined. The former approachhas the advantage that it utilizes, rather thangenerating, hydrogen chloride, a product whosedisposal – generally as hydrochloric acid – hasbecome increasingly difficult for chlorinatedhydrocarbon producers. Moreover, this methodleads to a single target product, monochloro-methane, in contrast to methane chlorination (cf.Figs. 4 and 5). As a result of the ready and low-cost availability of methanol (via the low pres-sure methanol synthesis technique) and its faciletransport and storage, the method also offers theadvantage of avoiding the need for placing pro-duction facilities in the vicinity of a methanesupply.

Since in the chlorination of methane eachsubstitution of a chlorine atom leads to genera-tion of an equimolar amount of hydrogen chlo-ride – cf. Eqs. 2 – 5 – a combination of the two

Figure 6. Thermodynamic equilibrium 2 CCl4�C2Cl4þ 2Cl2a) 0.1 MPa; b) 1 MPa; c) 10 MPa


methods permits a mixture of chlorinatedmethanes to be produced without creating largeamounts of hydrogen chloride at the same time;cf. Eq. 6.

Monochloromethane production from metha-nol and hydrogen chloride is carried out catalyti-cally in the gas phase at 0.3 – 0.6 MPa (3 – 6 bar)and temperatures of 280 – 350 �C. The usualcatalyst is activated aluminum oxide. Excesshydrogen chloride is introduced in order to pro-vide a more favorable equilibrium point (located96 – 99%on the side of products at 280 – 350 �C)and to reduce the formation of dimethyl ether as aside product (0.2 to 1%).

The raw materials must be of high purity inorder to prolong catalyst life as much as possible.Technically pure (99.9%) methanol is em-ployed, along with very clean hydrogen chloride.In the event that the latter is obtained fromhydrochloric acid, it must be subjected to specialpurification (stripping) in order to remove inter-fering chlorinated hydrocarbons.

Process Description. In a typical produc-tion plant (Fig. 7), the two raw material streams,hydrogen chloride and methanol, are warmedover heat exchangers and led, after mixing andadditional preheating, into the reactor, whereconversion takes place at 280 – 350 �C and ca.0.5 MPa (5 bar).

The reactor itself consists of a large number ofrelatively thin nickel tubes bundled together and

filled with aluminum oxide. Removal of heatgenerated by the reaction (33 kJ/mol) is accom-plished by using a heat conduction system. A hotspot forms in the catalyst layer as a result of theexothermic nature of the reaction, and thismigrates through the catalyst packing, reachingthe end as the latter’s useful life expires.

The reaction products exiting the reactor arecooled with recycled hydrochloric acid (> 30%)in a subsequent quench system, resulting in sep-aration of byproduct water, removed as ca. 20%hydrochloric acid containing small amounts ofmethanol. Passage through a heat exchangereffects further cooling and condensation of morewater, as well as removal of most of the excessHCl. The quenching fluid is recovered and sub-sequently returned to the quench circulation sys-tem. The gaseous crude product is led from theseparator into a 96% sulfuric acid column, wheredimethyl ether and residual water (present in aquantity reflective of its partial vapor pressure)are removed, the concentration of the acid dimin-ishing to ca. 80% during its passage through thecolumn. In this step, dimethyl ether reacts withsulfuric acid to form ‘‘onium salts’’ and methylsulfate. It can be driven out later by furtherdilution with water. It is advantageous to use therecovered sulfuric acid in the production of fer-tilizers (superphosphates) or to direct it to asulfuric acid cleavage facility.

Dry, crude monochloromethane is subse-quently condensed and worked up in a high-

Figure 7. Production of monochloromethane by methanol hydrochlorinationa) Heat exchangers; b) Heater; c) Multiple-tube reactor; d) Quench system; e) Quench gas cooler; f) Quenching fluid tank;g) Sulfuric acid column; h) CH3Cl condensation; i) Intermediate tank; j) CH3Cl distillation column


pressure (2 MPa, 20 bar) distillation column togive pure liquid monochloromethane. The gas-eous product emerging from the head of thiscolumn (CH3ClþHCl), along with the liquiddistillation residue – together making up ca.5 – 15% of the monochloromethane productmixture – can be recovered for introduction intoan associated methane chlorination facility. Theoverall yield of the process, calculated on thebasis of methanol, is ca. 99%.

The commonly used catalyst for vapor-phasehydrochlorination of methanol is g-aluminumoxide with an active surface area of ca. 200m2/g. Catalysts based on silicates have notachieved any technical significance. Catalystaging can be ascribed largely to carbon deposi-tion. Byproduct formation can be minimizedand catalyst life considerably prolonged bydoping the catalyst with various componentsand by introduction of specific gases (O2) intothe reaction components [44]. The life of thecatalyst in a production facility ranges fromabout 1 to 2 years.

Liquid-Phase Hydrochlorination. Theonce common liquid-phase hydrochlorinationof methanol using 70% zinc chloride solutionat 130 – 150 �C and modest pressure is currentlyof lesser significance. Instead, new productiontechniques involving treatment of methanolwith hydrogen chloride in the liquid phasewithout the addition of catalysts are becomingpreeminent. The advantage of these methods,apart from circumventing the need to handlethe troublesome zinc chloride solutions, is thatthey utilize aqueous hydrochloric acid, thusobviating the need for an energy-intensivehydrochloric acid distillation. The disadvantageof the process, which is conducted at 120 –160 �C, is its relatively low yield on a space –time basis, resulting in the need for large reactionvolumes [45–47].

Other Processes. Other techniques for pro-ducing monochloromethane are of theoreticalsignificance, but are not applied commercially.

Monochloromethane is formed when a mix-ture of methane and oxygen is passed into theelectrolytes of an alkali chloride electrolysis[48]. Treatment of dimethyl sulfate with alumi-num chloride [49] or sodium chloride [50] resultsin the formation of monochloromethane. Meth-

ane reacts with phosgene at 400 �C to give CH3Cl[51]. The methyl acetate –methanol mixture thatarises during polyvinyl alcohol synthesis can beconverted to monochloromethane with HCl at100 �C in the presence of catalysts [52]. It hasalso been suggested that monochloromethanecould be made by the reaction of methanol withthe ammonium chloride that arises during sodi-um carbonate production [53].

The dimethyl ether which results from meth-ylcellulose manufacture can be reacted withhydrochloric acid to give monochloromethane[54]. The process is carried out at 80 – 240 �Cunder sufficient pressure so that water remains asa liquid. Similarly, cleavage of dimethyl etherwith antimony trichloride also leads to mono-chloromethane [55].

In methanolysis reactions for the manufactureof silicones, monochloromethane is recoveredand then reintroduced into the process of silaneformation [56]:

SiCl2ðCH3Þ2þ2CH3OH!SiðOHÞ2ðCH3Þ2þ2CH3Cl ð10ÞSiþ2CH3Cl!SiCl2ðCH3Þ2 ð11Þ

4.3. Production of Dichloromethaneand Trichloromethane

The industrial synthesis of dichloromethane alsoleads to trichloromethane and small amounts oftetrachloromethane, as shown in Figure 4 andFigure 5. Consequently, di- and trichloromethaneare prepared commercially in the same facilities.In order to achieve an optimal yield of theseproducts and to ensure reliable temperaturecontrol, it is necessary to work with a largemethane and/or monochloromethane excess rel-ative to chlorine. Conducting the process in thisway also enables the residual concentration ofchlorine to be kept in the fully reacted product atan exceptionally low level (< 0.01 vol%), whichin turn simplifies workup. Because of the largeexcess of carbon-containing components, theoperation is customarily accomplished in a re-cycle mode.

Process Description. One of the oldest pro-duction methods is that of Hoechst, a recyclechlorination which was introduced as early as1923 andwhich, apart frommodifications reflect-


ing state-of-the-art technology, continues essen-tially unchanged, retaining its original impor-tance. The process is shown in Figure 8.

The gas which is circulated consists of amixture of methane and monochloromethane.To this is added fresh methane and, as appropri-ate, monochloromethane obtained from metha-nol hydrochlorination. Chlorine is then intro-duced and the mixture is passed into the reactor.The latter is a loop reactor coated with nickel orhighalloy steel inwhich internal gas circulation isconstantlymaintained bymeans of a coaxial inlettube and a valve system. The reaction is con-ducted adiabatically, the necessary temperatureof 350 – 450 �C being achieved and maintainedby proper choice of the chlorine to startingmaterial (CH4þCH3Cl) ratio and/or by pre-warming the mixture [57]. The fully reacted gasmixture is cooled in a heat exchanger and passedthrough an absorber cascade in which dilutehydrochloric acid and water wash out the result-ing hydrogen chloride in the form of 31% hydro-chloric acid. The last traces of acid and chlorineare removed by washing with sodium hydroxide,after which the gases are compressed, dried, andcooled and the reaction products largely con-densed. Any uncondensed gas – methane and tosome extent monochloromethane – is returned tothe reactor. The liquified condensate is separatedby distillation under pressure into its pure com-ponents, monochloromethane, dichloromethane,trichloromethane (the latter two being the prin-cipal products), and small amounts of tetrachlor-omethane. The product composition is approxi-

mately 70 wt% dichloromethane, isapproximately 27 wt% trichloromethane, and3 wt% tetrachloromethane.

Methane chlorination is carried out in a simi-lar way by Chemische Werke H€uls AG, whosework-up process employs prior separation ofhydrogen chloride by means of an adiabaticabsorption system. After the product gas hasbeen washed to neutrality with sodium hydrox-ide, it is dried with sulfuric acid and compressedto ca. 0.8 MPa (8 bar), whereby the majority ofthe resulting chloromethanes can be condensedwith relatively little cooling (at approximately�12 to � 15 �C). Monochloromethane is recycledto the chlorination reactor. The subsequent work-up to pure products is essentially analogous tothat employed in the Hoechst process.

Other techniques, e.g., those of Montecatiniand Asahi Glass, function similarly with respectto drying and distillation of the products.

The loop reactor used by these and othermanufacturers (e.g., Stauffer Chem. Co.) [58]has been found to give safe and trouble-freeservice, primarily because the internal circula-tion in the reactor causes the inlet gases to bebrought quickly to the initiation temperature,thereby excluding the possibility of formationof explosive mixtures. This benefit is achieved atthe expense of reduced selectivity in the conductof the reaction, however (cf. Figs. 4 and 5). Incontrast, the use of an empty tube reactor withminimal axial mixing has unquestionable advan-tages for the selective preparation of dichloro-methane [59, 60]. The operation of such a reactor

Figure 8. Methane chlorination by the Hoechst method (production of dichloromethane and trichloromethane)a) Loop reactor; b) Process gas cooler; c) HCl absorption; d) Neutralization system; e) Compressor; f) First condensation step(water); g) Gas drying system; h) Second condensation system and crude product storage vessel (brine); i) Distillation columnsfor CH3Cl, CH2Cl2, and CHCl3


is considerably more complex, however, espe-cially from the standpoint of measurement andcontrol technology, since the starting gases needto be brought up separately to the ignition tem-perature and then, after onset of the reaction withits high enthalpy, heatmust be removed bymeansof a cooling system. By contrast, maintenance ofconstant temperature in a loop reactor is relative-ly simple because of the high rate of gas circula-tion. A system operated by Frontier Chem. Co.employs a tube reactor incorporating recycledtetrachloromethane for the purpose of tempera-ture control [61].

Reactor Design. Various types of reactorsare in use, with characteristics ranging betweenthose of fully mixing reactors (e.g., the loopreactor) and tubular reactors. Chem. Werke H€ulsoperates a reactor that permits partial mixing,thereby allowing continuous operation with littleor no preheating.

Instead of having the gas circulation takeplace within the reactor, an external loop canalso be used for temperature control, as, e.g., inthe process described by Montecatini [62] andused in a facility operated by Allied ChemicalCorp. In this case, chlorine is added to the reactedgases outside of the chlorination reactor, neces-sary preheating is undertaken, and only then isthe gas mixture led into the reactor.

The space – time yield and the selectivity ofthe chlorination reaction can be increased byoperating two reactors in series, these beingseparated by a condensation unit to removehigh-boiling chloromethanes [63].

Solvay [64] has described an alternativemeans of optimizing the process in respect toselectivity, whereby methane and monochloro-methane are separately chlorinated in reactorsdriven in parallel. The monochloromethane pro-duced in the methane chlorination reactor isisolated and introduced into the reactor for chlo-rination of monochloromethane, which is alsosupplied with raw material from a methanolhydrochlorination system. The reaction is carriedout at a pressure of 1.5 MPa (15 bar) in order tosimplify the workup and separation of products.

Because of its effective heat exchange char-acteristics, a fluidized-bed reactor is used byAsahi Glass Co. for methane chlorination [65].The reaction system consists of two reactorsconnected in series. After separation of higher

boiling components, the low-boiling materialsfrom the first reactor, including hydrogen chlo-ride, are further treated with chlorine in a secondreactor. Reactors of this kindmust be constructedof special materials with high resistance to botherosion and corrosion. Special steps are required(e.g., washing with liquid chloromethanes) toremove from the reaction gas dust derived fromthe fluidized-bed solids.

Raw Materials. Very high purity standardsmust be applied to methane which is to bechlorinated. Some of this methane is derivedfrom petrochemical facilities in the course ofnaphtha cleavage to ethylene and propene,whereas some comes from low-temperature dis-tillation of natural gas (the Linde process). Com-ponents such as ethane, ethylene, and higherhydrocarbons must be reduced to a minimum.Otherwise, these would also react under theconditions of methane chlorination to give thecorresponding chlorinated hydrocarbons, whichwould in turn cause major problems in the puri-fication of the chloromethanes. For this reason,every effort is made to maintain the level ofhigher hydrocarbons below 100 mL/m3. Inertgases such as nitrogen and carbon dioxide (butexcluding oxygen) have no significant detrimen-tal effect on the thermal chlorination reaction,apart from the fact that their presence in exces-sive amounts results in the need to eliminateconsiderable quantities of off-gas from the re-cycling system, thus causing a reduction in prod-uct yield calculated on the basis of methaneintroduced.

Chlorine with a purity of ca. 97% (residue:hydrogen, carbon dioxide, and oxygen) is com-pressed and utilized just as it emerges fromelectrolysis. Newer chlorination procedures aredesigned to utilize gaseous chlorine of higherpurity, obtained by evaporization of previouslyliquified material.

Similarly, monochloromethane destined forfurther chlorination is a highly purified productof methanol hydrochlorination, special proce-dures being used to reduce the dimethyl ethercontent, for example, to less than 50 mL/m3.

Depending on the level of impurities presentin the starting materials, commercial processesincorporating recycling can lead to productyields of 95 – 99% based on chlorine or 70 –85% based on methane. The relatively low


methane-based yield is a consequence of the needfor removal of inert gases, although the majorityof this exhaust gas can be subjected to furtherrecovery measures in the context of some asso-ciated facility.

Off-Gas Workup. The workup of off-gasfrom thermal methane chlorination is relativelycomplicated as a consequence of the methaneexcess employed. Older technologies accom-plished the separation of the hydrogen chlorideproduced in the reaction through its absorption inwater or azeotropic hydrochloric acid, leading toordinary commercial 30 – 31% hydrochloricacid. This kind of workup requires amajor outlayfor materials of various sorts: on the one hand,coatings must be acid-resistant but at the sametime, materials which are stable against attack bychlorinated hydrocarbons are required.

A further disadvantage frequently plaguesthese ‘‘wet’’ processes is the need to find a usefor the inevitable concentrated hydrochloric acid,particularly given that the market for hydrochlo-ric acid is in many cases limited. Hydrogenchloride can be recovered from the aqueoushydrochloric acid by distillation under pressure,permitting its use inmethanol hydrochlorination;alternatively, it can be utilized for oxychlorina-tion of ethylene to 1,2-dichloroethane. Disadvan-tages of this approach, however, are the relativelyhigh energy requirement and the fact that the

hydrogen chloride can only be isolated by distil-lation to the point of azeotrope formation (20%HCl).

Newer technologies have as their goal workupof the chlorination off-gas by drymethods. Thesepermit use of less complicated construction ma-terials. Apart from the reactors, in which nickeland nickel alloys are normally used, all otherapparatus and components can be constructed ofeither ordinary steel or stainless steel.

Hydrogen chloride can be removed from theoff-gas by an absorption – desorption system de-veloped byHoechst AG and utilizing awashwithmonochloromethane, in which hydrogen chlo-ride is very soluble [66]. A similar procedureinvolving HCl removal by a wash with trichlor-omethane and tetrachloromethane has been de-scribed by Solvay [64].

Other Processes. The relatively complicat-ed removal of hydrogen chloride from methanecan be avoided by adopting processes that beginwith methanol as raw material. An integratedchlorination/hydrochlorination facility (Fig. 9)has been developed for this purpose and broughton stream on a commercial scale by StaufferChem. Co. [67].

Monochloromethane is caused to react withchlorine under a pressure of 0.8 – 1.5MPa (8 – 15bar) at elevated temperature (350 – 400 �C) withsubsequent cooling occurring outside of the

Figure 9. Chlorination of monochloromethane by the Stauffer process [68]a) Chlorination reactor; b) Quench system; c) Multistage condensation; d) Crude product storage vessel; e) Drying;f) Distillation and purification of CH2Cl2 and CHCl3; g) Hydrochlorination reactor; h) Quench system; i) H2SO4 dryingcolumn; j) Compressor


reactor. The crude reaction products are separat-ed in a multistage condensation unit and thenworked up by distillation to give the individualpure components. Monochloromethane is re-turned to the reactor. After condensation, gas-eous hydrogen chloride containing smallamounts of monochloromethane is reacted withmethanol in a hydrochlorination system corre-sponding to that illustrated in Figure 7 for theproduction of monochloromethane. Followingits compression, monochloromethane is returnedto the chlorination reactor. This process is distin-guished by the fact that only a minimal amountof the hydrogen chloride evolved during thesynthesis of dichloromethane and trichloro-methane is recovered in the form of aqueoushydrochloric acid.

As a substitute for thermal chlorination at hightemperature, processes have also been developedwhich occur by a photochemically-initiated rad-ical pathway. According to one patent [68],monochloromethane can be chlorinated selec-tively to dichloromethane at � 20 �C by irradia-tion with a UV lamp, the trichloromethane con-tent being only 2 – 3%.A corresponding reactionwith methane is not possible.

Liquid-phase chlorination of monochloro-methane in the presence of radical-producingagents such as azodiisobutyronitrile has beenachieved by the Tokuyama Soda Co. The reac-tion occurs at 60 – 100 �C and high pressure [69].The advantage of this low-temperature reactionis that it avoids the buildup of side productscommon in thermal chlorination (e.g., chlorinat-ed C2-compounds such as 1,1-dichloroethane,1,2-dichloroethene, and trichloroethene). Heatgenerated in the reaction is removed by evapo-ration of the liquid phase, which is subsequentlycondensed. Hydrogen chloride produced duringthe chlorination is used for gas-phase hydro-chlorination of methanol to give mono-chloromethane, which is in turn recycled forchlorination.

It is tempting to try to avoid the inevitableproduction of hydrogen chloride by carrying outthe reaction in the presence of oxygen, as in theoxychlorination of ethylene or ethane. Despiteintensive investigations into the prospects, how-ever, no commercially feasible applications haveresulted. The low reactivity of methane requiresthe use of a high reaction temperature, but this inturn leads to undesirable side products and an

unacceptably high loss of methane throughcombustion.

In this context, the ‘‘Transcat’’ process of theLummus Co. is of commercial interest [70]. Inthis process, methane is chlorinated and oxy-chlorinated in two steps in a molten salt mixturecomprised of copper(II) chloride and potassiumchloride. The starting materials are chlorine, air,and methane. The process leaves virtually noresidue since all of its byproducts can berecycled.

Experiments involving treatment of methanewith other chlorinating agents (e.g., phosgene,nitrosyl chloride, or sulfuryl chloride) have failedto yield useful results. The fluidized-bed reactionof methane with tetrachloromethane at 350 to450 �C has also been suggested [71].

The classical synthetic route to trichloro-methane proceeded from the reaction of chlorinewith ethanol or acetaldehyde to give chloral,which can be cleaved with calcium hydroxideto trichloromethane and calcium formate [72].Trichloromethane and calcium acetate can alsobe produced from acetone using an aqueoussolution of chlorine bleach at 60 – 65 �C. Adescription of these archaic processes can befound in [73].

4.4. Production ofTetrachloromethane

Chlorination of Carbon Disulfide, Thechlorination of carbon disulfide was, until thelate 1950s, the principal means of producingtetrachloromethane, according to the followingoverall reaction:

CS2þ2Cl2!CCl4þ2S ð12ÞThe resulting sulfur is recycled to a reactor for

conversion with coal or methane (natural gas) tocarbon disulfide. A detailed look at the reactionshows that it proceeds in stages corresponding tothe following equations:

2CS2þ6Cl2!2CCl4þ2S2Cl2 ð13ÞCS2þ2S2Cl2�CCl4þ6S ð14Þ

The process developed at the Bitterfeldplant of I.G. Farben before World War II wasimproved by a number of firms in the UnitedStates, including FMC and the Stauffer Chem.


Co. [74–76], particularly with respect to purifi-cation of the tetrachloromethane and the result-ing sulfur.

In a first step, carbon disulfide dissolved intetrachloromethane is induced to react with chlo-rine at temperatures of 30 – 100 �C. Either iron oriron(III) chloride is added as catalyst. The con-version of carbon disulfide exceeds 99% in thisstep. In a subsequent distillation, crude tetra-chloromethane is separated at the still head. Thedisulfur dichloride recovered from the still pot istransferred to a second stage of the process whereit is consumed by reaction with excess carbondisulfide at ca. 60 �C. The resulting sulfur isseparated (with cooling) as a solid, which hasthe effect of shifting the equilibrium in the reac-tions largely to the side of tetrachloromethane.Tetrachloromethane and excess carbon disulfideare withdrawn at the head of a distillation appa-ratus and returned to the chlorination unit. Aconsiderable effort is required to purify the tetra-chloromethane and sulfur, entailing hydrolysis ofsulfur compounds with dilute alkali and subse-quent azeotropic drying and removal from themolten sulfur by air stripping of residual disulfurdichloride. Yields lie near 90% of the theoreticalvalue based on carbon disulfide and about 80%based on chlorine. The losses, which must berecovered in appropriate cleanup facilities, resultfrom gaseous emissions from the chlorinationreaction, from the purification systems (hydroly-sis), and from the molten sulfur processing.

The carbon disulfide method is still employedin isolated plants in the United States, Italy, andSpain. Its advantage is that, in contrast to chlorinesubstitution on methane or chlorinating cleavagereactions, no accumulation of hydrogen chlorideor hydrochloric acid byproduct occurs.

Perchlorination (Chlorinolysis). Early inthe 1950s commercial production of tetrachlor-omethane based on high-temperature chlorina-tion of methane and chlorinating cleavage reac-tions of hydrocarbons (�C3) and their chlorinat-ed derivatives was introduced. In processes ofthis sort, known as perchlorinations or chlorino-lyses, substitution reactions are accompanied byrupture of C –C bonds. Starting materials, inaddition to ethylene, include propane, propene,dichloroethane, and dichloropropane. Increasinguse has been made of chlorine-containing by-products and the residues from other chlorination

processes, such as those derived from methanechlorination, vinyl chloride production (via ei-ther direct chlorination or oxychlorinationof ethylene), allyl chloride preparation, etc.The course of the reaction is governed by theposition of equilibrium between tetrachloro-methane and tetrachloroethene, as illustratedearlier in Figure 6, whereby the latter alwaysarises as a byproduct. In general, these processesare employed for the production of tetrachlor-oethylene (see ! Chloroethanes and Chlor-oethylenes, Section 2.5.3. and [77]), in whichcase tetrachloromethane is the byproduct. Mostproduction facilities are sufficiently flexible suchthat up to 70 wt% tetrachloromethane can beachieved in the final product [78]. The productyield can be largely forced to the side of tetra-chloromethane by recycling tetrachloroethyleneinto the chlorination reaction, although the re-quired energy expenditure is significant. Higherpressure [79] and the use of hydrocarbons con-taining an odd number of carbon atoms increasesthe yield of tetrachloromethane. When the reac-tion is carried out on an industrial scale, a tem-perature of 500 to 700 �C and an excess ofchlorine are used. The corresponding reactorseither can be of the tube type, operated adiabati-cally by using a recycled coolant (N2, HCl, CCl4,or C2Cl4) [80–82], or else they can be fluidized-bed systems operated isothermally [83, 84]. By-products under these reaction conditions includeca. 1 – 7%perchlorinated compounds (hexachlo-roethane, hexachlorobutadiene, hexachloroben-zene), the removal of which requires an addition-al expenditure of effort.

Pyrolytic introduction of chlorine into chlori-nated hydrocarbons has become increasinglyimportant due to its potential for consumingchlorinated hydrocarbon wastes and residuesfrom other processes. Even the relatively highproduction of hydrogen chloride can be tolerated,provided that reactors are used which operate athigh pressure and which can be coupled withother processes that consume hydrogen chloride.Another advantage of the method is that it can beused for making both tetrachloromethane andtetrachloroethylene. The decrease in demand fortetrachloromethane in the late 1970s and early1980s, a consequence of restrictions (related tothe ozone hypothesis) on the use of chlorofluor-ocarbons prepared from it, has led to stagnationin the development of new production capacity.


Hoechst High-Pressure Chlorinolysis.The high-pressure chlorinolysis method devel-oped and put in operation by Hoechst AG has thesame goals as the process just described. It can beseen in Figure 6 that under the reaction condi-tions of this process – 620 �C and 10 to 15 MPa(100 to 150 bar) – the equilibrium

2CCl4�C2Cl4þ2Cl2

lies almost exclusively on the side of tetra-chloromethane, especially in the presence of anexcess of chlorine [41, 42, 85]. This methodutilizes chlorine-containing residues from otherprocesses (e.g., methane chlorination and vinylchloride) as rawmaterial, although these must befree of sulfur and cannot contain solid or poly-merized components.

The conversion of these materials is carriedout in a specially constructed high-pressure tubereactor which is equipped with a pure nickel linerto prevent corrosion. Chlorine is introduced inexcess in order to prevent the formation of by-products and in order maintain the final reactiontemperature (620 �C) of this adiabatically con-ducted reaction. If hydrogen-deficient startingmaterials are to be employed, hydrogen-richcomponents must be added to increase the en-thalpy of the reaction. In this way, even chlorine-containing residues containing modest amountsof aromatics can be utilized. Hexachloroben-zene, for example, can be converted (albeit rela-tively slowly) at the usual temperature of thisprocess and in the presence of excess chlorine totetrachloromethane according to the equilibriumreaction:

C6Cl6þ9Cl2�6CCl4

The mixture exiting the reactor is comprisedof tetrachloromethane, the excess chlorine, hy-drogen chloride, and small amounts of hexa-chlorobenzene, the latter being recycled. Thismixture is quenched with cold tetrachloro-methane, its pressure is reduced, and it is subse-quently separated into crude tetrachloromethaneand chlorine and hydrogen chloride. The crudeproduct is purified by distillation to give tetra-chloromethane meeting the required specifica-tions. This process is advantageous in thosesituations in which chlorine-containing residuesaccumulatewhichwould otherwise be difficult todeal with (e.g., hexachloroethane from methane

chlorination facilities and high-boiling residuesfrom vinyl chloride production).

Anumber of serious technical problems had tobe overcome in the development of this process,including perfection of the nickel-lined high-pressure reactor, which required the design ofspecial flange connections and armatures.

Multistep Chlorination Process. Despitethe fact that its stoichiometry results in highyields of hydrogen chloride or hydrochloric acid,thermal chlorination of methane to tetrachloro-methane has retained its decisive importance.Recent developments have assured that the re-sulting hydrogen chloride can be fed into otherprocesses which utilize it. In principle, tetra-chloromethane can be obtained as the majorproduct simply by repeatedly returning all of thelower boiling chloromethanes to the reactor. It isnot possible to employ a 1 : 4 mixture of thereactantsmethane and chlorine at the outset. Thisis true not only because of the risk of explosion,but also because of the impossibility of dealingwith the extremely high heat of reaction. Unfor-tunately, the simple recycling approach is alsouneconomical because it necessitates the avail-ability of a very large workup facility. Therefore,it is most advantageous to employ several reac-tors coupled in series, the exit gases of each beingcooled, enriched with more chlorine, and thenpassed into the next reactor [86]. Processes em-ploying supplementary circulation of an inert gas(e.g., nitrogen) have also been suggested [87].

The stepwise chlorination of methane and/ormonochloromethane to tetrachloromethane isbased on a process developed in the late 1950sand still used by Hoechst AG (Fig. 10) [88].

The first reactor in a six-stage reactor cascadeis chargedwith the full amount ofmethane and/ormonochloromethane required for the entire pro-duction batch. Nearly quantitative chlorine con-version is achieved in the first reactor at 400 �C,using only a portion of the necessary overallamount of chlorine. The gas mixture leavingthe first reactor is cooled and introduced into thesecond reactor alongwith additional chlorine, themixture again being cooled after all of the addedchlorine has been consumed. This stepwise ad-dition of chlorine with intermittent cooling iscontinued until in the last reactor the componentratio CH4 : Cl2¼ 1 : 4 is reached. The reactorsthemselves are loop reactors with internal circu-


lation, a design which, because of its efficientmixing, effectively shifts the product distributiontoward more highly chlorinated materials. Thegas mixture leaving the reactors is cooled in twostages to � 20 �C, in the course of which themajority of the tetrachloromethane is liquified,along with the less chlorinated methane deriva-tives (amounting to ca. 3% of the tetrachloro-methane content). This liquid mixture is thenaccumulated in a crude product storage vessel.

The residual gas stream is comprised largelyof hydrogen chloride but contains small amountsof less highly chlorinated materials. This is sub-jected to adiabatic absorption of HCl using eitherwater or azeotropic (20%) hydrochloric acid,whereby technical grade 31% hydrochloric acidis produced. Alternatively, dry hydrogen chlo-ride can be withdrawn prior to the absorptionstep, which makes it available for use in otherprocesses which consume hydrogen chloride(e.g., methanol hydrochlorination). The steamwhich arises during the adiabatic absorption iswithdrawn from the head of the absorption col-umn and condensed in a quench system. Themajority of the chloromethanes contained in thisoutflow can be separated by subsequent coolingand phase separation. Wastewater exiting from

the quench system is directed to a strippingcolumn where it is purified prior to being dis-carded. Residual off-gas is largely freed fromremaining traces of halogen compounds by low-temperature cooling and are subsequently passedthrough an off-gas purification system (activatedcharcoal) before being released into the atmo-sphere, by which point the gas consists mainly ofnitrogen along with traces of methane.

The liquids which have been collected in thecrude product containment vessel are freed ofgaseous components – Cl2, HCl, CH3Cl – bypassage through a degassing/dehydrating col-umn, traces of water being removed by distilla-tion. Volatile components are returned to thereaction system prior to HCl absorption. Thecrude product is then worked up to pure carbontetrachloride in a multistage distillation facility.Foreruns (light ends) removed in the first columnare returned to the appropriate stage of the reactorcascade. The residue in the final column (heavyends), which constitutes 2 – 3 wt% of thetetrachloromethane production, is made up ofhexachloroethane, tetrachloroethylene, trichlo-roethylene, etc. This material can be convertedadvantageously to tetrachloromethane in a high-pressure chlorinolysis unit.

Figure 10. Production of tetrachloromethane by stepwise chlorination of methane (Hoechst process)a) Reactor; b) Cooling; c) First condensation (air); d) Second condensation (brine); e) Crude product storage vessel;f) Degassing/dewatering column; g) Intermediate tank; h) Light-end column; i) Column for pure CCl4; j) Heavy-end column;k) HCl stream for hydrochlorination; l) Adiabatic HCl absorption; m) Vapor condensation; n) Cooling and phase separation;o) Off-gas cooler


Overall yields in the process are ca. 95%based on methane and> 98% based on chlorine.

Other Processes. Oxychlorination as a wayof producing tetrachloromethane (as well as par-tially chlorinated compounds) has repeatedlybeen the subject of patent documents [89–91],particularly since it leads to complete utilizationof chlorine without any HCl byproduct. Pilot-plant studies using fluidized-bed technologyhave not succeeded in solving the problemof the high rate of combustion of methane.On the other hand the Transcat process, a two-stage approach mentioned in page 13 and em-bodying fused copper salts, can be viewed morepositively.

Direct chlorination of carbon to tetrachloro-methane is thermodynamically possible at atmo-spheric pressure below 1100K, but the rate of thereaction is very low because of the high activa-tion energy (lattice energy of graphite). Sulfurcompounds have been introduced as catalysts inthese experiments. Charcoal can be chlorinatedto tetrachloromethane in the absence of catalystwith a yield of 17% in one pass at 900 to 1100 Kand 0.3 – 2.0 MPa (3 – 20 bar) pressure. None ofthese suggested processes has been successfullyintroduced on an industrial scale. A review ofdirect chlorination of carbon is found in [92].

In this context it is worth mentioning thedismutation of phosgene

2COCl2!CCl4þCO2

another approach which avoids the formation ofhydrogen chloride. This reaction has been stud-ied by Hoechst [93] and occurs in the presence of10 mol% tungsten hexachloride and activatedcharcoal at 370 to 430 �C and a pressure of 0.8MPa. The process has not acquired commercialsignificance because the recovery of the WCl6 isvery expensive.

5. Quality Specifications

5.1. Purity of the CommercialProducts and their Stabilization

The standard commercial grades of all of thechloromethanes are distinguished by their highpurity (> 99.9 wt%). Dichloromethane, the sol-

vent with the broadest spectrum of applications,is also distributed in an especially pure form(> 99.99 wt%) for such special applications asthe extraction of natural products.

Monochloromethane and tetrachloromethanedo not require the presence of any stabilizer.Dichloromethane and trichloromethane, on theother hand, are normally protected from adverseinfluences of air and moisture by the addition ofsmall amounts of efficient stabilizers. The fol-lowing substances in the listed concentrationranges are the preferred additives:

Ethanol 0.1 – 0.2 wt%

Methanol 0.1 – 0.2 wt%

Cyclohexane 0.01 – 0.03 wt%

Amylene 0.001 – 0.01 wt%

Other substances have also been described asbeing effective stabilizers, including phenols,amines, nitroalkanes, aliphatic and cyclic ethers,epoxides, esters, and nitriles.

Trichloromethane of a quality correspondingto that specified in the Deutsche Arzneibuch, 8thedition (D.A.B. 8), is stabilized with 0.6 – 1 wt%ethanol, the same specifications as appear in theBritish Pharmacopoeia (B.P. 80). Trichloro-methane is no longer included as a substance inthe U.S. Pharmacopoeia, it being listed only inthe reagent index and there without anyspecifications.

5.2. Analysis

Table 6 lists those classical methods for testingthe purity and identity of the chloromethanes thatare most important to both producers and con-sumers. Since the majority of these are methodswith universal applicability, the corresponding

Table 6. Analytical testing methods for chloromethanes

Method

Parameter DIN ASTM

Boiling range 51 751 D 1078

Density 51 757 D 2111

Refraction index 53 491 D 1218

Evaporation residue 53 172 D 2109

Color index (Hazen) 53 409 D 1209

Water content (K. Fischer) 51 777 D 1744

pH value in aqueous extract – D 2110


Deutsche Industrie Norm (DIN) and AmericanSociety for the Testing of Materials (ASTM)recommendations are also cited in the Table.

Apart from these test methods, gas chroma-tography is also employed for quality controlboth in the production and shipment of chloro-methanes. Gas chromatography is especiallyapplicable to chloromethanes due to their lowboiling point. Even a relatively simple chromato-graph equipped only with a thermal conductivity(TC) detector can be highly effective at detectingimpurities, usually with a sensitivity limit of afew parts per million (mg/kg).

6. Storage, Transport, and Handling

Dry monochloromethane is inert with respect tomost metals, thus permitting their presence dur-ing its handling. Exceptions to this generaliza-tion, however, are aluminum, zinc, and magne-sium, as well as their alloys, rendering theseunsuitable for use. Thus most vessels for thestorage and transport of monochloromethane arepreferentially constructed of iron and steel.

Since it is normally handled as a compressedgas, monochloromethane must, in the FederalRepublic of Germany, be stored in accord withAccident Prevention Regulation (Unfall-verh€utungsvorschrift, UVV) numbers 61 and62 bearing the title ‘‘Gases Which Are Com-pressed, Liquified, or DissolvedUnder Pressure’’(‘‘Verdichtete, verfl€ussigte, oder unter Druckgel€oste Gase’’) and issued by the Trade Federa-tion of the Chemical Industry (Verband derBerufsgenossenschaften der chemischen Indus-trie). Additional guidelines are provided by gen-eral regulations governing high-pressure storagecontainers. Stored quantities in excess of 500 talso fall within the jurisdiction of the EmergencyRegulations (St€orfallverordnung) of the GermanFederal law governing emission protection.

Gas cylinderswith a capacity of 40, 60, 300, or700 kg are suitable for the transport of smallerquantities of monochloromethane. Shut-offvalves on such cylinders must be left-threaded.Larger quantities are shipped in containers, rail-road tank cars, and tank trucks, these generallybeing licensed for a working pressure of 1.3MPa(13 bar).

The three liquid chloromethanes are also nor-mally stored and transported in vessels con-

structed of iron or steel. The most suitable mate-rial for use with products of very high purity isstainless steel (material no. 1.4 571). The use instorage and transport vessels of aluminum andother light metals or their alloys is prevented byvirtue of their reactivity with respect to thechloromethanes.

Storage vessels must be protected against theincursion of moisture. This can be accomplishedby incorporating in their pressure release systemscontainers filled with drying agents such as silicagel, aluminum oxide, or calcium chloride. Alter-natively, the liquids can be stored under a dry,inert gas. Because of its very low boilingpoint, dichloromethane is sometimes stored incontainers provided either with external watercooling or with internal cooling units installed intheir pressure release systems.

Strict specifications with respect to safetyconsiderations are applied to the storage andtransfer of chlorinated hydrocarbons in order toprevent spillage and overfilling. Illustrative is thedocument entitled ‘‘Rules Governing Facilitiesfor the Storage, Transfer, and Preparation forShipment of Materials Hazardous to Water Sup-plies’’ (‘‘Verordnung f€ur Anlagen zum Lagern,Abf€ullen und Umschlagen wassergef€ahrdenderStoffe’’, VAwS). Facilities for this purposemust be equipped with the means for safelyrecovering and disposing of any material whichescapes [103].

Shipment of solvents normally entails the useof one-way containers (drums, barrels) made ofsteel and if necessary coated with protectivepaint. Where product quality standards are un-usually high, especially as regards minimal resi-due on evaporation, stainless steel is the materialof choice.

Larger quantities are shipped in containers,railroad tank cars, tank trucks, and tankers of boththe transoceanic and inland-waterway variety. Sothat product specifications may be met for mate-rial long in transit, it is important during initialtransfer to ensure high standards of purity and theabsence of moisture.

Rules for transport by all of the various stan-dard modes have been established on an interna-tional basis in the form of the following agree-ments: RID, ADR, GGVSee, GGVBinSch,IATA-DGR. The appropriate identification num-bers and warning symbols for labeling as haz-ardous substances are collected in Table 7.


The use and handling of chloromethanes –both by producers and by consumers of thesubstances and mixtures containing them – aregoverned in the Federal Republic of Germany byregulations collected in the February 11, 1982version of the ‘‘Rules Respecting Working Ma-terials’’ (‘‘Arbeitsstoff-Verordnung’’). To someextent, at least, these have their analogy in otherEuropean countries as well. Included are stipula-tions regarding the labeling of the pure sub-stances themselves as well as of preparationscontaining chloromethane solvents. The centralauthorities of the various industrial trade orga-nizations issue informational and safety bro-chures for chlorinated hydrocarbons, and theseshould be studied with care.

The standard guidelines for handling mono-chloromethane as a compressed gas are the‘‘Pressure Vessel Regulation’’ (‘‘Druckbeh€al-ter-Verordnung’’) of February 27, 1980, with therelated ‘‘Technical Rules for Gases’’ (‘‘Tech-nische Regeln Gase’’, TRG) and the ‘‘TechnicalRules for Containers’’ (‘‘Technische RegelnBeh€alter’’, TRB), as well as ‘‘Accident Preven-tion Guideline 29 – Gases’’ (‘‘Unfallver-h€utungsvorschrift [UVV] 29, Gase’’).

For MAK values, TLV values, and considera-tions concerning toxicology and ecotoxicologyof the chloromethanes see Chapter 139.

7. Behavior of Chloromethanesin the Environment

Chloromethanes are introduced into the environ-ment from both natural and anthropogenicsources. They are found in the lower atmosphere,and tetrachloromethane can even reach into thestratosphere. Trichloromethane and tetrachloro-methane can be detected in many water supplies.

The chloromethanes, like other halogenatedhydrocarbons, are viewed aswater contaminants.Thus, they are found in both national and inter-

national guidelines related to water quality pro-tection [94, 95].

There are fundamental reasons for needing torestrict chlorocarbon emissions to an absoluteminimum. Proven methods for removal of chlor-omethanes from wastewater, off-gas, and resi-dues are

Vapor stripping with recyclingAdsorption on activated charcoal and recyclingRecovery by distillationReintroduction into chlorination processes [96]Combustion in facilities equipped with offgas

cleanup

7.1. Presence in the Atmosphere

All four chloromethanes are emitted to the atmo-sphere from anthropogenic sources. In addition,large quantities of monochloromethane are re-leased into the atmosphere by the combustionof plant residues and through the action ofsunlight on algae in the oceans. Estimates ofthe extent of nonindustrial generation of mono-chloromethane range from 5� 106 t/a [97] to28� 106t/a [98].

Natural sources have also been considered fortrichloromethane [99] and tetrachloromethane[100] on the basis of concentration measure-ments in the air and in seawater (Table 8).

The emission of chloromethanes from indus-try is the subject of legal restrictions in manycountries. The applicable regulations in the Fed-eral Republic of Germany are those of the TALuft [101].

The most important sink for many volatileorganic compounds is their reaction in thelower atmosphere with photochemicallygenerated OH radicals. The reactivity of mono-chloromethane, dichloromethane, and trichloro-methane with OH radicals is so high that inthe troposphere these substances are relativelyrapidly destroyed.

Table 8. Atmospheric concentration of chloromethanes (in 10�10vol.

%) [99]

Compound Continents Oceans Urban areas

CH3Cl 530 . . . 1040 1140 . . . 1260 834

CH2Cl2 36 35 <20 . . . 144

CHCl3 9 . . . 25 8 . . . 40 6 . . . 15 000

CCl4 20 . . . 133 111 . . . 128 120 . . . 18 000

Table 7. Identification number and hazard symbols of chloromethanes

Identification

Product number Hazard symbol

Monochloromethane UN 1063 H (harmful)

IG (inflammable gas)

Dichloromethane UN 1593 H (harmful)

Trichloromethane UN 1888 H (harmful)

Tetrachloromethane UN 1846 P (poison)


By contrast, the residence time in the tropo-sphere of tetrachloromethane is very long, withthe result that it can pass into the stratosphere,where it is subjected to photolysis from hardultraviolet radiation. The Cl atoms released inthis process play a role in the ozone degradationwhich is presumed to occur in the stratosphere(Table 9) [102].

7.2. Presence in Water Sources

Seawater has been found to contain relativelyhigh concentrations of monochloromethane(5.9 – 21�10�9 mL of gas/mL of water) [98], inaddition to both trichloromethane (8.3 – 14�10�9 g/L) [99] and tetrachloromethane (0.17 –0.72�10�9 g/L) [99]. Dichloromethane, on theother hand, could not be detected [97].

Chloromethanes can penetrate both surfaceand groundwater through the occurrence of ac-cidents or as a result of improper handling duringproduction, transportation, storage, or use (Table10). Groundwater contamination by rain whichhas washed chlorinated hydrocarbons out of theair is not thought to be significant on the basis ofcurrent knowledge. One frequent additionalcause of diffuse groundwater contamination thatcan be cited is defective equipment (especiallyleaky tanks and wastewater lines) [103].

The chloromethanes are relatively resistant tohydrolysis. Only in the case of monochloro-methane in seawater is abiotic degradation ofsignificance, this compound being subject in

weakly alkaline medium to cleavage with theelimination of HCl.

The microbiological degradability of dichlor-omethane has been established [106–112]. Thisis understood to be the reason for the absence oronly very lowconcentrations of dichloromethanein the aquatic environment [103].

Since trichloromethane and tetrachloro-methane are stable compounds with respect toboth biotic and abiotic processes, their disap-pearance is thought to be largely a consequenceof transfer into the atmosphere by natural strip-ping phenomena.

Treatment with chlorine is a widespread tech-nique for disinfecting drinking water. In theprocess, trihalomethanes result, largely trichlor-omethane as a result of the reaction of chlorinewith traces of organic material. A level of 25 mg/L of trihalomethanes is regarded in the FederalRepublic of Germany as the maximum accept-able annual median concentration in drinkingwater [113].

8. Uses and Economic Aspects

As a result of very incomplete statistical recordsdetailing production and foreign trade by indi-vidual countries, it is very difficult to describeprecisely the world market for chloromethanes.The information which follows is based largelyon systematic evaluation of the estimates ofexperts, coupledwith data found in the secondaryliterature, as well as personal investigations andcalculations.

The Western World includes about 40 produ-cers who produce at least one of the chlorinatedC1 hydrocarbons. No authoritative information isavailable concerning either the production ca-pacity or the extent of its utilization in theComecon nations or in the People’s Republic ofChina. It can be assumed, however, that a largepart of the domestic requirements in these coun-tries is met by imports. In reference to productioncapacity, see [114].

In comparing the reported individual capaci-ties it is important to realize that a great manyfacilities are also capable of producing otherchlorinated hydrocarbons. This situation is aresult of the opportunities for flexibility both inthe product spectrum (cf. Section 4.1) and in thevarious manufacturing techniques (e.g., tetra-

Table 9. Velocity of decomposition of chloromethanes in the atmo-

sphere [97]

Reaction velocity

with OH radicals

kOH�1012 cm3 Half-life,

Compound molecule�1 s�1 weeks

CH3Cl 0.14 12

CH2Cl2 0.1 15

CHCl3 0.1 15

CCl4 <0.001 >1000

Table 10. Chloromethane concentration in the Rhine river (mg/L)[104, 105]

Compound Date Mean value Max. value

CH2Cl2 1980 not detected

CHCl3 1980 4.5

CHCl3 1982 0.4 . . . 12.5 50.0

CCl4 1982 <0.1 . . . 3.3 44.4


chloromethane/tetrachloroethene, cf. Section4.4). If one ignores captive use for further chlo-rination (especially of monochloromethane), itcan be concluded that the largest portion of theworld use of chloromethanes (ca. 34%) can beattributed to tetrachloromethane. The mostimportant market, accounting for over 90% ofthe material produced, is that associated with theproduction of the fluorochlorocarbons CFC 11(trichloromonofluoromethane) and CFC 12(dichlorodifluoromethane). These fluorochloro-carbons possess outstanding properties, such asnonflammability and toxicological safety, andare employed as refrigerants, foaming agents,aerosol propellants, and special solvents.

The production level of tetrachloromethane isdirectly determined by the market for its fluori-nated reaction products CFC 11 and CFC 12. Theappearance of the so-called ozone theory, whichasserts that the ozone layer in the stratosphere isaffected by these compounds, has resulted since1976 in a trend toward reduced production oftetrachloromethane. This has been especiallytrue since certain countries (United States,Canada, Sweden) have imposed a ban onaerosol use of fully halogenated fluorochlorocar-bons. However, since 1982/1983 there has beena weak recovery in demand for tetrachloro-methane in the production of fluorine-containingcompounds.

Outside Europe, a smaller amount of tetra-chloromethane finds use as a disinfectant and as afungicide for grain.

Monochloromethane and dichloromethaneeach account for about 25% of the world marketfor chloromethanes (Table 11). The demand formonochloromethane can be attributed largely(60 – 80%) to the production of silicones. Its useas a starting material for the production of thegasoline anti-knock additive tetramethyllead isin steep decline.

The most important use of dichloromethane,representing ca. 40 – 45% of the total market, isas a cleaning agent and paint remover. An addi-tional 20 – 25% finds application as a pressuremediator in aerosols. One further use of dichlor-omethane is in extraction technology (decaffei-nation of coffee, extraction of hops, paraffinextraction, and the recovery of specialtypharmaceuticals).

In all of these applications, especially thoserelated to the food and drug industries, the puritylevel requirements for dichloromethane are ex-ceedingly high (> 99.99 wt%).

Trichloromethane holds the smallest marketshare of the chloromethane family: 16%. Itsprincipal application, amounting to more than90% of the total production, is in the productionof monochlorodifluoromethane (CFC 22), acompound important on the one hand as a refrig-erant, but also a key intermediate in the prepara-tion of tetrafluoroethene. The latter can be poly-merized to give materials with exceptional ther-mal and chemical properties, including PTFE,Hostaflon, Teflon, etc.

Chloroform is still used to a limited extent asan extractant for pharmaceutical products. Due toits toxicological properties, its use as an inhala-tory anaesthetic is no longer significant. Smallamounts are employed in the synthesis of ortho-formic esters.

Table 12 provides an overview of thestructure of the markets for the various chlori-nated C1 compounds, subdivided according toregion.

9. Toxicology

Monochloromethane. Chloromethane [74-87-3], methyl chloride, is an odorless gas and,except for freezing the skin or eyes due to

Table 11. Production capacities of chloromethanes 1000 t/year [139]

Western Europe (FRG) United States Japan

1981 1993 1981 1993 1981 1993

Monochloromethane 265 295 (100) 300 274 70 106

Dichloromethane 410 237 (170) 370 161 65 86

Trichloromethane 140 247 (60) 210 226 65 53

Tetrachloromethane 250 182 (150) 380 140 70 40


evaporation, inhalation is the only significantroute of exposure. It acts mainly on the centralnervous system with well documented cases ofexcessive human exposure, leading to injury andeven death [132]. The symptoms of overexposureare similar to inebriationwith alcohol (a shufflinggait, incoordination, disorientation, and changein personality), but last much longer, possiblypermanent in severe exposures. According toexperimental results, excessive exposure tomethyl chloride was carcinogenic in mice andalso affected the testes of male rats and fetuses ofpregnant female rats [138]. It is mutagenic incertain in vitro test systems. Available referencesindicate that methyl chloride may increase therate of kidney tumors in mice in conjunctionwithrepeated injury to this organ. The TLV and theMAK (1985) are both 50 ppm (105 mg/m3).

Dichloromethane. Dichloromethane [75-09-2], methylene chloride, is the least toxic of

the chlorinated methanes. It is moderate in tox-icity by ingestion, but the liquid is quite painful tothe eyes and skin, particularly if confined on theskin [132–134]. Absorption through the skin isprobably of minor consequence if exposure iscontrolled to avoid irritation.

Inhalation is the major route of toxic expo-sure. The principal effects of exposure to highconcentrations (greater than 1000 ppm) areanesthesia and incoordination. Exposure tomethylene chloride results in the formation ofcarboxyhemoglobin (COHb) caused by its me-tabolism to carbon monoxide. This COHb is astoxic as that derived from carbon monoxideitself. However, at acceptable levels of expo-sure to methylene chloride, any probable ad-verse effects of COHb will be limited to per-sons with pronounced cardiovascular orrespiratory problems. Other possible toxic ef-fects of carbon monoxide itself would not beexpected.

Methylene chloride is not teratogenic in ani-mals [135] and has only limited mutagenicactivity in Salmonella bacteria. It does not appearto be genotoxic in other species. Available re-ports of lifetime studies at high concentrationshave produced inconsistent results in hamsters,rats, and mice. No tumors, benign or malignant,were increased in hamsters; rats developed only adose related increase in commonly occurringnonmalignant mammary tumors; white mice,both sexes, had a large increase in cancers ofthe livers and lungs. Available epidemiologicaldata do not indicate an increase in cancer inhumans; they do indicate that the current occu-pational standards are protective of employeehealth [136, 137].

Trichloromethane. Trichloromethane [67-66-3], chloroform, is only moderately toxic fromsingle exposure, but repeated exposure can resultin rather severe effects [132–134]. Its use as asurgical anesthetic has become obsolete, primar-ily because of delayed liver toxicity and thedevelopment of anesthetics with a greater marginof safety.

Ingestion is not likely to be a problem unlesslarge quantities are swallowed accidentally ordeliberately. Chloroform has a definite solventaction on the skin and eyes and may be absorbedif exposure is excessive or repeated. Its recog-nized high chronic toxicity requires procedures

Table 12. Demand and use pattern of chloromethanes (1983)

Western United

Europe States Japan

Monochloromethane 230 000 t 250 000 t 50 000 t

Silicone 52% 60% 83%

Tetramethyllead 12% 15% –

Methylcellulose 15% 5% 1%

Other methylation

reactions, e.g., tensides,

pharmaceuticals ca. 21% ca. 20% ca. 16%

Dichloromethane 210 000 t 270 000 t 35 000 t

Degreasing and paint

remover

46% 47% 54%

Aerosols 18% 24% 19%

Foam-blowing agent 9% 4% 11%

Extraction and

other uses

27% 25% 16%

Trichloromethane 90 000 t 190 000 t 45 000 t

CFC 22 production 78% 90% 90%

Other uses,

e.g., pharmaceuticals,

intermediate

22% 10% 10%

Tetrachloromethane 250 000 t 250 000 t 75 000 t

CFC 11/12 production 94% 92% 90%

Special solvent for

chemical reactions

6% 8% 10%


and practices to control ingestion, skin, andeye contact, as well as inhalation exposure ifliver and kidney injury, the most likely conse-quence of excessive exposure, is to beprevented.

In animals, chloroform is fetotoxic (toxic tothe fetus of a pregnant animal) but only weaklyteratogenic if at all [135]. It does not appear tobe mutagenic by common test procedures, butincreases the tumor incidence in certain ratsand mice. There is considerable evidence thatthe tumors in rat kidneys and mice livers arethe result of repeated injury to these organs andthat limiting exposure to levels that do notcause organ injury will also prevent cancer. Itis, therefore, very important that humanexposure be carefully controlled to preventinjury.

Tetrachloromethane. Tetrachloromethane[56-23-5], carbon tetrachloride, was once recom-mended as a ‘‘safety solvent.’’ Misuse and itsrather high liver toxicity, as well as the readyavailability of alternate safe solvents, have elim-inated its application as a solvent. Single expo-sures are not markedly injurious to the eyes andskin or toxic when small quantities are ingested.However, repeated exposure must be carefullycontrolled to avoid systemic toxicity, particularlyto the liver and kidneys [132–134]. In humans,injury to the kidney appears to be the principalcause of death.

Inhalation can produce anesthesia at highconcentrations, but transient liver as well askidney injury result at much lower concentra-tions than those required to cause incoordina-tion. There appears to be individual suscepti-bility to carbon tetrachloride, with some hu-mans becoming nauseated at concentrationsthat others willingly tolerate. Ingestion ofalcohol is reported to enhance the toxicity ofcarbon tetrachloride. Such responses should notoccur, however, if exposures are properly con-trolled to the recommended occupationalstandards.

Carbon tetrachloride is not teratogenic inanimals [135] nor mutagenic in common testsystems, but does increase liver tumors inmice, probably as a result of repeated injury tothat organ. Therefore, it is very important thathumanexposurebecarefullycontrolled topreventliver injury.

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112 T. Leisinger, Experientica 39 (1983) 1183–1191.

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116 Kirk-Othmer,5, pp. 714–762; 23, pp. 764–798, 865–885.

117 L. Scheflan:TheHandbook of Solvents, D. vanNostrand,

New York 1953.

118 G. Hawley: The Condensed Chemical Dictionary, Van

Nostrand, New York 1977.

119 S. Patai:TheChemistry of theC-HalogenBond, Part 1–2,

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oethane, Trichloroethylene’’ in Chemical Economics

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5A–5C; Vinyl Chloride, Stanford Research Institute,

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123 E. W. Flick: Industrial Solvents Handbook, 3rd ed.,

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2, Menlo Park, California, USA, 1984.

126 Stanford Research Institute: Chem. Econ. Handbook,

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ogy, 3rd ed., vol. 2 B, Wiley-Interscience, New York

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133 American Conference of Governmental Industrial

Hygienists Inc. Documentation of the Threshold Limit

Values 1980 (WithAnnual Supplements) 6500Glenway

Bldg. D-5, Cincinnati, OH 45211.

134 Commission for Investigation of the Health Hazards of

Chemical Compounds in the Work Environment: Tox-

ikologisch-arbeitsmedizinische Begr€undung von MAK-

Werten, Verlag Chemie, Weinheim 1984.

135 J. A. John, D. J. Wroblewski, B. A. Schwetz:

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andReviews in Teratology, vol. 2, Plenum, 1984, p. 267–

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136 American Conference of Governmental Industrial

Hygienists. TLV’s�

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Chemical Substances in theWork Environment Adopted

by the ACGIH for 1985–1986. ACGIH 6500 Glenway

Bldg. D-5, Cincinnati, OH 45211.

137 Deutsche Forschungsgemeinschaft: MAK, Verlag Che-

mie, Weinheim 1984.

138 ‘‘The Chemical Industry Institute of Toxicology’’: Un-

published data on methyl chloride. Research Triangle

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139 K. Weissermehl, H.—J. Arpe, Industrielle Organische

Chemie, 5. Auflage Wiley-VCH, 1998.

Further Reading

J. de Boer (ed.): Chlorinated Paraffins, Springer, Berlin

Heidelberg 2010.

G. Kreysa, M. Sch€utze (eds.): Corrosion Handbook, 2nd ed.,

DECHEMA/Wiley-VCH, Weinheim 2009.

K. A. Marshall: ‘‘Chlorocarbons and Chlorohydrocarbons,

Survey’’, Kirk Othmer Encyclopedia of Chemical Tech-

nology, 5th edition, John Wiley & Sons, Hoboken, NJ,

online DOI: 10.1002/0471238961.1921182218050504.

a01.pub2.

R. A. Meyers, D. K. Dittrick (eds.): Encyclopedia of Envi-

ronmental Pollution and Cleanup, Wiley, New York

1999.

P. Patnaik (ed.): A Comprehensive Guide to the Hazardous

Properties of Chemical Substances, 3rd ed., Wiley, Ho-

boken, NJ 2007.

C. E. Wilkes, J. W. Summers, C. A. Daniels (eds.): PVC

Handbook, Hanser Gardner, Cincinnati, OH 2005.


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