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Reaction of Dihydroxymethyl Radical with Molecular Oxygen in Aqueous Solution

Eberhard Bothe and Dietrich Schulte-Frohlinde* Institut für Strahlenchemie im Max-Planck-Institut für Kohlenforschung, Stiftstraße 34-36, D-4330 Mülheim a. d. Ruhr

Z. Naturforsch. 35b, 1035-1039 (1980); eingegangen am 7. März 1980

Dihydroxymethyl Radical, Oxydation with Oxygen, Conductivity, Aqueous Solution

Dihydroxymethyl radical (1) was found to react with oxygen diffusion controlled (k = 4.5 x 109 M _ 1 s - 1 ) . The final products of the reaction are formic acid and 1/2 hydrogen peroxide. The precursor of H2O2 was shown to be O2' - which is formed with G(02" -) = 6.3 (a value which includes the 02"~ formed by H atoms and oxygen (G(H) = 0.55)). If it is assumed that dihydroxymethylperoxyl radicals are formed as intermediates the rate constant of 0 2 ' ~ formation must be > 10 6s - 1 . A bimolecular reaction of two dihydroxy-methylperoxyl radicals forming tetroxides does not occur at the dose rates used. The rate constants for the establishment of the equilibrium

CH 2 (0H) 2 + H2O2 CHaOH(OOH) + H a O k-3

have been determined at p H 5.7 and 19 °C to be k 3 = 1 . 9 x 10~2 M ^ s - 1 and k_3 = 1.5 X 10~6 M _ 1 s _ 1 in aqueous solution.

Introduction The decay of aliphatic a-hydroxyperoxyl radicals

in aqueous solution has been studied extensively during the past decade [1-12]. If present in high concentrations these radicals decompose in second order reactions. At low radical concentrations first order reactions are predominant leading to alde-hydes (or ketones) and 02"~ radicals. In the first order reactions an OH_-induced reaction, equation (1), has been distinguished from a spontaneous decay, equation (2).

H2C(0H)02- + OH- H2CO + 0 2 - + H 2 0 (1)

H2C(0H)02- -> H2CO + H+ + 0 2 - (2)

The spontaneous first order elimination rate con-stant increases with increasing methyl substitution, e.g. kei = < 5 , 50 and 600 s - 1 for the decay of a-hydroxyperoxyl radicals derived from methanol, ethanol and isopropanol, respectively [3].

In the present work the influence of an additional OH as substituent on the decay rate, on the forma-tion of 02"_ and on the final products was studied. The influence of a second OH or alkoxy group on the elimination rate is of interest since an unusual high rate for the formation of 02 '~ (k > 7.0 X 105 s"1) was observed with a similar radical derived from glucose [4],

* Reprint requests to Prof. Dr. D. Schulte-Frohlinde. 0340-5087/80/0800-1035/$ 01.00/0

Experimental Aqueous solutions of formaldehyde hydrate

(10~3-10 -2 M) were irradiated with 1 or 2 ys electron pulses (0.1-2 krad) from a van de Graaff accelerator (2.8 MeV). Solutions containing H 2 0 2 (10~2M) and formaldehyde hydrate (5 X 10~3 M) were photolyzed with UV flashes (half width of the flash ~ 5 ys, X > 250 nm). The experimental set up employed has been described earlier [13]. The solutions were prepared from formaldehyde which was freshly produced by heating paraformaldehyde and dis-solving the released gaseous formaldehyde in argon saturated water. It should be emphasized that this procedure was found to be necessary in order to obtain formaldehyde hydrate free of polymers. Distillation of aqueous solutions of formaldehyde did not give solutions free of polymers. The hydrate of formaldehyde in aqueous solution is formed spontaneously from the aldehyde within one second at room temperature [14]. pH Values were adjusted with NaOH or HCIO4. Unless otherwise stated the solutions were saturated with a mixture of N2O/O2 (80/20 v/v) for electron irradiation and with oxygen for flash photolysis. With one exception all measure-ments were performed at 2 1 ± 2 C . The yield of formic acid was determined conductometrically or by titration with NaOH and the yield of H2O2 and organic peroxides was measured iodometrically as described by Allan et al. [15]. The yield of H021

radicals was determined by time dependent conduc-tivity measurement after the electron pulse.

Results 1. Flash photolysis

Solutions containing H2C(OH)2 (5 x 10~3 M) and H2O2 (IO-2 M) at pH 4.5-6.5 showed a fast initial

1036 E. Bothe-D. Schulte-Frohlinde • Reaction of Dihy droxy lmethyl Radical with Oxygen 1036

increase of conductivity within flash duration. After the flash the conductivity decayed within some hundred ms to a constant value which was higher than the level prior to the flash, i.e. transient and permanent conductivity changes were produced (Figure 1). The ratio of the jaelds of permanent to transient conductivity was observed to decrease with decreasing pH (range 4.4-6.5) and with in-creasing flash intensity (range varied within a factor 5). An upper limit of unity was reached with small flash intensities, i.e. at low radical concentra-tions, and at a pH-value of 6.5. The half-life of the decay of the transient conductivity was found to decrease with increasing flash intensity. A bimolec-ular rate constant of approximately 2 k = 2 x 107 M - 1 s - 1 at pH = 6.5 was calculated from this dependence assuming homogeneous distribution of the decaying ions. This rate constant is similar to that found for the decay of H+ and 02'~ at pH = 6.5 [16].

2. Pulse radiolysis

In aqueous solutions of H2C(OH)2 (5 X 10 - 3 M) at pH 3.5-6.5 an increase in conductivity with a half-life of a few /us is observed after the decay of H+ and O H - produced by the electron pulse. The kinetics of the build up was of first order with a rate constant which was smaller when the concentration of oxygen in the solution was lowered. A straight line was obtained when the reciprocal half-life of the build up

Fig. 1. Conductivity sig-nal (ordinate, 50 mV/div) versus time (abszissa, 20 ms/div) as observed in flash photolysis of an oxygenated aqueous solu-tion containing 10 - 2 M H 2 0 2 and 5 x 10 - 3 M for-maldehyde at 21 °C and pH — 6.5.

(0.55 < ti/2 < 6 . 5 jus) wTas plotted versus the oxygen concentration (2 x 10 -5 < [02 ] < 2.7 X 10 - 4M). From the slope of the line a rate constant of 4.5 X 109

M _ 1 s - 1 db 15% was calculated for the reaction of an intermediate with oxygen leading to a conducting species. Experiments at + 1-5 °C showed that the rate determining step for the formation of the conductivity is still the reaction of the radicals with oxygen. From the value reached after the end of this reaction the conductivity decayed to a permanent level that was higher than the value prior to the pulse, i. e. a permanent and a transient conductivity change are produced by the pulse with G values Gend and Gtrans. The G values were calcu-lated from the conductivity changes using 323 and 55 cm2 i2 -1 equivalent -1 for the equivalent conduc-tivity of H+ and the anion respectively. Figure 2 shows Gend (curve B), Gtrans (curve C) and Gmax = Gend + Gtrans (curve A) as determined from the results of our conductivity measurements in the pH range 3.5 < p H < 6 . 5 (errors ± 10%).

The decay of the transient conductivity was predominantly of second order with a rate constant of 2k ~ 2 X 107 M-1 s - 1 at pH = 6.5. This constant was derived from the slope of the linear plot of ti/2-1 versus dose rate (0.2 krad pulse -1 < dose rate < 2 krad pulse -1), neglecting a small first order contri-bution of the decay. Similar results have been observed previously for the decay of transient conductivity caused by H+ and 02* ~ [17]. Thus we

E. Bothe-D. Schulte-Frohlinde • Reaction of Dihy droxy lmethyl Radical with Oxygen 1037

1 2 -

G(H+) -

8 -

U-

0 4 5 6 7

• pH

Fig. 2. G-values for the formation of protons calculated from conductivity changes as observed after 1 ys electron pulses in aqueous solutions containing form-aldehyde (3 x 10~3 M), N 2 0 (2.2 X IO-2 M) and 0 2 (2.8 X 10 - 4 M) as a function of pH at room tem-perature; dose per pulse « 0 . 3 krad. Curve A : maximum yield (Gmax) observed 10 /is after the pulse. Curve B : final yield of stable acid (Gend) measured 300 ms after the pulse. Curve C: transient conductivity (Gtrans) obtained by substracting curve B from curve A. Curve D : yield of total - stable and transient -conductivity corrected for the undissociated forms of the acids involved (HCOOH and H 0 2 - ) ; see text.

attribute the transient conductivity to the dis-sociated hydroperoxyl radicals. This is further supported by the fact, that the pK value of the un-stable acid responsible for Gtrans agrees with the pK of H0 2 - , Fig. 2, curve C, (pK(H02-) = 4.75 [18]).

The permanent conductivity is due to a stable acid. Since the pK value of the maximum yield of conductivity (Gmax, Fig. 2, curve A) does not agree with the pK value of H0 2 ' , we conclude that the stable acid must be formed already during the initial build up of the conductivity. If one excludes the formation of an acid in a reaction of the peroxyl radical with formaldehyde hydrate, one has to assign the permanent conductivity to an acid with one C-atom because it is not possible to form an acid with two or more C-atoms within the few /us of the build up in a second order reaction at the dose rates used. The only stable acid with one C-atom with a pK value < 6 is formic acid. Thus we attribute the permanent conductivity change to the dissociated formic acid. Further evidence for this assignment is gained from the fact, that the yield of the final conductivity decreased from 5.5 at pH = 6.5 to half of this value at pH = 3.7, the pK value of formic acid (Fig. 2, curve B).

Because only the dissociated forms of the acids are detected in conductivity measurements, the G-values in Fig. 2 have to be corrected for the undissociated forms in order to obtain the total yield of the acids formed. The corrected values (obtained on the basis of pK (HCOOH) = 3.7 and p K (H02-) = 4.75) are shown in Fig. 2, curve D. The fact that the corrected yields are independent of pH-value (Fig. 2) provides further evidence that the assignment of the conductivity change to the formation of H0 2 - and HCOOH is correct.

3. Product analysis

For product analysis irradiations have been per-formed with solutions containing N 2 0 / 0 2 (80/20 v/v) and formaldehyde hydrate (2 x 10 -3) at pH = 6.5. The irradiations were carried out either with repeti-tive pulses from the accelerator (frequency of pulses 1 Hz, dose rate 1 krad ys_1) or at the 60Co-y-source (dose rate 2.5 krad min -1). The total absorbed dose in both cases varied from 15 to 50 krad. Both methods gave the same G values within the limits of experimental errors which shows the absence of dose rate effects. From the linear yield/dose dia-gramms we obtained the G-values G(HCOOH = 5.5 and G(H202) = 3.6 (errors ± 10%).

4. Formation of organic peroxides

In the iodometrical test a fast (ti/2 < 10 sec) and a slow (ti/2 ~2.8 min) build up of I3 werej observed in the irradiated solutions. The fast build up is attributed to H 2 0 2 and the slow build up to the presence of organic hydroperoxides or peroxides. However, also unirradiated solutions containing H 2 0 2 (10-4) and formaldehyde (2 x 10~3 to 1.4 X IO -2 M) showed both, the fast and the slow formation of I 3 when the iodometric test for per-oxides was carried out after a period of storage of the solution. The following results show that the slow build up found in H202 /CH2 (0H)2 solutions is due to the presence of hydroperoxides formed in a thermal process:

i. At constant concentration of H 20 2 , the yield of the fast build up decreases with increasing concentration of formaldehyde and with in-creasing time after preparing the solutions.

ii. A constant value for the yield of the fast build up was reached after 5 h standing of the solution. This value remained constant within the limits of experimental error (15%) for 10 h.

1038 E. Bothe-D. Schulte-Frohlinde • Reaction of Dihy droxy lmethyl Radical with Oxygen 1038

iii. The sum of the yields of the fast build up and the slow build up was approximately constant, neglecting a 15% decrease observed more than 10 h after preparing the solutions at the highest concentration of formaldehyde used.

These observations are explained by assuming the establishment of the equilibrium (3).

k3 CH2(OH)2 + H 2 0 2 . < = • - CH2OH(OOH) + H 2 0 (3)

k_3

The rate constants determined were k3 = 1.9 X l o - 2 M-i s-i ± 30% and k_3 = 1.5 X 10~6 M"1 s"1 ± 30% at pH 5.7. These values are considerably higher than those reported earlier ( k 3 = 1.3 X 10~3 M - 1 s_1

and k_3 = 7 . 2 x l O - ' M - i s - 1 ) [19]. This difference could be due to the fact that the authors prepared their solutions by dilution of 36% A. R. grade aqueous stock solution of formaldehyde which is usually not free of polymers.

Since the yield of the slow build up in the irra-diated solutions was found to be identical with that of unirradiated solutions with appropriate concen-trations, we conclude that no organic hydroper-oxides and peroxides are formed by irradiation of aqueous fo maldehyde solutions under our condi-tions.

Discussion The reactive species produced in the radiolysis of

N 2 0 containing aqueous solutions are OH radicals (G = 5.6-6.0) and hydrogen atoms (G = 0.55). Photolysis of hydrogen peroxide in dilute aqueous solutions gives rise to OH radicals. The quantum yield for OH formation is approximately one [20]. Formaldehyde is present in freshly prepared dilute aqueous solutions predominantely in the form of monomeric methylene-glycol, CH2(OH)2. The OH radicals react with CH2(OH)2 to give dihydroxy-methyl radicals 1, equation (4). The rate constant [21] for the reaction of OH with formaldehyde in aqueous solution was measured to be k = 2 X 109 M-i s-1.

CH2(OH)2 + OH ^ CH(OH)2 x H 2 0 (4) 1

From this it follows that at concentrations of CH2(OH)2 higher than 10 - 3 M, reaction (4) is prac-tically complete within 1 //s after pulse or flash. Hydrogen atoms react under our experimental con-ditions at a diffusion controlled rate with oxygen

and form H0 2 - , equation (5). Due to the pK value of the hydroperoxyl radical (pK = 4.75) H02" dis-sociates within ~1 ^s into H+ and 0 2 , _ , equation (6), thus resulting in a conductivity increase with G = 0.55 which is complete within 1 /us.

H + 0 2 H02- (5) H02- <± H+ + 02— (6)

Since 1 has a pK value of 9.5 [22], it cannot give rise to conductivity changes in the /us time range under our conditions at pH values < 7 . In the presence of oxygen 1 reacts with 0 2 . This reaction which is responsible for conductivity has a rate constant of 4.5 X 109 M-1 s - 1 (see previous section). Since the half-life for the build up of the conductivity is 0.55 jus with pure 0 2 independent of dose, no bi-molecular reactions involving two peroxyl radicals (2) can contribute to the formation of conductivity. Hence the formation of conductivity via tetroxides is excluded and the conducting species are expected to be the product of the reaction of 0 2 - f 1 only. The rate of 0 2 - _ formation from (CH2(OH)2 + 0 2 ) is unusual large in comparison with that from the reaction of 0 2 with CH2OH. In the latter case a peroxyl radical is formed as an intermediate which decomposes slowly (eq. (2)) with a rate constant of < 5 s _ 1 . If also for the production of 02.~ from 1 + 0 2 the formation of a peroxyl radical as an intermediate is assumed (eq. (7)) a rate constant of > 106 s - 1 for its decay follows from the smallest half-life of 0.55 IUS for the conductivity build-up.

1 + 0 2 HC(0H) 2 00- (7) 2

The reason for this big increase is neither a favour-able conformation due to the presence of the sub-stituents, nor release of sterical strain due to elimination since the a-peroxyl radical of isopropanol produces 0 2 - - with a rate constant at least three orders of magnitude smaller than the rate constant for the 02"_ formation from 0 2 + l . A very fast formation of 0 2 - _ has also been observed from a

reaction of CH2OHCH(CHOH)3ÖOH (glucose C-l radical) with 0 2 (k > 7 X 105 M"1 s-1) [6]. This shows that the second OH group can be replaced by an alkoxy group without significant alteration in the unusual high rate of 02-~ formation. The mechanism favoured at present which may explain the fast rate constants is based on the assumption that the second OH or OR group strongly increases the stability of

E. Bothe-D. Schulte-Frohlinde • Reaction of Dihy droxy lmethyl Radical with Oxygen 1039

the transition state due to a better stabilization of positive charge produced by charge separation during elimination of 02" ~ from the peroxvl radical 2. The proton may be eliminated from the transition state simultaneously or subsequently. The strong stabilization power of two adjacent methoxy groups for a positive charge has recently been demonstrated

by the observation that the radical cation (CH30)26-CH2 in aqueous solution does react surprisingly slowly with water. This radical cation disappears mainly bimolecularly [23].

We thank Mr. F. Schwörer and Mr. K. H. Toepfer for making possible the successful use of the pulse radiolysis set-up.

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