Radiation-induced oxidation of 2-propanol by hydrogen peroxide in aqueous solutions1

C. E. BURCHILL AND 1. S. GINNS Departn~et~t of Clzetnistry, University of Manitoba, Witlnipeg 19, Marlitobn

Received December 15, 1969

The radiation-induced oxidation of 2-propanol by hydrogen peroxide in neutral deaerated aqueous solution has been investigated. 2-Propanol is oxidized to acetone, and hydrogen peroxide reduced in stoichiometrically equivalent high yields. The yields are independent of hydrogen peroxide concentration in the range 5 x to M and linearly dependent on alcohol concentration in the range 0.13 to 1.05 M . The reaction yields increased with decreasing dose rate.

The results are explained by a chain mechanism in which initiation occurs via H-atom abstraction from 2-propanol to form either (CH,) ,COH (1) or CH3 CHOH CH2 (2). 1 reacts with H 2 0 2 in a chain propagating reaction

1 + H201 -> acetone 4- H 2 0 + O H 2 may abstract the a hydrogen from the parent alcohol

2 + 2-propanol -t 2-propanol 4- 1 or undergo bimolecular termination. A lower limit of 53 + I0 1 mole-' s - ' is estimated for the rate con- stant for this radical conversion reaction. Canadian Journal of Chemistry, 48, 1232 (1970)

Introduction The peroxidation of 2-propanol has been

studied using a variety of radical initiation techniques. Merz and Waters (1) have studied the chain oxidation of 2-propanol using Fenton's reagent. The photolysis of hydrogen peroxide in aqueous solutions of 2-propanol has been investigated (2, 3). The reaction of H,O, has been studied in solution in pure 2-propanol using both photochemical (4) and thermal (5) initiation. The disappearance of hydrogen peroxide by a chain process during they irradiation of aqueous solutions of 2-propanol has been described by Allan and Beck (6).

Mechanisms have been proposed for these oxidations and for the similar radiation-induced oxidation of ethanol by hydrogen peroxide in aqueous solution (7). During a preliminary study of the radiation-induced reaction of 2-propanol and hydrogen peroxide, we observed features of the dependence of the yields on solute concentra- tions which were inconsistent with the accepted mechanism. We now present the results of an extensive study of this reaction and a revised mechanism to explain them.

Experimental 2-Propanol (Fisher Certified Reagent) and hydrogen

peroxide (30% unstabilized Fisher Certified Reagent)

were used as received. Water was redistilled from alkaline K M n 0 4 solution and acidic KZCr2O7 solution. Analytical reagents were of reagent grade and used without further treatment.

Solutions were prepared using freshly distilled water, irradiated, and analyzed on the same day. Ten-ml aliquots of the solutions were placed in 25 ml Pyrex bulbs fitted with demountable vacuum-seated stopcocks. Before irradiation the samples were degassed under vacuum by successive freeze-pump-thaw cycles.

Most samples were irradiated a t room temperature in a 60Co Gammacell 220 source (Atomic Energy of Canada, Ltd.) at an average dose rate of 1.70 x 1019 eV I- ' s-'. Lead shields supplied with the Gammacell were used to provide attenuation to a nominal 10%. For studies at lower dose rates, an Eldorado Model A 60Co source a t the Manitoba Cancer Treatment and Research Foundation was used. Dose rates were estimated using the Fricke dosimeter and assuming G(Fe3+) = 15.6. Fo r all solutions it was assumed that the absorbed dose was proportional to the electron density, and the dose rates estimated for the solutions were adjusted for variations in electron density with solution composition.

Acetone was determined spectrophoton~etrically using the salicylaldehyde method of Berntsson (8). Hydrogen peroxide was determined by the iodide method (9).

Results The reduction of hydrogen peroxide during

irradiation in the presence of a constant (0.52 M ) concentration of 2-propanol is shown in Fig. 1. The figure is a composite of the results from a series of ex~eriments with different initial con- centrations of hydrogen peroxide. It is clear that - - A

'This work was supported in part by the Defence the rate of hydrogen peroxide reduction is Research Board of Canada, Grant No. 9530-78, and in part by Atomic Energy of Canada, Limited, Commercial of i ts concentration in the range Products Division. 5 x lo-* to M H 2 0 2 . G ( - H 2 0 2 ) is found

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BURCHILL A N D GINNS: OXIDATION OF 2-PROPANOL 1233

TABLE 1

G(acetone) and G(- H202) from water-2-propanol solutions*

2-Propanol (mole I - ' ) G(acetone) G(- H202)

Pure 16.3

*Constant initial [H,02] = 0.01 M, dose rate = 1.70 x I O L g eV I-' s-'.

from the slope of this graph and is 51.8 + 2.0 for this concentration of 2-propanol. Additional G(-H20,) values for various concentrations of 2-propanol are given in Table 1.

Acetone formation yields for a constant initial 401i!!!c- 3 o concentration of 0.01 M H 2 0 2 and various con- o 0.5 I .o 1.5

centrations of 2-propanol are also shown in 1 2 - PROPANOLI (mole r1 I

Table 1. The stoichiometric equivalence of the 2. variation of yields wi th bpropanol concen-

tration. Initial [H20,] = 0.01 M, dose rate = 1.70 x

5 0 i i / 10" eV I - ' I - ' . 0, G(-H20,); 0, G(acetone).

acetone formation and peroxide reduction yields is evident for those systems in which both quan-

4 0 - - tities were measured. This equivalence is also illustrated in Fig. 2, which further demonstrates that both yields increase linearly with increasing

- alcohol concentration in the range 0.13 to 1.05 - 3 0 - - M 2-propanol. Extrapolation of both yields as - 0 E - shown in Fig. 2 leads to the result Go (H,02) =

33.4 + 1.0 and Go (acetone) = 31.2 f 1.0 at $ - zero 2-propanol concentration. x 2 0 - - As the 2-propanol coiicentration is increased

,.7 beyond 1.05 M, the initial yield of acetone con- P tinues to increase but it is no longer a linear

function of alcohol concentration. G(acetone) 10- - reaches a maximum of -- 100 at a concentration

of 2-propanol - 3.7 M and then decreases to 16 f 1 in pure 2-propanol. Initial yields of acetone formation are shown as a function of

o mole fraction 2-propanol in Fig. 3. 0 2 0 4 0 60 The effect of varying dose-rate on the oxidation

x DOSE l e v I - ' I of 2-propanol by hydrogen peroxide is shown in FIG. 1. Radiation-induced reduction of H 2 0 2 in Fig. 4 for s o ~ u t i b n s o . o r ~ in H202 and 0.52 M

deaerated aqueous 2-propanol (0.52 MI solutions. Dose i n 2-propanol. increase in qacetone) with rate = 1.70 x I O l 9 eV I - I s-'. Initial [H202]: 0, 0.05; 0, 0.03; n, 0.02; 0, 0.01; A, 5 x M. decreasing dose rate is clearly demonstrated.

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1234 CANADIAN JOURNAL OF CHEMISTRY. VOL. 48, 1970

I I I I I 2 0 4 0 6 0 8 0 100

MOLE PERCENT 2-PROPANOL

FIG. 3. Initial acetone formation yields as a function of mole percent 2-propanol. Initial [H20,] = 0.01 M, dose rate = 1.70 x lOI9 eV I- ' s-I.

Table 2 contains data demonstrating the effect of varying dose-rate for other concentrations of 2-propanol.

At concentrations of H202 < 10-3 M, the rate of peroxide reduction in the presence of 0.52 M 2-propanol is dependent on the concentration of H202, as is demonstrated in Fig. 5. The figure is again a composite of the results obtained from experiments with different initial concentrations of H202. The peroxide concentration approaches a steady-state value of -- 3 x M. G(acetone) at the steady-state concentration of H202 is 0.97 f 0.05.

TABLE 2

Dose-rate effect on G(acetone) from water-2-propanol solutions*

-~ pppp-.--p-

G(ace tone)

2-Propanol Unattenuated Dose rate (mole I - ' ) dose rate attenuated to 26%

FIG. 4. Variation of G(acetone) with dose rate. [2- Propanol] = 0.52, initial [H20z] = 0.01 M.

Discussion 0-1.0 M 2-Propanoll.5 x 10- 2-10-3 M H,02

In this concentration region, the significant features of the results are that the yields of acetone formation and hydrogen peroxide reduc- tion are essentially equal, linearly dependent upon the alcohol concentration, and independent of the peroxide concentration. The linear plot of G vs. 2-propanol concentration extrapolates to a significant chain value at zero alcohol concentra- tion and the radiation-chemical yields show an inverse dependence upon dose rate.

In this range of solute concentrations, we may assume without serious error that the y radiation interacts primarily with the solvent water to form the species e,,-, H, OH, H,O+, H202 , H,. The hydrated electron will react rapidly with the solute hydrogen peroxide

I e,,- + Hz02 + .OH + OH- 56.5 138.3 73.7 Since

Pure 2-propanol 16.3 22.0 ---- ~ O H + ( C H , ) ~ C H O H > ~ O H + H , O ~

'Initial [ H 2 0 2 ] = 0.01 M , unattenualed dose rate - 1.70 x 10" eV 1-1 S-'. k~ + ( C H ~ ) Z C H O H ' k ~ + H z O z (lo)

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BURCHILL AND GINNS: OXIDATION OF 2-PROPANOL 1235

lo-" x DOSE l e v I" I

FIG. 5. Reduction of HzOz at low concentrations. [2-Propanol] = 0.52 M, dose rate = 1.70 x 10'' eV I-' s-'. Initial [H20,]: 0, 5 x .,2.5 x A, 10-3; m, 3 x ~ o - ~ M .

and [2-propanol] >> [H~o,]

all H and OH will react with 2-propanol. Pre- vious authors (1-5) have assumed reactions proceed entirely by abstraction of an H atom from the ci position of the alcohol.

[2 1 H + (CH3)2CHOH -> Hz + 1

where 1 represents the radical (CH,),COH. The chain reaction has been attributed to the pro- pagating reaction [4] with termination of the

chain by bimolecular reaction of 3

This simple reaction scheme, however, may be shown to predict a first-order dependence of the reaction rate on the concentration of H202 and no dependence on alcohol concentration, whereas we have observed that the reaction rate under irradiation is independent of peroxide concentra-

tion and linearly dependent upon the alcohol concentration. We propose the following modifi- cation of the reaction scheme to explain the observed results

HzOl^rte , , - , H, OH, H 3 0 + , H20z , Hz

[1 1 e,,- + HzOz -> OH- + OH

[2] H + (CH3)ZCHOH + Hz + (CH~)ZCOH ( C H ~ ) ~ C O H = 1

[7] H + (CH3)2CHOH -> Hz + C H ~ C H O H C H ~ CH~CHOHCH~ = 2

[3] OH + (CH3),CHOH + Hz0 + 1

[8] OH + (CH3)2CHOH + Hz0 + 2

[4 1 1 + H 2 0 z + (CH3)2C0 + HzO + OH

[9] 2 + (CH3)2CHOH + (CH3)zCHOH + 1

[lo] 2 + 2 -> 2,5-hexanediol or

(CH3)ZCO + (CH3)zCHOH

The novel features of this proposal are the inclusion of abstraction reactions from the methyl groups of the alcohol, reactions [7], [S], and the radical conversion reaction [9].

The rate constants for reactions [3] and [7] have been assessed (lo), and it is clear from their values that the abstraction from the ci position is not entirely specific and that reaction [7] must be considered in addition to reaction [3]. No rate data are available for the abstraction of ci and P hydrogens from simple alcohols by OH radicals, although i t might be argued that if abstraction by H atoms is not entirely specific to the ci

position, that by OH radicals would be even less specific by virtue of its greater exothermicity. Allan and Beck (6) have suggested that the O H radical may not react specifically at the ci position of 2-propanol. More convincing evidence for the lack of specificity is provided by the observation of electron spin resonance (e.s.r.) spectra attri- buted to both radicals 1 and 2 in the photo- initiated reaction of H20, with 2-propanol (1 1). These radicals, whose formation was attributed to OH radical abstraction of H atoms from the alcohol, were present in concentrations within the same order of magnitvde, and the spectrum attributed to radical 2 became increasingly prominent as the system was diluted with water. We therefore contend that the abstraction of H atoms from the p position of the alcohol cannot be neglected in any mechanistic description of these free-radical reactions.

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1236 CANADIAN JOURNAL O F CHEMISTRY. VOL. 48, 1970

The radical conversion reaction [9] was suggested, in part, by the form of the experimental results. The linear dependence of the radiolytic yields on alcohol concentration suggests that the chain length is determined by the competition between a radical terminating reaction and the - reaction of that radical with the alcohol. In view of the high rate of its reaction with the alcohol (10) and the large concentration of the alcohol, it is unlikely that the steady-state concentration of O H is sufficient for it to play any role in chain termination. Chain termination involving radical 1 would be in competition with the propagating reaction [4] and would lead to some dependence of chain length on H,O, concentration. There remains the possibility of reaction [9] as the rate-controlling propagation step in competition with reaction [lo] as the terminating step. While this reaction appears to predict the appropriate kinetic form of the results, this is not sufficient to conclude that the reaction occurs. Examination of the literature, however, has brought to light considerable evidence that such a reaction may occur.

Thomas (12) has observed the reaction of methyl radicals with 2-propanol, presumably by abstraction of the a hydrogen of the alcohol. Radical 2, being an alkyl radical, may well un- dergo a similar reaction, although in view of the greater degree of ordering required in the forma- tion of a transition rate, the rate constant might well be less than the value 3.4 x lo3 M-' s-' observed for the methyl radical. Livingston and

Zeldes (1 1, 13) observed the e.s.r. spectrum of radical 2 during the photolysis of H,O, in 2-propanol-water mixtures, but its intensity was reduced in mixtures of high alcohol concentration and the spectrum was not observed in pure 2-propanol. This may be argued as evidence for the occurrence of reaction [9]. With increasing alcohol concentration, the rate of reaction [9] would increase with a consequent decrease in the steady-state concentration of radical 2. It was also noted (1 1) that as the solution (75 "/, 2-pro- panol, 25 "/, H,O) was cooled from 30 to - 5 "C the suectrum of 2 was enhanced. This observation is also consistent with the occurrence of a reaction such as [9], a relatively slow reaction, with a significant energy of activation. With a decrease in temperature, the rate of the reaction would be reduced and the steady-state concentration of 2 increased. Norman and West (14) have recently suggested reaction [9] to explain the variation in intensity of e.s.r. spectra with changing substrate concentration in the metal-ion catalyzed reaction of organic compounds with hydrogen peroxide.

The presence of 2,5-hexanediol as a reaction product was demonstrated qualitatively by vapor-phase chromatography, using flame-ioni- zation detection and a 6 ft column (2 % Carbowax 20 M on 60180 chromosorb W).

Returning then to the proposed mechanism, we may derive an expression for the radiation- chemical yields in the system using the normal steady-state approximations

Clll k2 (k3 + 1~8) k3

+ GH{k, + k, (Gcnq- +

Gc,g- + G~ + G~~

I + ('+ 2)k9( 2kl,D [2-p ropanol]

(D is the dose rate in units of 6.02 x eV I - ' s-'). G(acetone) differs only marginally from G(- H,O,), depending upon the exact stoichi- ometry of the terminating reaction, [lo].

Note that this mechanism predicts a linear dependence of the yields on 2-propanol concen- tration, no dependence on H,O, concentration, and a non-zero intercept at zero alcohol concen- tration corresponding to the first portion of the yield equation (in square brackets). This con-

forms precisely to the experimental results as represented in Figs. 1 and 2.

Equating the G(- H,O,) intercept from Fig. 2 (33.4 +_ 1.0) to the portion of eq. [ I I ] in square brackets, we may estimate a value of k3/k8 using accepted values for the primary yields in irradi- ated water (1 5) and the literature values fork,, k,, and (k, + k,) (10). The ratio of k3/k8 so calcu- lated is 6.2. Assuming the value of k, + k8 to be 1.4 x log l mole-' s- ' gives

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BURCHILL AND GINNS: OXIDATION OF 2-PROPANOL 1237

The second part of eq. [ l l ] , as previously noted, predicts a linear dependence of radiation- chemical yields on 2-propanol concentration, and the coefficient of [2-propanol] corresponds to the slope of the line in Fig. 2. However, this coefficient includes a dose-rate dependent term which predicts an inverse square-root dependence on the dose rate and extrapolation to the zero- alcohol intercept at high dose rates. As seen from Fig. 4, the yield is not the predicted linear function of ( I / D ) & , although the yield decreases with increasing dose rate and the curve can, without difficulty, be extrapolated to the zero-alcohol intercept. The fact that there is a significant dose- rate effect confirms that there is a bimolecular termination, but the curvature of Fig. 4 indicates that there is, in addition, an apparent first-order termination which becomes increasingly impor- tant at low dose rates. If it is assumed that at the highest dose rate used (1.75 x 1019 eV 1-' s-'), termination is primarily by the bimolecular process [lo], then it is possible to estimate a lower limit for the rate constant of the conversion reaction [9]. Using k3/k, = 6.2, assuming that the bimolecular termination reaction has a rate constant 2k1, = 2.0 x lo9 1 mole-' s-' and using Buxton's (15) values for the primary radical yields, k, can be calculated from the slope of G(-H202) vs. [2-propanol] (Fig. 2). The lower limit of k, so estimated is 53 + 10 1 mole-' s-'. This estimate is nearly two orders of magnitude less than the observed rate constant for the corresponding methyl-radical reaction (12), but this, in view of the highly ordered transition state required for such a reaction, may not be unreasonable.

In the low dose-rate studies, a reduction in the yield of acetone was observed on increasing the concentration of hydrogen peroxide, suggesting that a reaction such as [12] may be the competing "first-order" terminating reaction.

[12] CH3CHOHcHZ + HzOz + (CH3),CHOH + HOZ

High Propanol Concentration /0.01 M H202 As the concentration of 2-propanol is increased

to values greater than 1.0 M, the yield of acetone increases less rapidly, reaching a maximum in the region of 3.7 M. This levelling-off in the yield of

acetone could be attributed to the increasing rate of reaction [9] with increasing alcohol con- centration. If reaction [9] becomes sufficiently rapid, it may no longer be exclusively rate con- trolling and the chain length would be determined in part by competition between reactions [4] and [6]

This would result in a transition to a situation in which the reaction becomes inde~endent of alcohol concentration and first-order in hydrogen peroxide. In fact, in this region, the yield-dose curve for hydrogen peroxide reduction becomes non-linear, although at none of the higher alcohol concentrations is it completely first order.

At alcohol concentrations in excess of 4.0 M, the chain yield of acetone decreases to a limiting value of 16 + 1 in pure 2-propanol. There are at least two possible explanations for this decrease. The argument presented above suggests that, with increasing alcohol concentration, the yield should increase to a maximum limiting value and then remain constant. This assumes, however, that the primary yields of free-radical inter- mediates remain unchanged, which is highly improbable as the system is changed from pure water as solvent to pure 2-propanol. A decrease in the primary yields with increasing alcohol concentrations could account for the decreasing yield. However in a study of the photo-induced reaction, Barrett et a[. (3) found an almost iden- tical change in the quantum yield for hydrogen peroxide reduction, although the primary quan- tum yield of peroxide homolysis is said to remain constant in this concentration range. The de- creasing yield was attributed by Barrett et al. to reaction of the product OH radicals with alcohol molecules in the "primary photochemical cage". The parallel between the radiation-chemical results reported here and the photochemical results makes this explanation unlikely, since the radiation chemical "spur" is not analogous to the photochemical "cage". We suggest that with increasing alcohol concentration, and consequent increase in the rate of reaction [9], reaction [4] becomes the principle rate-controlling propaga- tion step and that the rate constant for [4] decreases with increasing alcohol concentration as a result of the decrease in dielectric constant.

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CANADIAN JOURNAL OF CHEMISTRY. VOL. 48, 1970

The reaction of the polar radical 1 with the polar relative rates of terminating and propagating molecule H,O, would involve the formation of reactions, but we found this quantity to be rather a polar transition state and would be expected to irreproducible, varying from one experiment to decreasein rate withdecreasing dielectricconstant another by as much as a factor of two. (16).

At high 2-propanol concentrations, there is a Summary significant dose-rate effect on the yields of acetone The abstraction of hydrogen atoms from

2). This dose-rate effect but 2-propanol by OH radicals is suggested to be less never equals, a simple inverse square-root depen- specific as to the site of reaction than previously dence, suggesting that even in pure 2 - ~ r o ~ a n o 1 believed. The ratio of the rates of abstraction at termination by reaction [lo] may still be signi- the a and position is ficant .

Sherman (17) has reported the formation of k, /k , = 6.2 methane and acetic acid in chain yields in addition to acetone in the radiation-induced It is proposed that the radical formed by abstrac-

oxidation of alkaline 2-propanol by nitrous tion of a p hydrogen, CH3CHOHCH2, can

oxide. The formation of methane and acetic acid react with 2-propanol to abstract the a hydrogen,

was accounted for by the stoichiometric reaction forming the a radical (CH3),COH. A lower limit for the rate constant for this reaction is estimated

[I31 (CHJ)~CHOH + N 2 0 -> CH3COzH as 53 + 10 1 mole-' S-'. + Nz + CH4

We detected no significant formation of methane in the oxidation of neutral 2-propanol by hydro- gen peroxide, thus ruling out a reaction with H,O, analogous to reaction [13].

Aqueous bPropanol/< 10- M Hydrogen Peroxide

At a constant concentration of 2-propanol (0.52 M), the rate of reduction of hydrogen peroxide becomes dependent on its concentration (Fig. 3) and becomes essentially first-order in H,O, below M. We take this to indicate that the rate of reaction [4] has become sufficiently slow to become rate determining at these low peroxide concentrations.

When the concentration of H,O, reaches its steady-state value, the requirement of oxidation- reduction balance in the system predicts that

[14] G(net oxidation) = G(net reduction)

This latter value is expected to be of the order of 1.0 (15), in good agreement with the experimental value of G(acetone) = 0.97 + 0.05.

The magnitude of the steady-state concentra- tion of H,O, could be used to determine the

The authors gratefully acknowledge the cooperation of the Manitoba Cancer Treatment and Research Foundation and, particularly, the assistance of Dr. D. V. Cormack in performing the low dose-rate irradiations.

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