K.-D. Asmus, H. Mockel,and A. Henglein
1218
Pulse Radiolytic Study of the Site of OH. Radical Attack on Aliphatic Alcohols in Aqueous Solution K.-D.Asmus,* H. Mockel, and A. Henglein Hahn-Meitner-lnstitut fur Kernforschung Beriin GmbH, Sektor Strahlenchemie, 7 Beriin 39, West Germany (Received December 27, 1972) Publication costs assisted by Hahn-Meitner lnstitut GmbH Berlin
The formation of reducing and oxidizing radicals has been investigated in the reaction of hydroxyl radicals with formate, alcohols, and diols in pulse-irradiated aqueous solutions. Reducing radicals (C02 - and a-alcohol radicals) were identified by their reaction with tetranitromethane, oxidizing alkoxy radicals by their reaction with iodide. The reaction of hydroxyl radicals with formate and ethylene glycol exclusively leads to a radical with reducing properties. The principal reducing radical from methanol, CH20H, is formed from only 93% of the reacting hydroxyl radicals. The remaining 7% are accounted for by methoxy CH3OH H20 + CH30.. Relative probabilities for hydrogen radicals formed in the reaction OHatom abstraction from the a position, from the OH group, and from other positions of alcohols and diols by OH radicals are derived from the experimental data.
+
Introduction The hydroxyl radical is known to react with aliphatic alcohols via abstraction of a hydrogen atom from a C-H bond.1 The reaction
OH.
+ CH30H
---*
H20
+ CH20H
(1)
for example, leads to the formation of a hydroxymethyl radical. In case of higher alcohols abstraction can occur a t any carbon atom to give a-alcohol radicals (RzCOH), palcohol radicals (-CR-CR,OH), radicals with the unpaired electron even further away from the a-carbon atom (R = organic residue or H), though abstraction of an H atom from the a position seems in general to be preferred. Optical spectra, p K values, esr spectra, and the polarographic behavior of a number of a-alcohol radicals have been investigated.2-6 They can easily be distinguished from p-alcohol radicals (or 7-,6- etc., radicals) by their physical and chemical properties. a-Alcohol radicals, for example, readily transfer an electron to acceptors such as nitrobenzene,? hexacyanoferrate(III),s and tetranitromethane,9J* while p-alcohol radicals (etc.) do not undergo such reactions. Two pulse radiolytic studies have been reported where the extent of a-hydrogen atom abstraction from various aliphatic alcohols by hydroxyl radicals was investigated.7.8 These studies were carried out with N20 saturated (2 x 10-2 M ) aqueous solutions at high alcohol concentrations (10-2 to 1 M ) , and ca. 5 X 10-4 A4 of either nitrobenzene or hexacyanoferrate(II1) as an electron acceptor. Under these conditions the hydrated electrons formed during the irradiation were converted to OH radieaqN2 + OHO H - ) , and both OH. cals (N2O and H. rapidly attacked the alcohol to form the various types of alcohol radicals. The electron acceptors were subsequently reduced exclusively by the a-alcohol radicals thereby producing CeHsN02- or Fe(CTu')e4-. The yields of these electron transfer reactions could easily be determined from the large changes in absorption resulting from these reactions. The result,s are listed in Table I. In both cases, it was assumed that the reaction of OH. with methanol exclusively occurs via eq 1; Le., the relative
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The Journal of Physical Chemistry, Voi. 77, No. 10: 1973
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yield of reduction was taken as 100% for methanol. Less reduction occurs when higher alcohols are used as OH radical scavengers, and the relative yields obtained were interpreted as relative probabilities of hydrogen atom abstraction from the a-carbon atom of these alcohols. The data of the two independent investigations are seen to agree well. Burchill and Ginns,ll however, recently derived relative abstraction probabilities from kinetic data on radiationinduced oxidation of several alcohols by hydrogen peroxide in aqueous solution which are lower than those given in Table I. They found that a-hydrogen abstraction by OH radicals occurs to only 90 and 86% from ethanol and !&propanol, respectively. Walling and Kato12 in their study on the oxidation of 2-propanol by Fenton's reagent also found a lower yield (85%) of a abstraction. The values derived from the hydrogen peroxide experiments are also seen to be internally consistent, and the observed discrepancies between these and the pulse radiolysis data may be attributed to a different interpretation of the results. In a private communication, Dr. Burchill pointed out that methanol in its reaction with OH radicals might not exclusively produce CH2OH radicals. This would mean that all the relative yields given in Table I are too (1) See, for example: (a) M. S. Matheson and L. M. Dorfman in "Pulse Radiolysis," The MIT Press, Cambridge, Mass., 1969; (b) A. Henglein, W. Schnabel, and J. Wsndenburg in "Einfuhrung in die Strahlenchemie," Verlag Chemie. Weinheim. 1969. (2) I. A. Taub and L, M. Dorfman, J . Amer. Chem. SOC., 84, 4053 (1962). (3) K.-D. Asmus, A. Henglein, A. Wigger, and G. Beck, Ber. Runsen.. ges. Phys. Chem., 70, 756 (1966). (4) M. Simic, P.Neta, and E. Hayon,J. Phys. Chem., 73, 3794 (1969). (5) R. Livinaston and H. Zeldes. J. Chem. Phvs.. 44, 1245 (1966). (6) M. GraGel, A. Henglein, J. Liiie, and M.'Scheffler, Ber. Bunsenges. Phys. Chem., 76, 67 (1972). (7) K.-D. Asmus, A. Wigger. and A. Henglein, Ber. Bunsenges. Phys. Chem., 70, 862 (1966). (8) G. E. Adams and R. L. Willson. Trans. Faradav SOC., 65, 2981 (1969). (9) K.-D. 'Asmus, A. Henglein, M. Ebert, and J. P. Keene, Eer. Bunsen: ges. Phys. Chem., 68, 657 (1964). (10) J. Rabani. W. A. Mulac, and M. S. Matheson, J . Phys. Chem., 69, 53 11965). (11) C. 'E. Burchill and I. S. Ginns, Can. J. Chem., 48, 1232, 2628 (1970). (12) C. WallingandS. Kato, J.Amer. Chem. Soc., 93,4275 (1971).
OH.
Radical Attack on Aliphatic Alcohols
1219
TABLE I:Percentage of H Atom Abstraction from the a-Carbon Atom of Various Alcohols According to Earlier Pulse Radiolytic Investigationsa
psec duration (200-700 rads) were generally used. The dose was monitored with a secondary emission foil. The chemical standard was an aqueous solution saturated with M ) , and containing 2 X 10-1 M NzO (ca. 2 x YOAbstraction HCOONa and 1.2 X 10-4 M C(NO2)4. Tetranitromethane in such a solution is reduced to nitroform via reaction 4 Nitrobenzene Hexacyanoferrate( I I I) Alcohol method method with G = G(eaqOH. H.) = 6.1 ( G = number of species/100 eV of absorbed energy), Methanol 100 100 The solutions were prepared from triply distilled water. Ethanol 97 97 Alcohols, sodium formate, and potassium iodide were of 1-Propanol 68 61;65 reagent grade. Traces of oxygen in nitrous oxide were re2-Propanol 96 89;95 moved by passing the N20 through a column filled with 1-Butanol 35 34 aqueous Cr2+ solution. Tetranitromethane was purified by a See ref 7 and 8 washing several times with triply distilled water. A satuM ) served as a stock rated solution of C(N02)4 (6 X high. There are, in fact, earlier observations which could solution. The water was generally deaerated with argon cast some doubt on the role of methanol as the “100% aprior to saturating with N20 and/or addition of other solabstraction standard”; it was observed that the COz- radutes (which had been deaerated separately). All experiical anion formed by attack of OH radicals on formate reduces tetranitromethanel3 and he~acyanoferrate(II1)~~ments were performed at room temperature (ca. 20”). with a somewhat higher yield than the products of the OH Evaluation of Data radical reaction with methanol. The calculation of radiation chemical yields is based on The present pulse radiolysis study, therefore, has been the generally accepted values on the formation of primary undertaken to redetermine the yield of reducing species in species in aqueous solutions: G(eaq-) = 2.7, G(OH.) = the reaction of hydroxyl radicals with various alcohols and 2.8, and G(H.) = 0.6. Hydrated electrons were converted formate. As an electron acceptor tetranitromethane was to hydroxyl radicals by NzO. Thus in NzO saturated soluused which is known to react with a-alcohol radicals and tions a total of G(0H.) = 5.5 and G(H-) = 0.6 were COz- via9,10,13 available.1 >COH C(NO,), The individual error for an optical single pulse experiC(NO,),NO, H’ > C O (2) ment is ca. *lo%, mainly given by the uncertainty in the and monitored dose. A much higher accuracy is necessary for the correct interpretation of our data. Therefore, every re.COz- + C(NOz), C(NOz)B- + NO, + COZ (3) sult reported here is a mean of some 20 to 30 individual experiments. For the comparison of relative yields of The stable nitroform anion was detected by its strong abC(NO2)4 reduction by several radicals, it is a prerequisite sorption a t 350 nm (e 1.50 X lo4 M - I cm-I). that always the same fraction of radicals react with tetraExperiments were also carried out to determine whether nitromethane. Therefore, concentrations were chosen to or not methoxy radicals are produced as a second possible ensure that the product of rate constant and scavenger species in the reaction of hydroxyl radicals with methanol concentration remained practically constant for all reacvia tions involved. Further, direct comparisons were made OH. + CH,OH CH,O* HZO (4) only between results obtained a t the same dose. Taking For identification of methoxy radicals the oxidation of ioall the uncertainties into account, the total error limits dide ions15116 can be given with *2% for the C(N02)4 reduction and &lo% for the I- oxidation results.
+
+
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+
+
+
+
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-
+
was used. Similar experiments were also carried out with higher alcohols. Finally, it should be mentioned that hydrogen atoms which are produced during the irradiation of aqueous solutions can also abstract H atoms from the various positions of an alcohol. The yield of hydrogen atoms in NzO saturated solutions is much lower, however, than the yield of hydroxyl radicals (G(H) = 0.6; G ( 0 H ) = 5.5).1 The data reported in this paper, therefore, represent abstraction probabilities by a 9 : l radical mixture of OH. and H., but even if the relative reactivities of hydroxyl radicals and hydrogen atoms were different with respect to the position of the attacked hydrogen atom in the alcohol molecule, the figures would still reflect in the main the reaction probabilities of the OH radical owing to its much higher concentration in the radical mixture.
Experimental Section The technique and equipment of pulse radiolysis have already been described.lb.17 Electron pulses of 0.3-1.0
Results and Discussion Relative Yields of Reducing Species. Figure 1 shows the optical absorption a t 350 nm as a function of time, observed in pulse irradiated solutions containing 2 x 10-2 M nitrous oxide and 1.2 x M tetranitromethane plus, respectively, 2 X 10-1 M of sodium formate (Figure l a ) , ethanol (Figure l b ) , and 1-propanol (Figure IC).Under these conditions the conversion of the hydrated electrons by N20 and the reaction of hydroxyl radicals (and hydrogen atoms) with formate or alcohols go to completion during the 1-psec pulse. The subsequent reaction of COZand a-alcohol radicals with tetranitromethane occurs with rate constants of the order of lOg-lO10 M-1 sec-1-9JOJ3 (13) A. Fojtik. G. Czapski, and A. Henglein, J. Phys. Chem., 74, 3204 (1970). (14) G. E. Adams, private communication. (15) F. S. Dainton, I. V. Janovsky, and G. A. Salmon, J. Chem. SOC. D, 335 (1969); Proc. Roy. SOC.,Ser. A, 327, 305 (1972). (16) D. H. Ellison, G. A. Salmon, and F. Wilkinson, Proc. Roy. SOC. Ser. A, 328, 23 (1972). (17) A. Hengiein,Ai/g. Prakt. Chem., 17, 296 (1966). The Journal of Physicai Chemistry, Voi. 77, No. 10, 1973
K.-D. Asmus, H. Mockel. and A. Henglein
1220
pulsed solutions of diols (ea. 2 X 10-1 M) and relatively low tetranitromethane (TNM) concentrations (10-5 to 10-4 M): k(TNM + ethylene glycol radical) = 1.7 X 109 M - 1 sec-', k(TNM + 1,2-propanediol radical) = 3.2 X lo9 M-' sec-I, and k(TNM + 2.3-butanediol radical) = 3.3 X lo9 M-1 sec-1. Accordingly, the tetranitromethane
I
C
.-0 c
The figures given in Table I1 are lower than those listed in Table I. The relative yield for 2-propanol is now quite close to that measured by Burchill and Ginns." The most interesting result, however, appears to be that for metha-
Q
5 ul
a a time
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Figure 1. Formation of nitroform in pulse-irradiated aqueous solutions containing N20 (2 X M).C(NO2)d (1.2 X W ' M ) . and OH. radical scavengers: a. HCOONa (2 X lo-' M ) ; b. C2H50H (2 X lo-' M ) ; c, n-C3H70H (2 X lo-' M ) . Pulse length. 1 psec (ca. 700 rads); time scale, 50 psecllarge division.
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TABLE II: Relative Yields of H Abslraclion lrom the a Postlion, lrom the OH Groups, and Irom B (or y. 6, etc.) Posilions 01
Various Alcohols
while the remaining 7% must be accounted for by some other reaction, the most logical of which is the formation of methoxy radicals as a second product uia reaction 4. This is supported by the results for a solution containing 1.2 X 10-4 M tetranitromethane, 2 M CHsOH, and 5 X M (CH&CHOH, where the relative yield of C(N0.h- absorption increased to 100%, although hydroxyl radicals are still scavenged directly by the methanol. Probably the CH30. radical undergoes an abstraction reaction CHZO.
Alcohol lor formate)
HCOO-
OH
n Abstraction abstraction 100 93.0 84.3 53.4 85.5 41.0
CHsOH C2H50H
n-C3H70H (CH42CHOH n-C4H90H
lerl-C4H90H (CHzOH)z CHZOH-CHOH-CH~
CHs-CHzOH-CHpOH-CH3
100 79.2 71.0
B (7. 6elc.l abstraction
13 where the basic form 0- of the hydroxyl radical exists. The results were the same within the limits of error indicating that 0- forms CHzOH and CH30. in its reaction with methanol with the same relative probabilities as OH radicals. A compilation of alkoxy radical yields from several alcohols and diols measured by the iodide method is listed in the third column of Table 11. The highest yields were observed for methanol and 2-methyl-2-propano1, i.e., compounds carrying exclusively CH3 groups. The formation of alkoxy radicals becomes less important with increasing number of secondary and tertiary hydrogen atoms in the alcohol molecules. The fourth column of Table I1 contains the difference between 100% and the sum of the percentages given in columns 2 and 3. This difference has to be attributed to the relative yield of hydrogen atom abstraction by hydroxyl radicals from carbon atoms in the p, y , etc. positions. (19) M. Anbar and P Neta, Int. J. Appi. Radiat. Isotopes, 18, 493 (1963).
The Joornaiof Physical Chemistry, Voi. 77, No. 10, 1973
