The Reactivity of Aromatic Compounds toward Hydroxyl Radicals

All other reagents were used without further purifica- ... grade and sodium borate was a Baker Analyzed rea- ... (4) W. T. Dixon and R. 0. .... Org. C...
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The Reactivity of Aromatic Compounds toward Hydroxyl Radicals

by M. Anbar, D. Meyerstein, and P. Neta The We’eizmannInstitute of Science and the Soreq Nuclear Research Centre, Rehovoth, Israel (Received February 96,1966)

The relative rates of reaction of OH radicals with a series of substituted benzene and benzoate ions have been determined. The rates show a good agreement with Hammett’s up relation for electrophilic substitution with a p = -0.41.

Introduction The reactions of benzene and substituted benzenes with hydroxyl radicals have been the subject of several recent ~tudies.l-~It was suggested that this reaction involves the addition of the OH radical to the aromatic ring. The reaction rates of hydroxyl radicals with the various aromatic compounds are high and approach the diffusion controlled limit ; still, some effects of substituents could be ~ b s e r v e d . ~It~was ~ of interest to investigate more extensively the effects of substituents on this reaction. From the effects of substituents on the rates of reaction of OH radicals with aromatic compounds, it was hoped to infer the nature of this reaction, by coniparison with reactions of conventional electrophilic and nucleophilic reactants. I n a recent investigation, the effects of substituents on the reactivity of aromatic compounds toward hydrated electrons were correlated with the u values of Hammett’s equation.6 It was decided to oarry out an analogous study on the reactivity of OH radicals with monosubstituted benzenes and para-substituted benzoate ions. The specific rates were derived from competition kinetics using the method developed by Kraljic and Tr~mbore.~

Experimental Section Materials. Triply distilled water was used as solvent. p-Xitrosodimethylaniline (Matheson Coleman and Bell) was recrystallized four times from water. All other reagents were used without further purification. n’itrobenzene, benzonitrile, aniline, N,N-dimethylanilint:, phenylacetic acid, and p-chlorobenzoic acid were of Fluka puriss grade. Acetophenone, p nitrobenzoic, terephthalic, p-fluorobenzoic, p-aminobenzoic, and p-hydroxybenzoic acids were of Fluka purum grade. Benzoic acid, p-bromobenzoic acid, The Journal of Physical Chemistry

and phenol were of B.D.H. analytical grade. Benzamide, acetanilide, anisole, and p-toluic acid were B.D.H. laboratory reagents. Anisic acid, benzenesulfonamide, and phenyl acetate were of Eastman pure grade and sodium borate was a Baker Analyzed reagent. Procedure. The experimental technique followed the procedure developed by Kraljic and Trumhore7 using p-nitrosodimethylaniline as competitive substrate. It has been shown that the decomposition of p-nitrosodimethylaniline is due solely to OH radicals.’ The relative rates obtained by this method were independent of the relative concentrations of the reactants and the time of irradiation (0.4-4 X 10’’ ev ml-I); ie., they were independent of the extent of reaction, within the experimental range (l-lO% consumption of p nitrosodimethylaniline). The relative specific rates obtained by this method are c ~ m p a r a b l ewith ~ , ~ results obtained by other competition kinetics methods, including those using pulsed radiolysis t e c h n i q ~ e s , ~ and cases in which the products of the OH reactions were i ~ l o a t e d . ~ The Corn y source had a dose rate of 4.1 X 101’ ev ml-l rnin-’ and the time of irradiation was 30 sec. (1) L. M. Dorfman, I. A. Taub, and R . E. Btihler, J . Chem. Phys., 36, 3051 (1962). (2) L. M. Dorfman, I. A. Taub, and I). A. Harter, ibid., 41, 2954

(1964). (3) R. W.Matthews and D . F. Sangster, J . Phys. Chem., 69, 1938 (1965). (4) W.T. Dixon and R. 0. C. Norman, Proc. Chem. Soc., 97 (1963). (5) G. E. Adams, J. W. Boag, J. Currant, and B. D . Michael in “Pulse Radiolysis,” J. H. Baxendale, M. Ebert, J. P. Keene, and A. J. Swallow, Ed., Academic PressInc., New York, N. Y., 1965,p 131. (6) M.Anbar and E. J. Hart, J . Am. Chem. SOC.,86, 5633 (1964). (7) I. Kraljic and C. N. Trumbore, ibid., 87, 2547 (1965). (8) M. Anbar, D. Meyerstein, and P . Neta, to be published.



The irradiations were carried out a t 25”. A doublebeam Beckman DB spectrophotometer was used for the direct measurement of the difference in the optical density between the irradiated and the nonirradiated samples. The initial concentration of p-nitrosodimethylaniline was determined using a Hilger Uvispec spectrophotometer; the initial optical densities were in the range of 1.1-1.2 (A, 4400 A, B 34,200 1. mole-’ cm-l).

Results and Discussion

of q valuesl8where q = log [ k O H + C s H s X / k O H + C s H s ] . Comparable values of q were obtained for substituted benzoate ions. Table I1 : T h e Reaction R a t e Constants of Hydroxyl Radicals with para-Substituted Benzoate Ions k0Htsubstrat.e


( ~ 1 0 - 9M - 1 sec -1)

1.17 1.85 1.92 1.92 2.07 2.50 2.62 2.97 3.24 4.72

The reaction rate constants of hydroxyl radicals with substituted benzene and benzoate ion are summarized in Tables I and 11. The absolute rate constants were

Table I: T h e Reaction R a t e Constants of Hydroxyl Radicals with Substituted Benzenes v* = log kOH+substrate

kOH+substrate (x10-*M-’ UeC-l)

‘This Substituent



1.65 1.76 2.0 2‘. 15




. ( .

Ref 2

Ref 3‘

Ref 5


2 .5 2.6 2.6 2.86






3.0 , . ,


51 . 6 5.1 5.3 5.3

3.3 3.0



6.2 4.8


-0.30 -0.27 -0.22 -0.19 (-0.20) -0.12 -0.10 -0.10 -0.06 -0.04 (-0.05) -0.04 0.00 ( -0.04) 0.04 0.19 0.21 0.21

Derived from t h e relative rates determined in ref 3 taking 1.1 X 109 M-1 sec-l.l Calculated using t h e rate const’ants determined in this work and ~ O H + C ~ = H ~ 3.3 X 109 M-* sec-’.* T h e values in parentheses are for t h e results of ref 2-5 relative to k O E + C s H s obtained in each study. ~ O H + C ~ H ~ O=H




-0.328 -0.125 -0.109 -0.109 -0.081 0,000 0.025 0.079 0.117 0.290



1.5 2.3



The q values, which indicate the relative reactivity of an aromatic compound carrying a given substituent, could be correlated with the effect of the same substituent on the reactivity of the same compounds toward electrophilic reagents. A quantitative evaluation showed that the OH P h X systems follow Hammett’s equation, = UP.^-'^ When the q values are plotted against the u valuesl5 (taken from ref 13), a satisfactory correlation was attained for all the substituents studied (Figures 1 and 2). For substituents having a deactivalues were taken. vating effect both upalaand urnmela For the other substituents only the uparavalues were taken, as it has been shown from product analysis that the attack of OH radicals on aromatic compounds is electrophilic in nature;16-18 thus, its attack on the meta position of aromatic compounds carrying orthopara directing substituents may be neglected.lg


(9) L. P. Hammett, “Physical Organic Chemistry,” McGraw-Hill Book Co., Inc., New York, N. Y., 1940, Chapter 7. (10) H. H. JaffB, Chem. Rev., 5 3 , 191 (1953). (11) R. W. Taft, N. C. Deno, and P. S. Skell, Ann. Rev. Phys.

Chem., 9 , 287 (1958).

calculated from the measured relative rates using the value k O H + c 2 H s o n = 1.1 X 10’ M-’ Sec-’.5 The values of relative rates were highly reproducible and show small standard deviations (coefficient of variaValues obtained by other methods tion < * 5 % ) . are presented in Table I for comparison. The rates of reaction vary from 1.17 X lo9 M-1 sec-* for p-nitrobenzoate ion to 5.3 X lo9 M - l sec-l for aniline. The specific reaction rates of substituted benzene have been related to that of benzene and expressed in terms

(12) H. Van Bekkum, P. E. Verkade, and B. M. Wepster, Rec.

Trav. Chim., 78, 815 (1959). (13) J. Hine, “Physical Organic Chemistry,” McGraw-Hill Book Co., Inc., New York, N. Y., 1962, p 87. (14) L. M. Stock and H. C. Brown, Advan. Phys. Org. Chem., 1, 36 (1963). (15) The

u and not u + values were taken, as the latter are appropriate for reactions with p values smaller than -0.6 (Y. Okamato and H. C. Brown, J. Org. Chem., 2 2 , 485 (1957)), whereas OH radicals show a p value of about -0.4 (vide infra). (16) G. Stein and J. Weiss, J. Chem. SOC.,3245 (1949). (17) H. Loebl, G. Stein, and J. Weiss, ibid., 405 (1951). (18) J. J. Weiss, Radiation Res. Suppl., 4 , 149 (1964).

Volume 70,Number 8 August 1066



rl Figure 1. q aa a function of c for monosubstituted benzenes: 7‘ OJ









’1 Figure 2.

q aa a function of c for p-substituted benzoate ions:



latter with OH radicals could be neglected, as their contribution could not exceed 5% of the over-all rate.8 It may be seen from Figures 1 and 2, that at first approximation the same p value, namely p = -0.41, fits both monosubstituted benzenes and para-substi tuted benzoate ions. This means that the u values in disubstituted benzenes are additive. It may be concluded that the mechanism of attack of hydroxyl radicals on aromatic compounds is analogous to an electrophilic substitution. This is in accord with the assumption that in most cases of electrophilic attacks on aromatic compounds the rate-determining step involves the addition of the electrophilic reactant t#othe aromatic ring.z0 These conclusions corroborate the suggestion that the primary step in the attack of hydroxyl radicals on aromatic compounds involves a n addition of the OH radicals to the aromatic ring.’-4 It has been shown that OH radicals are noncharged speciesz1and that HzOf, if it exists at all, is limited to the spur only.22 The electrophilic nature of the OH radical, which has been inferred from the relative yields of ortho-, meta-, and para-substituted benzoic acids,16-18 does not necessarily imply that it is positively charged. The electrophilic behavior of the noncharged OH radicals is not surprising in view of the high electron affinity of this species. The absolute value of p obtained for the reactions of OH radicals is small compared with the respective values for conventional electrophilic substitutions.l 4 This is probably due to the high reactivity of OH radicals, which also makes them less selective. The reactions of hydrogen atom with aromatic compounds, which proceed with specific rates of the order of lo9 M-l sec-l, show a comparable absolute p value ( p -0.7).23 On the other hand, hydrated electrons which react with certain aromatic compounds at diffusion controlled ratessrz4 exhibit a much higher absolute p value than OH radicals. This suggests that the PhX eaQ- reactions, although governed by the electron distribution of the aromatic substrate as expressed by the u function, are intrinsically different in nature from an aromatic substitution reaction.

When the specific rate constants for the reaction of

OH radicals with aromatic compounds carrying aliphatic groups were considered, the reactivities of the

The Journal of Physical Chemistry

(19) A. Sakumoto and G. Tsuchihashi, Bull. Chem. SOC.Japan, 34, 663 (1961). (20) L. Melander, “Isotope Effects on Reaction Rates,” The Ronald Press Co., New York, N. Y., 1960, p 107. (21) C. J. Hochanadel, Radiation Res., 17, 286 (1962). (22) M. S. Matheson, Radiation Res. Suppl., 4, 1 (1964). (23) M. Anbar, D. Meyerstein, and P. Neta, Nature, 209, 1348 (1966). (24) M. Anbar, “Reactions of the Hydrated Electron,” Advances in Chemistry Series, No. 50, American Chemical Society, Washington, D. C., 1965, p 55.