Reactions of Alkoxy Radicals: I. Hydrogen Atom Abstraction From Substituted Toluenes

Abstract

The relative rates of hydrogen atom abstractio~l from ten substituted toluenes by t-butoxy radicals in carbon tetrachloride a t 40 OC have been measured and the data fitted to the Hammett equation. A much better correlation is obtained with the a+ constants than with the u constants of the substituents. In this respect, therefore, the reaction is similar to the majority of hydrogen atom abstraction reactions by radicals of moderate and high electron affinity. That is, the polar properties of the substituents are much more important than their stabilizing effect on the benzyl radical formed in the reaction.The cumyloxy radical has a similar reactivity to the t-butoxy radical. The p-nitrocumyloxy radical appears to be slightly more reactive and also slightly more susceptible to the polar elfects of substituents than are t-butoxy or cu~nyloxy radicals.The relative rates of hydrogen atom abstraction from compounds of the type XC6H4YH by free radicals can frequently be correlated by means of the Hamlnett p u relationship. Data for a \vide variety of such relations were sulnnlarized in a recent paper from this laboratory (1). I t was shown that with few exceptions, notably hydrogen abstraction by t-butoxy radicals fro111 substituted toluenes (2,3), a better correlation was usually obtained with (I+ constants rather than with u constant^.^ Further support for this generalization has been provided by Russell and Willianlson (5) who specifically pointed out that for the t-butoxy radical the proper substituents had not been exanlined to distinguish clearly betxveen a U-and a u+-correlation. With this point in mind we decided to reinvestigate this reaction with particular attention to those substituents having significantly different (I and 6 constants. While this \vork was in progress a similar study by Gilliom and Ward appeared (6). However, we feel that our results and conclusions differ sufficiently from theirs to warrant the present communication. EXPERIMENTAL ReagentsToluene, 112-xylene, p-sylene, ethylbenzene, and cumene were Phillips Research Grade materials. m-Chlorotoluene, p-chlorotoluene, vz-nitrotoluene, p-nitrotoluene, and p-methoxytoluene were obtained from Eastman. These compou~lds were used a s supplied, their purity being checked by gas-liquid chromatography (g.1.c.). Toluene-0-ds was obtained fro111 Merck, Sharp & Dohme; it contained over 95% trideuterotoluene. We are grateful to the Benzol Products Company for a generous sample of p-methoxybenzyl chloride.p-Phenoxytoluene was prepared by the method of Russell and Williamson (5). p-Cyanotoluene was prepared by the dehydration of p-tolualdehyde oxime with acetic anhydride (cf. ref. 7). t-Butyl hypochlorite was prepared as described by Teeter and Bell (8). Cumyl hypochlorite and p-nitrocumyl hypochlorite were prepared from the corresponding alcohols by the method of Zavitsas and Seltzer (9). The p-nitrocu~nyl alcohol was prepared from cumene by nitration (10) followed by oxidation of the purified p-nitrocuinene by the method of Icwart and Francis (...

Oxidation of HMG‐CoA Reductase Inhibitors by tert‐Butoxyl and 1,1‐Diphenyl‐2‐picrylhydrazyl Radicals: Model Reactions for Predicting Oxidatively Sensitive Compounds During Preformulation

“…This suggestion is consistent with work that has shown that substituent effects can be adequately correlated with + constants. 16 The fact that steric effects dominate over electronic effects in our system can be explained by a closer examination of our substrates. The compounds studied here have a bulky group that can hinder the approach of the abstracting radical, thus raising the energy of the transition state sufficiently to compensate for any electronic stabilization of the transition state by the electrondonating groups.…”

“…The notion of a polar transition state (polar effect) has been used to explain the reactivity of substituted toluenes toward tert-butoxyl radical. 15,16 These data suggest that the transition state for hydrogen atom abstraction by the tert-butoxyl radical from substituted toluenes involves some polar character (i.e., a zwitterionic state). This suggestion is consistent with work that has shown that substituent effects can be adequately correlated with + constants.…”

Correlation analysis in the chemistry of free radicals

“…31 In our opinion, the use of s + -constants is really more effective as far as the properties of radical cations are concerned but this is not that obvious for neutral radicals. Nevertheless, radical bromination of substituted toluenes with bromine, 24 N-bromosuccinimide 24 and bromotrichloromethane 32 as well as the quantitative interpretation of abstraction of a hydrogen atom by alkoxy radicals, 11,12,33 chlorine atoms, 34 and peroxy radicals 34 have become classical examples of successful use of the s + constants in free-radical chemistry. Experimental results obtained by one of the authors of this review have confirmed the exceptionally high efficiency of the s + constants in correlations with single-electron reduction potentials of various aromatic molecules and radicals.…”

“…The new topological approach developed on the basis of the above models, which allows the substituent effect to be expressed in terms of discrete atomic contributions, was first reported in our publication and called r 72 -analysis. 220 Since both the steric and inductive constants were expressed in Eqns (11) and (12) as functions of 1/r 72 , the two-parameter Taft equation…”

Hydrogen atom abstraction from substituted toluenes by tert-butoxy radical

“…A recent report by Kennedy and Ingold (1) states that a Hammett po relationship affords a better correlation with ot constants than with o constants for hydrogen abstraction by tertbutoxy radicals from substituted toluenes. This result differs from earlier observations (2,3).…”