Nucleophilic character of the tert‐butyl radical. Absolute rate constants for the reactions with substituted toluenes

Abstract

The decay of photochemically generated tert-butyl radicals is studied at 48°C in 11 n-and p -substituted toluenes by time-resolved electron spin resonance spectroscopy. It is governed by the second-order self-termination perturbed by a pseudo-first-order reaction of the radical with the toluenes. The first-order lifetimes yield the rate constants k A for hydrogen transfer from toluenes to tert-butyl. Substituent effects on the rate constants confirm the nucleophilic character of the radical.Rate constants for the abstraction of a benzylic hydrogen atom from mand p-substituted toluenes by many neutral radicals are known to obey, at least approximately, the Hammett p a relation [l] (1) h i is the rate constant for unsubstituted toluene, and a is a substituent parameter. This correlation is often interpreted in terms of polar resonance structures contributing to the transition state and lowering the activation energy. The sign of the reaction parameter p classifies radicals as electro-( p < 0) or nucleophilic ( p > 0). For most radicals a negative p is found [I].It has been argued [2] that for this case the correlation can also be explained in terms of substituent effects on the dissociation energy of the benzylic CH bond without resorting to polar transition states. The few published examples for radicals with positive p values [3-71 are of deciding importance, therefore. Usually correlation (1) is based not on absolute rate constants but on relative rate constants derived from relative product yields in reaction systems where the hydrogen abstraction reaction from the toluene competes with the reaction of the radical with a suitable added standard. Using deuterated thiols as standards, Pryor et al. [3,4] obtained the hitherto highest positive p reported, p = 1.0, for the tert-butyl radical at log(KA/K;) = p -a

Reactions of Alkyl Radicals with Substituted Toluenes and the Effect of Substituents on Dissociation Energies of Benzyl CH Bonds

“…Because of experimental problems found by Tanner and co-workers [14] with the work that reported 1 = 1.0 for tert-butyl radicals, Pryor and co-workers re-investigated the reactivity of tert-butyl radicals from azoisobutane (= bis(2-methylpropyl)diazene) photolysis with toluenes and reported 1 = 0.49 [17] ascribed to benzyl-H abstraction, in good agreement with the 1 value reported by Dütsch and Fischer [1]. However, there was serious disagreement in the reactivities relative to toluene of many substituents, e.g., p-(tert-butyl) 1.22 [1] and 0.78 [17], p-CN 3.28 [1] and 2.23 [17], etc.…”

“…The kinetic measurements of Dütsch and Fischer with tert-butyl radicals [1], however, provided data inconsistent with those described above in several important ways and generated the impetus for much of the work to be reported here. The absolute rate constants reported indicate that only ca.…”

“…On the basis of Hammett correlations of 13 C, 1 H-NMR coupling constants of toluenes and other evidence, it had been postulated that electron-withdrawing substituents strengthen the bond and electron-donating substituents weaken it. The same was postulated for anilines on the basis of 15 N, 1 H coupling constants.…”

“…On the basis of Hammett correlations of 13 C, 1 H-NMR coupling constants of toluenes and other evidence, it had been postulated that electron-withdrawing substituents strengthen the bond and electron-donating substituents weaken it. The same was postulated for anilines on the basis of 15 N, 1 H coupling constants. If this were the case, then all H abstractions from toluenes would have negative 1 values, as was the case up to then for abstractions by electrophilic radicals such as chlorine, bromine, trichloromethyl, phenyl, etc., radicals.…”

“…-In 1982, Dütsch and Fischer published Nucleophilic Character of the tert-Butyl Radical. Absolute Rate Constants for the Reactions with Substituted Toluenes [1]. tert-Butyl radicals were produced by pulsed photolysis of di(tert-butyl) ketone at 488.…”