Radical-nucleophilic (SRN1) reactions: electron spin resonance studies of electron-capture processes. Part 5. p-Nitrobenzyl and p-nitrocumyl systems

Abstract

Electron spin resonance spectroscopy has been used to probe two of the steps postulated for the radical-nucleophilic substitution (S_RN1) mechanism for p-nitrobenzyl and p-nitrocumyl systems. These two steps are electron-capture by p-nitrobenzyl and p-nitrocumyl derivatives to form radical-anions and their dissociation to yield radicals and anions [equations (1) and (2) in Scheme 1]. A range of radical-anions (p-NO₂C₆H₄CH₂X)^–˙, with X = I, Br, Cl, and SCN and (p-NO₂C₆H₄CMe₂X)^–˙, with X = Br and NO₂ have been unambiguously identified by e.s.r. spectroscopy and shown to be infinitely long-lived at low temperature. Our results indicate that the p-nitrobenzyl radical-anions do not dissociate to give p-nitrobenzyl radicals at the temperatures used (77 K to ca. 160 K). However, dissociation of the p-nitrocumyl radical-anions (p-NO₂C₆H₄CMe₂X)^–˙, with X = Br and NO₂, to p-nitrocumyl radicals (p-NO₂C₆H₄ĊMe₂) and bromide and nitrite anions respectively was clearly observed. The results are compared with those obtained for solution S_RN1 reactions for which these radical-anions and radicals are postulated as intermediates.