Radical cations of benzyl chlorides and bromides formed by radiolysis. An electron spin resonance study

Abstract

Exposure of dilute solutions of benzyl chloride, benzyl bromide and various derivatives thereof in trichlorofluoromethane to ⁶⁰Co γ-rays at 77 K gave the radical cations, characterised by their e.s.r. spectra. The major species obtained from C₆H₅CH₂Cl had hyperfine coupling to three inequivalent protons; assigned to the para ring proton (12 G) and the two methylene protons (26 and 21.5 G), with no detectable coupling to the chlorine nucleus. Only a single isotropic feature was obtained from C₆D₅CD₂Cl, confirming the absence of significant coupling to chlorine. It is suggested that the preferred conformation has the chlorine atom close to, but not in, the plane of the benzene ring, with a significant barrier for the in-plane site.

In contrast, only cations with strong hyperfine coupling to bromide nuclei were obtained from benzyl bromide. The p-methyl derivative gave only one structure [A_∥(⁸¹Br)= 162 G] but benzyl bromide gave two major species [A_∥(⁸¹Br)= 193 and 133 G] thought to be two alternative conformations of the parent cation. All these cations were sensitive to visible light, but we were unable to detect any well defined radical photoproducts.

Reasons for the formation of different conformations are discussed and the results are compared with optical studies for matrix-isolated cations and with e.s.r. results for β-chloro-and β-bromo-alkyl radicals.