Free Radicals in the Radiolysis of Benzyl Halides

Abstract

ULTRA-VIOLET absorption spectroscopy can be used to study the production of radicals by photolysis or radiolysis if solutions in rigid media, such as M P (2 parts methyl-cyclohexane, 3 parts isopentane) at 77° K, are used. The photolysis of toluene, benzyl chloride and similar compounds, to give benzyl and related radicals, has been studied in detail¹. It was soon realized² that γ-radiolysis gave spectra different from those given by ultra-violet photolysis and this was later confirmed by spectrographic study of the series—C₆H₅CH₃, C₆H₅CH₂Cl, C₆H₅CHCl₂, C₆H₅CCl₃ (ref. 3). The results seemed to show that γ-radiolysis led to breaking of the α-C—H bond while photolysis broke the C—Cl bond. Gallivan and Hamill⁴, however, have found strong evidence that the dominant process in the radiolysis of these halides is dissociative electron attachment; for example A positive ion of the solvent is also trapped. They observed the same spectral differences and therefore suggested that the spectrum is that of the benzyl radical perturbed by the neighbouring chloride ion which does not escape from the solvent cage. Surprisingly, chloride, bromide and iodide gave identical spectra, but it may be that the nature of the anion is not important.