Pulse-Radiolysis Study of the Formation of Benzyl Radicals from Benzyl Chloride in Cyclohexane in the Gas Phase

Abstract

The yield of benzyl radical (Bz) formed by electron‐beam pulse radiolysis of benzyl chloride (BzCl) in cyclohexane vapor (1.3 atm/120°) has been studied by optical absorption measurements. Electron scavengers lower the benzyl radical (Bz) absorption by competing with dissociative electron capture by BzCl. A study of this competition gave the relative rate constants for electron capture at 120°C (SF₆ = 1) : BzCl = 1.6 × 10⁻³, CCl₄ = 0.3, CH₃Br = 5 × 10⁻⁵, C₆H₅NO₂ ∼ 2 × 10⁻². The maximum depression of the absorption, Δ(Bz)_max, which was independent of the electron scavenger used and of the BzCl concentration (0.2 to at least 4 mole %), is believed to correspond to the yield of Bz from dissociative capture. With benzyl bromide, acetate, and formate in cyclohexane, Δ(Bz)_max was the same as with benzyl chloride. Bz is also formed by processes other than dissociative capture which are not influenced by electron scavengers. The yield from these processes increases with increasing BzCl concentration. Using electron beam radiolysis, the decomposition of BzCl due to dissociative capture, measured by chemical analysis, $\mathrm{G(-\hspace{0.17em}}\mathrm{BzCl}{)}^e\text{\hspace{0.17em}=\hspace{0.17em}2.6\hspace{0.17em}±\hspace{0.17em}0.4}$ molecules per 100 eV. ⁶⁰Co γ‐radiolysis gave $\mathrm{G(-\hspace{0.17em}}\mathrm{BzCl}{)}^e\text{\hspace{0.17em}=\hspace{0.17em}4.3\hspace{0.17em}±\hspace{0.17em}0.1}$ , which is close to the value expected if all of the electrons react by dissociative capture. Possible reasons for the low value with electron radiolysis are discussed.