Disproportionation‐combination ratios of small alkyl radicals formed by the mercury‐photosensitized hydrogenation of olefins in the gas phase

Disproportionation‐combination ratios have been obtained for small alkyl radicals produced in the mercury‐photosensitized hydrogenation of ethylene, propylene, butene‐1, butene‐2, and isobutene. For conditions favoring thermalized radicals, kd/kc (ethyl, ethyl) = 0.14, kd/kc (isopropyl, isopropyl) = 0.69, kd/kc (isopropyl, n‐propyl) = 0.41, kd/kc (2‐butyl, 2‐butyl) = 0.77, kd/kc (2‐butyl, n‐butyl) = 0.45, and kd/kc (t‐butyl, t‐butyl) = 3.1. The results are consistent with kd/kc (n‐propyl, n‐propyl) = 0.15 and kd/kc (n‐butyl, n‐butyl) = 0.14. The general agreement between these disproportionation‐combination ratios and those obtained where photolytic radical sources have been used indicates that under appropriate conditions, H atom addition to olefins is a suitable source for the study of thermalized alkyl radicals and may be especially useful for larger alkyl radicals for which clean photolytic precursors are not available.