Isomerization of <i>n</i>‐hexyl and <i>s</i>‐octyl radicals by 1,5 and 1,4 intramolecular hydrogen atom transfer reactions

Abstract

Abstract n ‐Hexyl and s ‐octyl radical isomerizations by intramolecular hydrogen atom shift have been studied in the presence of high methyl radical concentration where isomerized alkyl radicals reacted predominantly by combination and disproportionation reactions with methyl radicals. By assuming the rate coefficient of 1‐hexyl radical recombination to be equal to that of ethyl self‐combination, the rate coefficient of log( k 1 / s −1 ) = (9.5 ± 0.3) – (11.6 ± 0.3) kcal mol −1 / RT ln 10 has been derived for the 6 sp isomerization of n ‐hexyl radicals, 1‐hexyl → 2‐hexyl (1). Investigation of s ‐octyl radical isomerization was complicated by fast interconversion between 3‐octyl, 2‐octyl, and 4‐octyl radicals. Use of the methyl trapping technique and systematic variation of methyl radical concentration made possible the determination of log( k 2 / s −1 ) = (9.4 ± 0.7) − (11.2 ± 1.0) kcal mol −1 / RT ln 10 for the 6 ss isomerization of 3‐octyl and the estimation of log( k 3 / s −1 ) = 10.5–17 kcal mol −1 / RT ln 10 for the 5 ss isomerization of 2‐octyl radicals, where 3‐octyl → 2‐octyl (2), and 2‐octyl → 4‐octyl (3).