Reaction of acyclic hydrocarbons towards t-butoxy radicals. A study of hydrogen atom abstraction by using the radical trapping technique

The reaction of 3-methylpentane and 2,4-dimethylpentane toward t-butoxy rad icals has been investigated, in neat and benzene solutions, by using the ra dical trapping technique. Abstraction occurs principally from the tertiary and secondary C-H reaction sites of 3-methylpentane and the tertiary positi on of 2.4-dimethylpentane. The tertiary and in particular secondary C-H rea ction sites of 2,4-dimethylpentane are shown to be considerably less suscep tible towards t-butoxy radical facilitated abstraction compared with the eq uivalent reaction sites of 3-methylpentane. As a result, the latter is thre e times as reactive as 2,4-dimethylpentane as a neat hydrocarbon solution a nd seven times as reactive in a diluted mixture of benzene. Differences in selectivity and rate of hydrogen abstraction, between the substrates, are i nterpreted in terms of non-bonding interactions retarding t-butoxy radicals from approaching sterically demanding C-H reaction sites. The selectivity from 3-methylpentane is solvent-insensitive whereas abstraction from 2,4-di methylpentane is modified in benzene. Further, the rate of hydrogen abstrac tion, from either substrate, to t-butoxy radical beta-scission is considera bly smaller in benzene. Both observations are interpreted in terms of t-but oxy radical solvation by the aromatic solvent.