Flash photolysis studies of phenyl-substituted phenols, quinones, and corresponding free radicals. Part 4. Kinetics and mechanism of the elementary reaction of oxidation of aliphatic alcohols by aroxyl radicals

Abstract

The kinetics of the elementary reaction between the 2,6-diphenyl-4-stearoxyphenoxyl radical (PsO˙) with C₁–C₄ aliphatic alcohols in the corresponding alcohols or benzene–alcohol mixtures has been investigated. It has been found that k is ca. 10^–3–10^–6 l mol^–1 s^–1 at 338 K. The reactions have characteristically high activation energies E_a= 40–75 kJ mol^–1 and low pre-exponential factors A= 10¹–10⁷ l mol^–1 s^–1. An increase of the alcohol content in benzene inhibits the reaction due to formation of the less reactive PsO˙⋯ alcohol complexes. Using an N-benzylidene-t-butylamine oxide spin trap, alcohol radicals have been detected and the elementary reaction mechanism has been established: at 373 K primary alcohols and t-butyl alcohol are oxidized primarily by way of electron transfer, and secondary alcohols predominantly by hydrogen atom abstraction (the solvent is the corresponding alcohol). At 338 K the reaction is selective: in pure alcohols and benzene containing 5% alcohol the reaction route which had been dominant becomes the only possible one.