Selectivity in the halogenation of hexane by tertiary aminium radicals from the photodecomposition of N-halogenoammonium perchlorates

Abstract

The photochlorination of hexane with tertiary N-chloroammonium perchlorates in trifluoroacetic acid gives mono-cholorohexanes in high yield and with a striking preference for the 2-isomer. It is shown that free-radical chains are involved in which hydrogen-atom abstraction is by tertiary aminium radicals. The marked preference for 2-compared with 3-chlorination is attributed mainly to the bulky aminium radical attacking the relatively more accessible 2-position; an alternative explanation involving a reversible hydrogen-atom abstraction is ruled out. The magnitude of the primary deuterium kinetic isotope effect in these photochlorinations gives information about the dependence of the extent of C–H bond breakage in the transition state on the structure of the abstracting aminium radical.