Kinetic electron spin resonance spectroscopy. Part 6.—Formation and termination reactions of aliphatic radicals by reductive dissociation of halogeno-compounds

Abstract

The reactions of hydroxy-alkyl radicals with a variety of aliphatic halogeno-compounds RX have been studied. By examining the effects of varying concentrations of RX on the concentration and lifetime of the hydroxy-alkyl radicals it is shown that these are replaced by radicals R· produced by electron transfer to RX and subsequent loss of X^–. Peak intensity anomalies are satisfactorily explained in terms of a hyperfine energy induced E–A polarization plus a contribution from an initial (emissive) polarization. Self-termination rate constants for nine aliphatic radicals have been determined. These rate constants are in the range 3 × 10⁸ to 5 × 10⁹ dm³ mol^–1 s^–1 in propan-2-ol at 300 K and have an inverse correlation with molecular mass. The problems associated with measurements of rate constants for the electron transfer step are discussed. For the reaction of Me₂ĊOH with trichloroacetic acid the value (6.1 ± 1.9)× 10⁶ dm³ mol^–1 s^–1 was obtained in propan-2-ol at 293 K.