Rates and mechanism of the reactions of hydroxyl radicals with acetic, deuterated acetic, and propionic acids in the gas phase

Abstract

Rate constants for the reactions of hydroxyl radicals with the monomer and dimer of acetic acid, deuterated acetic acids, and propionic acid have been determined by a laser photolysis-resonance absorption technique. Hydroxyl radicals were generated by photolysis of the acids at 222 nm with a KrCl laser and their decay was followed by time-resolved resonance absorption. The monomers of acetic and deuterated acetic acids reacted with OH much faster than the dimers, whereas the monomer and dimer of propionic acid reacted with about equal rate constants. A primary isotope effect was observed when carboxylic but not alkyl hydrogen was substituted by deuterium in acetic acid. The results are entirely consistent with the two-channel mechanism that we proposed for the reaction of OH with formic acid. The results are interpreted in terms of the variations in C-H bond strengths and in equilibrium constants for adduct formation of the acids studied.