Pressure effects on the rates of some hydrogen/deuterium transfer reactions between 2,2-Diphenyl-1-picrylhydrazyl and substituted phenols

Abstract

The volumes of activation for hydrogen transfer reactions between 2,2- diphenyl-1-picrylhydrazyl (dpph) and a series of substituted phenols (2,4,6-tri-t-butylphenol, 2,6-di-t-butylphenol, 2,6-di-t-butyl-4- methylphenol, 2,4,6-trimethylphenol and 2,4,6-triphenylphenol) were measured in toluene at 25°C up to pressures of 2 kbar. The analogous deuterium transfer reactions were studied in the case of the first three phenols. The obtained volumes of activation, ΔV^?_expt proved to be independent of the nature of the phenol and the extent of deuteration. The overall mean value of ΔV^?_expt is -13±3 ml mol^-1 and can be virtually entirely attributed to intrinsic molecular changes, ΔV^?_intr. From results of the effect of pressure on the first overtone of the O-H stretching frequency for a number of selected phenols in CS₂, and from the lack of influence of pressure on the primary kinetic isotope effect, it is concluded that ΔV^?_ntr is essentially independent of moderate pressures.