A piezokinetic study of the solvolyses of benzyl chlorides.

Abstract

Rate constants for the solvolysis of benzyl chloride, p-methyl benzyl chloride, p-nitro benzyl chloride and p-chloro benzyl chloride were determined at four pressures at 50°C in an acetone-water mixture containing 0.1407 mole fraction acetone. Rate constants for the solvolysis of p-methyl benzyl chloride in four acetone-water mixtures were also determined at four pressures at 50°C. The partial molar volumes of this latter compound were determined in these same acetone-water mixtures at 25°C. The log10k-pressure data were fitted to polynomials of the first or second order and the volumes of activation and the parameters (delta DeltaV*/delta P)T were calculated. It was found that the volumes of activation were all negative and varied with substituent in the order p-CH3 > H > p-Cl > p-NO2 and the parameters (delta DeltaV*/delta P)T were zero or positive, varying in the order p-NO 2 > p-Cl > H > p-CH3. Several effects which could produce this behavior were explored but it was not possible to assign these reactions to either the extreme SN1 or SN 2 categories on the basis of these results. It was also found that the volumes of activation for the solvolysis of p-methyl benzyl chloride in different acetone-water mixtures passed through a minimum when plotted against solvent compositions. The partial molar volume of p-methyl benzyl chloride passed through a maximum as a function of solvent composition. This suggested that the minimum in the volumes of activation arose at least in part from a maximum in the partial molar volumes of the reactants. A linear correlation between the entropies and volumes of activation is observed. This is discussed in terms of simple dielectric theory.