Pressure Effects in the Solvolysis of Benzyl Chlorides

Rates of solvolysis of benzyl chloride and of substituted benzyl chlorides have been measured in an acetone-water mixture (acetone mole fraction 0.147) at pressures ranging from atmospheric to 1 kbar. Pressure studies have also been made for p-methyl benzyl chloride in various acetone-water mixtures. Measurements have also been made of the partial molar volumes of the reactants. The plots of log k against pressure are fitted to a second-degree polynomial in P, and values of ΔV^? and (δΔV^//δP)_T are obtained. The ΔV^? values are all negative, having values ranging from ?18 to ?24 cc/mole. The results are interpreted on the view that the mechanisms are S_N2(1), i.e. are towards the S_N1 end of the S_N2 spectrum of behavior. The ΔV^? values steadily become more negative in the series p? CH₃, H, p? Cl, pNO₂, and this is interpreted in terms of the greater spreading of positive charge in the p? CH₃ case and in terms of greater S_N2(2) character in the p? NO₂ case. The ΔV^? values go through a minimum as the solvent composition is varied, a result that is related to the existence of a corresponding maximum in the partial molar volumes of the reactant. The (δΔV^?/δP)_T values show a negative correlation with ΔV^?, suggesting, as expected, that the more compact activated complexes are the least compressible.