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 Δ V ‡ and (δΔ V / /δ P ) T are obtained. The Δ V ‡ 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 N 2(1), i.e. are towards the S N 1 end of the S N 2 spectrum of behavior. The Δ V ‡ values steadily become more negative in the series p CH 3 , H, p Cl, p NO 2 , and this is interpreted in terms of the greater spreading of positive charge in the p CH 3 case and in terms of greater S N 2(2) character in the p NO 2 case. The Δ V ‡ 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 (δΔ V ‡ /δ P ) T values show a negative correlation with Δ V ‡ , suggesting, as expected, that the more compact activated complexes are the least compressible.