The mechanism of alkylation reactions. Part 2. The effect of pressure and substituents on the reaction of phenacyl bromide with pyridine in methanol

Abstract

Rate constants have been determined for the reaction of p-bromophenacyl bromide with pyridine and three substituted pyridines in methanol at 313.2 K. The reaction of phenacyl bromide in methanol at 313.2 K was investigated for pressures up to 100 MPa and the effect of substituents on the volume of activation, ΔV*, determined. From the partial molar volumes of reactants and products, the values of ΔV* were interpreted in terms of the intrinsic volume of the transition state, solvent electrostriction in the transition state and differences in hydrogen bonding between the initial and the transition state. The results support a transition state in which (a) bond forming leads over bond breaking and (b) considerable charge dispersal occurs. A priori calculations of ΔV and ΔV* are compared with experimental values and their utility discussed.