Kinetics and mechanism of the quaternization of poly(4-vinyl pyridine) with alkyl and arylalkyl bromides in sulpholane

Abstract

Quaternization reactions of poly(4-vinyl pyridine) with several organic bromides show retardation of reaction in excess of that predicted by second-order kinetics. Kinetic expressions from a neighbouring-group model, in which a reaction is characterized by the rate constants ki for reaction of a pyridyl group having i= 0, 1 or 2 already reacted neighbours, have been applied to the experimental results. Quantities obtained include k0, K=k1/k0, L=k2/k0, activation energies E0(=E1=E2) and pre-exponential factors A0. The ratios K and L for the bromides range from 0.95 and 0.54, respectively, for ethyl bromide to 0.55 and 0.28 for 1-bromo-3-phenylpropane. On average K= 2L. The dependence of k0 values on the nature of the organic group of the bromides is discussed in terms of steric hindrance in the transition state; similar reasoning can be applied to molecules with a single pyridyl group. Linear empirical relationships have been found between K and L and the extended lengths of the alkyl and arylalkyl groups of the bromides. The deviation from second-order kinetics (K < 1, L < 1) is explained in terms of steric hindrance, involving the transition state of a reacting pyridyl group and the alkyl group on a neighbouring already reacted pyridyl group. Electrostatic effects are not thought to influence the reaction kinetics.