Quaternization of 1,3-di(4-pyridyl)propane with alkyl bromides in sulpholane

Abstract

1,3-Di(4-pyridyl)propane, DPP, has been quaternized with ethyl, n-butyl and n-hexyl bromides in sulpholane. The overall reaction rates are second order, i.e. there is no dependence of the reaction of the second pyridyl group of a DPP molecule on the prior quaternization of the first group. The course of the reactions is therefore as expected, but differs from the previously studied behaviour of the 1,3-pyridyl substituents of poly(4-vinyl pyridine). In the case of the polymer, there is retardation as reaction proceeds, i.e. a neighbouring-group effect operates. Quaternization reactions of DPP are not affected by the presence of N-ethyl pyridinium bromide, which rules out any significant electrostatic influence on the reactions. With DPP, the 1,3-substituents can adopt configurations such that there is no steric hindrance between the alkyl groups during quaternization of the second pyridyl group; in the polymer steric hindrance cannot be avoided.