Reaction kinetics and the spectroscopic and crystallographic nature of certain products from the quaternization of poly(4-vinyl pyridine) and of small pyridine molecules with alkyl bromides and iodides

Abstract

This extension of previous studies on the quaternization of poly-4-vinyl pyridine) and related small molecules, e.g. 4-methyl pyridine, deals with several topics. Kinetic measurements on quaternization of polymer with methyl iodide show negligible neighbouring-group retardation when one reacted neighbour is present, but some retardation by two reacted neighbours. The production of charges does not affect the kinetics. This supports the notion that steric hindrance is responsible for retardation of this type of reaction. In some cases coloured quaternization products are formed, e.g. 4-methyl-N-n-propyl pyridinium bromide (MPPBr) prepared at 333 K is red, but that prepared at 298 K is colourless. Analysis of u.v.–visible spectra does not indicate that charge-transfer interactions in solution are a source of colour. There is no evidence from ¹³C and ¹H n.m.r. spectra of impurities in the reaction products. It has been found that hydrogens undergo base-catalysed exchange with deuterium at rates depending on their position in the molecule. The crystal structure of MPPBr has been determined. There is no essential difference in structure between coloured and colourless samples, and no evidence for charge-transfer complex formation. The structure, with four molecules in each monoclinic unit cell of space group C2/c, reveals the presence of hydrogen-bonding between ring hydrogen atoms and Br^– ions.