Oxidation of pyridine with potassium peroxydisulphate. The role of pyridine cation-radicals as primary intermediates

Abstract

Detailed results of a study of kinetics, products, and some intermediates (e.s.r. spectroscopy) for reactions of pyridine with peroxydisulphate ion in aqueous media are presented. In neutral solution reaction rates are dependent on the first power of peroxydisulphate ion concentration and on the half power of pyridine concentration. Pyridylpyridinium ions are the major products with isomeric bipyridyls minor products. However, yields of the latter compounds increase rapidly (at the expense of pyridylpyridinium ions) with increasing initial pH of the reaction mixtures. Spin-trapping experiments indicate that pyridine cation-radicals and 2-pyridyl radicals are reaction intermediates. A chain reaction mechanism, involving initial electron transfer from pyridine, satisfactorily accounts for all the experimental observations. The effect of added base is thought to arise from either a competing deprotonation of pyridine cation-radical to pyridyl radicals, or stabilisation of intermediate radical ions produced by attack of sulphate anion-radical on the pyridine nitrogen atom.