Thiol-thione equilibria in nitrogenous Heterocyclic Mercapto-compounds

Abstract

Two different tautomeric forms may be written for monoaza-heterocyclic mereapto-compounds, one with the mobile hydrogen atom on sulphur and the other with the mobile hydrogen on nitrogen. For example 4-mercaptopyridine may exist in the tautomeric forms: (l; R = K) or' (II; R = H).Additional NH forms are possible for polyazaheterocyclic mereapto-compounds. The tautomerism of the mercapto-derivatives of six-membered rings has been little examined. Isolated qualitative studies of the tautomerism have been undertaken but no quantitative approach to the problem has previously been attempted. The present investigation was undertaken with the aim of elucidating the structure of simple nitrogenous heterocyclic mereapto-compounds in water because of their relationship to the biologically important 2-thiouracil derivatives which may be represented as (h i ). In the present work the mercapto - compounds have been studied by reference to fixed tautomeric forms in which the mobile hydrogen atom was replaced by an immobile methyl - group e.g. (I and II; R = Me). This can be done because changes in ultraviolet absorption spectra and ionization constants are small upon W- or Smethylation. Finally, by employing ionization constants, the ratio of the tautomers at equilibrium in rnonoaza - heterocyclic mercapto - compounds was calculated.