N-substituted heterocyclic cations. Part VIII. Substituent effects and the acidity of quinolinium ions. Hydroxide addition versus proton loss

Abstract

The effect of substituents on the equilibrium between N-substituted quinolinium ions and their pseudo-bases has been determined. Substituents in the 3-, 5-, 6-, 7-, and 8-positions have a large effect on the position of equilibrium, and substituents on the heteroatom have an even larger effect. Correlations with the Hammett equation show that both 6- and 7-substituents behave as if in a para-position, because of the possibility of conjugative interaction between the substituent and either the reaction centre or the hetero-atom. The substituent effects are so large that hydroxide ion addition is preferred to proton loss for all of the quinolinium ions studied. The more stable quinolinium ions add hydroxide ion at a pH similar to, or greater than, that at which their pseudo-bases lose the hydroxy-proton. The consequences of these equilibria are discussed.