Solution and gas-phase basicities of pyridines with bulky α-substituents

Abstract

Steric hindrance to solvation of pyridinium cations is largest for two α-t-butyl substituents, less for one t-butyl and one phenyl, less for two α-phenyl groups, and least for one phenyl and one alkyl. The effect of α-phenyl groups is reduced by buttressing. Gas-phase proton affinities of the pyridine determined by dimer dissociation do not correlate with the solution pK_a values. Steric accelerations of pyridinium dealkylation show fair correlation with solution pK_a; reasons for discrepancies are discussed.