Steric Effects in Forward and Reverse Menschutkin Reactions of Some Pyridines, Quinolines and Thiazoles

Abstract

The effects of ortho alkyl substituents on the rates of methylation of pyridine and demethylation of N-methylpyridinium iodides are compared (some quinolines are included as 'substituted pyridines'). For monosubstituted compounds, substituent steric effects are twice as important in the methylation (forward) reaction as in the demethylation reaction. In contrast, the ratio is 3 : 1 for disubstituted compounds, because the substituent effects are additive in the reverse reaction but greater than additive in the forward reaction. Some thiazole results are also reported and, here, steric effects on the forward reaction are small while no steric acceleration of the reverse reaction is evident.