Determination of a new scale of ortho-steric parameters S0 from N-methylation of pyridines

Abstract

The kinetics of quaternisation by methyl iodide of 33 substituted pyridines was measured in acetonitrile at 30 °C; the reactions were followed by n.m.r. with a competitive technique. The relative rate constants were similar to those observed in other polar aprotic solvents. Steric effects exist for all the ortho-substituents and can be estimated from the deviation from the Brønsted plot. A scale of ortho-steric parameters (S⁰) is proposed. There is no significant dependance of S⁰ on the Dougard–Decroocq solvent parameters and a complete lack of correlation between S⁰ and the electronic effects of substituents. The relative S⁰ values agree with other experimental estimations of the steric size of substituents. The S⁰ parameters are discussed in terms of substituent structure in connection with the Taft–Kutter–Hansch E₈ and Charton υ parameters. The general applicability of a unique scale of steric parameters is discussed.