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 i n other polar aprotic solvents. Steric effects exist for all the ortho-substituents and can be estimated from the deviation from the B r ~n s t e d plot. A scale of ortho-steric parameters (SO) is proposed. There is no significant dependance of SO on the Dougard-Decroocq solvent parameters and a complete lack of correlation between So and the electronic effects of substituents. The relative So values agree with other experimental estimations of the steric size of substituents. The So parameters are discussed in terms of substituent structure in connection with the Taft-Kutter-Hansch €. and Charton u parameters. The general applicability of a unique scale of steric parameters is discussed.
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