Determination of a new scale of ortho-steric parameters S 0 from N-methylation of pyridines
- 1 January 1980
- journal article
- research article
- Published by Royal Society of Chemistry (RSC) in Journal of the Chemical Society, Perkin Transactions 2
- No. 9,p. 1350-1355
- https://doi.org/10.1039/p29800001350
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.Keywords
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