Nucleophilic displacement reactions in aromatic systems. Part XI. Kinetics of the reactions of 2-chloro-, of 2-chloro-3-cyano-, and of 2-chloro-3-cyano-6-methyl-5-nitropyridine with aniline and substituted anilines in various solvents

Abstract

Rate coefficients have been measured for the reactions of aniline with 2-chloro-3-cyano-6-methyl- and with 2-chloro-3-cyano-5-nitropyridine in various protic solvents (mainly alkanols) and aprotic solvents at 20°. For selected systems rate coefficients have also been meausred at two other temperatures and activation parameters calculated. While dielectric constant as a measure of non-specific solvent effects is probably of some significance, the Lewis acid–base behaviour of the solvent appears to play the dominant role in governing reactivity. This is particularly clear for the series of protic solvents. Values of ΔH^‡ and ΔS^‡ vary widely, and for the reactions of 2-chloro-3-cyano-5-nitropyridine various pairs of alcohols show isokinetic temperatures within the range of experiment. Thus the pattern of k values depends critically on temperature. A limited study of the reactions of aniline with 2-chloro-5-nitropyridine in lower alcohols is also presented. Values of the Hammett ρ constant have been measured for the reactions of substituted anilines with 2-chloro-3-cyano-6-methyl-5-nitropyridine in several solvents. The ρ values depend but little on solvent, as suggested in related work.