Kinetic Studies of Bimolecular Nucleophilic Substitution. VIII. The Effect of Hydroxylic Solvents on the Nucleophilicity of Aliphatic Amines in the Menschutkin Reaction

Abstract

The rate of the reactions of methyl iodide with various aliphatic amines, e.g., methyl-, ethyl-, n-butyl-, ethanol- and benzyl-amines, have been measured in methanol solvent and compared with the previous results obtained in benzene solvent. Ethylamine, diethylamine, and triethylamine react slower in methanol than in benzene. In a similar fashion, butylamine, dibutylamine, tributylamine, methylamine and dimethylamine are less reactive in methanol than in benzene. These unusual results, e.g., the lower SN2 reactivity of amines in the more ionizing protic solvent, are attributed to the retardation by the higher activation energy in methanol solvent at the expense of the acceleration by higher values of the entropy of activation. The increase of the activation energy is ascribed to specific solvation, e.g., hydrogen bonding of methanol toward amines. Benzylamines and ethanolamines, which are comparatively less solvated by methanol exhibit the usual reactivity sequence toward methyl iodide, e.g., the higher SN2 reactivity in the more polar solvent.