Titrations in non-aqueous media. Part IV. Solvent effects on basicity of aliphatic amines

Abstract

The relative basicity orders of methyl-, ethyl-, propyl- and butylamines have been measured potentiometrically with perchloric acid in cyclohexane and hexane solvents. The basicity orders of the amines in the series, including ammonia, were found to be NH₃ > MeNH₂ > Me₂NH > Me₃N; NH₃ > EtNH₂ > Et₂NH > Et₃N; NH₃ > n-PrNH₂ > n-Pr₂NH > n-Pr₃N; NH₃ > n-BuNH₂ > n-Bu₂NH > n-Bu₃N; and n-BuNH₂ > i-BuNH₂ > sec-BuNH₂ > t-BuNH₂. In addition to these, some interesting orders have also been found among primary, secondary and tertiary amines, e.g., MeNH₂ > EtNH₂ > n-PrNH₂ > n-BuNH₂; Me₂NH > Et₂NH > n-Pr₂NH > n-Bu₂NH; Me₃N > Et₃N > n-Pr₃N > n-Bu₃N; and MeNH₂ > EtNH₂ > i-PrNH₂ > t-BuNH₂. From these findings four important results have been obtained as follows: (1) an increase in the number of alkyl group decreases the basicity; (2) an increase in the size of an alkyl group decreases the basicity; (3) amines containing unbranched alkyl groups are more basic than their branched analogues; and (4) an increase in the number of methyl substituents on the methyl group of methylamine decreases the basicity in a regular manner. These results contradict results reported in the literature and also earlier results obtained in this laboratory.