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 potentiometric-ally with perchloric acid in cyclohexane and hexane solvents. The basicity orders of the amines in the series, including ammonia, were found to be NH 3 MeNH 2 Me 2 NH > Me 3 N; NH 3 EtNH 2 Et 2 NH > Et 3 N; NH 3 n-PrNH 2 n-Pr 2 NH > n-Pr 3 N; NH 3 n-BuNH 2 n-Bu 2 NH > n-Bu 3 N; and n-BuNH 2 i-BuNH 2 sec-BuNH 2 f-BuNH 2. In addition to these, some interesting orders have also been found among primary, secondary and tertiary amines, e.g., MeNH 2 EtNH 2 n-PrNH 2 n-BuNH 2; Me2NH > Et 2 NH > n-Pr 2 NH > n-Bu 2 NH; Me 3 N > Et 3 N > n-Pr 3 N > n-Bu 3 N; and MeNH 2 EtNH 2 /-PrNH 2 f-BuNH 2. 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. © Royal Society of Chemistry.