Determination of Basic Strength of Aliphatic Amines through Ion Pair Formation in Some Ionic Liquid Solutions

To have an evaluation of the basic strength of aliphatic amines in ionic liquid solution, the stability constants relevant to the formation of amine/p-nitrophenol ion pairs were determined in different ionic liquids at 298 K. In particular, aliphatic (pyrrolidinium) and aromatic (imidazolium) ionic liquids were used. Imidazolium ions, bmim⁺ and bm₂im⁺, having different hydrogen bond donor abilities were taken into account. Anions were chosen ([BF₄⁻], [PF₆⁻], and [NTf₂⁻]; where NTf₂ = bis(trifluoromethansulfonyl)imide) showing different shape, size, and coordination ability. Several primary, secondary (cyclic or not), and tertiary amines were used to study the effect of amine structure on the ion pair stability. The comparison between data collected in this work and those previously obtained in ordinary molecular solvents evidence that ionic liquids are solvent media able to exalt the amine basicity. Furthermore, the experimental trends obtained in different ionic liquids can only be rationalized considering the whole of parameters affecting their tridimensional structure.