Amine basicity and ionic liquids

11 September 2015 - PhysOrgChem

Mao basicity ionic liquids 2015.PNGMao et al. in a recent JOC (DOI) took it on themselves to measure the basicity of 21 amines in an ionic liquid. The basicity range of amines in water or acetonitrile is well established. But what about IL's? The Mao crew think highly of themselves: 'The standard deviation was much superior to that sparsely reported elsewhere'. Competition, be ashamed! So now for the results. The ionic liquid was BmimNTf2 (a cousin of BMIM-PF6) and in this solvent the basicity was consistently lower compared to acetonitrile by a pKa difference of 3.6. The report summarises the results as H2O about equal to DMSO < ionic liquid < acetonitrile. The relationship with the relative permittivity (for unknown reasons the authors opt to call it 'polarity index') only goes that far. For water this value is 78, for acetonitrile 37 but for the IL it is only 12. The beta Kamlet-Taft parameter then? Does not help either, the ionic liquid is again at the bottom of the rankings. So with compliments for the accurate measurements, what is the main driver for amine basicity? Researchers research!