Low-Basicity and Low-Polarity Aprotic Solvents
Low-Basicity and Low-Polarity Aprotic Solvents
Low-basicity and low-polarity aprotic solvents, including importantly acetonitrile and tetrahydrofuran, are poor at solvating both anions and cations. Dissociation constants are consequently very low, especially for neutral acids, typically 10–12 orders of magnitude lower than in basic aprotic solvents, such as dimethylsulphoxide, and up to 18 log units lower compared with water. Homohydrogen-bond formation involving carboxylic acids, phenols, and amines, and intramolecular hydrogen bonding in mono-anions of suitable dicarboxylic acids, is extensive. Ion-pair formation is modest except in the low-dielectric solvent tetrahydrofuran. In tetrahydrofuran, acid–base equilibria are dominated by ion association formation, even at concentrations as low as 10‐5 M. In consequence, ion-pair acidity scales, based on solvent-separated lithium ion-pairs or contact caesium ion-pairs have been developed. For carbon acids, the ion-pair acidities correlate well with absolute pKa-values in dimethylsulphoxide.
Keywords: acetonitrile, tetrahydrofuran, propylene carbonate, acetone, carboxylic acids, phenols, amines, carbon acids, ion-association, hydrogen-bonding, ion-pair acidity
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