Donor number

In chemistry a donor number (DN) is a quantitative measure of Lewis basicity. A donor number is defined as the negative enthalpy value for the 1:1 adduct formation between a Lewis base and the standard Lewis acid SbCl₅ (antimony pentachloride), in dilute solution in the noncoordinating solvent 1,2-dichloroethane with a zero DN. The units are kilocalories per mole for historical reasons.^[1] The donor number is a measure of the ability of a solvent to solvate cations and Lewis acids. The method was developed by V. Gutmann in 1976.^[2] Likewise Lewis acids are characterized by acceptor numbers (AN, see Gutmann–Beckett method).

Typical solvent values are:^[3]

• acetonitrile 14.1 kcal/mol (59.0 kJ/mol)
• acetone 17 kcal/mol (71 kJ/mol)
• methanol 19 kcal/mol (79 kJ/mol)
• tetrahydrofuran 20 kcal/mol (84 kJ/mol)
• dimethylformamide (DMF) 26.6 kcal/mol (111 kJ/mol)
• dimethyl sulfoxide (DMSO) 29.8 kcal/mol (125 kJ/mol)
• ethanol 31.5 kcal/mol (132 kJ/mol)
• pyridine 33.1 kcal/mol (138 kJ/mol)
• triethylamine 61 kcal/mol (255 kJ/mol)

The donor number of a solvent can be measured via calorimetry, although it is frequently measured with nuclear magnetic resonance (NMR) spectroscopy using assumptions on complexation.^[4] A critical review of the donor number concept has pointed out the serious limitations of this affinity scale.^[5] Furthermore, it has been shown that to define the order of Lewis base strength (or Lewis acid strength) at least two properties must be considered.^[6] For Pearson qualitative HSAB theory, the two properties are hardness and strength,^[7] while for Drago's quantitative ECW model, the two properties are electrostatic and covalent.^[8]