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]
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]
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