Donor number

 ( Chemical Bonding ) 

In chemistry a donor number ( DN) is a quantitative measure of Lewis base. 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 per mole for historical reasons. The donor number is a measure of the ability of a solvent to Solvation and Lewis acids. The method was developed by V. Gutmann in 1976. Likewise Lewis acids are characterized by acceptor numbers (AN, see Gutmann–Beckett method).

Typical solvent values are:

• acetonitrile 14.1 kcal/mol (59.0 joule/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. A critical review of the donor number concept has pointed out the serious limitations of this affinity scale.Laurence, C. and Gal, J-F. Lewis Basicity and Affinity Scales, Data and Measurement, (Wiley 2010) p 51 ISBN 978-0-470-74957-9 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.Cramer, R. E., and Bopp, T. T. (1977) Great E and C plot. Graphical display of the enthalpies of adduct formation for Lewis acids and bases. Journal of Chemical Education 54 612–613. For Pearson qualitative HSAB theory, the two properties are hardness and strength, while for Drago's quantitative ECW model, the two properties are electrostatic and covalent.