Donor number explained

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]

References

1. Critical evaluation of stability constants and thermodynamic functions of metal complexes of crown ethers . Françoise Arnaud-neu . Rita Delgado . Sílvia Chaves . . 75 . 1 . 71–102 . 2003 . 10.1351/pac200375010071. 97549495 . free .
2. V. Gutmann . . 18 . 1976 . 225–255 . 10.1016/S0010-8545(00)82045-7 . Solvent effects on the reactivities of organometallic compounds . 2.
3. Book: D.T. Sawyer, J.L. Roberts . Experimental Electrochemistry for Chemists . John Wiley & Sons, Inc. . 1974.
4. KATAYAMA. Misaki. SHINODA. Mitsushi. OZUTSUMI. Kazuhiko. FUNAHASHI. Shigenobu. INADA. Yasuhiro. 2012. Reevaluation of Donor Number Using Titration Calorimetry. Analytical Sciences. 28. 2. 103–106. 10.2116/analsci.28.103. 22322801 . 0910-6340.
5. Laurence, C. and Gal, J-F. Lewis Basicity and Affinity Scales, Data and Measurement, (Wiley 2010) p 51 ISBN 978-0-470-74957-9
6. 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.
7. 10.1021/ed045p581. Hard and soft acids and bases, HSAB, part 1: Fundamental principles. Pearson, Ralph G.. J. Chem. Educ.. 1968. 45. 581–586. 1968. 1968JChEd..45..581P.
8. Vogel G. C. . Drago R. S . 1996. Journal of Chemical Education. 73. 8. 701–707. The ECW Model . 1996JChEd..73..701V. 10.1021/ed073p701.