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]

References

^ Françoise Arnaud-neu; Rita Delgado; Sílvia Chaves (2003). "Critical evaluation of stability constants and thermodynamic functions of metal complexes of crown ethers". Pure Appl. Chem. 75 (1): 71–102. doi:10.1351/pac200375010071. S2CID 97549495.
^ V. Gutmann (1976). "Solvent effects on the reactivities of organometallic compounds". Coord. Chem. Rev. 18 (2): 225–255. doi:10.1016/S0010-8545(00)82045-7.
^ D.T. Sawyer, J.L. Roberts (1974). Experimental Electrochemistry for Chemists. John Wiley & Sons, Inc.
^ KATAYAMA, Misaki; SHINODA, Mitsushi; OZUTSUMI, Kazuhiko; FUNAHASHI, Shigenobu; INADA, Yasuhiro (2012). "Reevaluation of Donor Number Using Titration Calorimetry". Analytical Sciences. 28 (2): 103–106. doi:10.2116/analsci.28.103. ISSN 0910-6340. PMID 22322801.
^ Laurence, C. and Gal, J-F. Lewis Basicity and Affinity Scales, Data and Measurement, (Wiley 2010) p 51 ISBN 978-0-470-74957-9
^ 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. doi:10.1021/ed054p612
^ Pearson, Ralph G. (1968). "Hard and soft acids and bases, HSAB, part 1: Fundamental principles". J. Chem. Educ. 1968 (45): 581–586. Bibcode:1968JChEd..45..581P. doi:10.1021/ed045p581.
^ Vogel G. C.; Drago R. S (1996). "The ECW Model". Journal of Chemical Education. 73 (8): 701–707. Bibcode:1996JChEd..73..701V. doi:10.1021/ed073p701.

References

Further reading