donor number

{{Acids and bases}}

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.{{cite journal | title = Critical evaluation of stability constants and thermodynamic functions of metal complexes of crown ethers |author1=Françoise Arnaud-neu |author2=Rita Delgado |author3=Sílvia Chaves | journal = Pure Appl. Chem. | volume = 75 | issue = 1 | pages = 71–102 | year = 2003 | doi = 10.1351/pac200375010071|s2cid=97549495 | doi-access = free }} 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.{{cite journal | author = V. Gutmann | journal = Coord. Chem. Rev. | volume = 18 | year = 1976 | pages = 225–255 | doi = 10.1016/S0010-8545(00)82045-7 | title = Solvent effects on the reactivities of organometallic compounds | issue = 2}} Likewise Lewis acids are characterized by acceptor numbers (AN, see Gutmann–Beckett method).

Typical solvent values are:{{cite book | author = D.T. Sawyer, J.L. Roberts | title = Experimental Electrochemistry for Chemists | publisher = John Wiley & Sons, Inc. | year = 1974}}

• 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.{{Cite journal|last1=KATAYAMA|first1=Misaki|last2=SHINODA|first2=Mitsushi|last3=OZUTSUMI|first3=Kazuhiko|last4=FUNAHASHI|first4=Shigenobu|last5=INADA|first5=Yasuhiro|date=2012|title=Reevaluation of Donor Number Using Titration Calorimetry|journal=Analytical Sciences|volume=28|issue=2|pages=103–106|doi=10.2116/analsci.28.103|pmid=22322801 |issn=0910-6340}} 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. {{doi|10.1021/ed054p612}} For Pearson qualitative HSAB theory, the two properties are hardness and strength,{{cite journal|doi=10.1021/ed045p581|title=Hard and soft acids and bases, HSAB, part 1: Fundamental principles|author= Pearson, Ralph G.|journal=J. Chem. Educ.| volume =1968|issue=45| pages= 581–586|year=1968|bibcode=1968JChEd..45..581P}} while for Drago's quantitative ECW model, the two properties are electrostatic and covalent.{{cite journal |author=Vogel G. C. |author2=Drago R. S |year=1996|journal=Journal of Chemical Education|volume=73|issue=8|pages=701–707|title=The ECW Model |bibcode=1996JChEd..73..701V|doi=10.1021/ed073p701}}