Editing Lewis acids and bases (section)

== Quantifying Lewis acidity==
Many methods have been devised to evaluate and predict Lewis acidity. Many are based on spectroscopic signatures such as shifts NMR signals or IR bands e.g. the [[Gutmann-Beckett method]] and the Childs<ref>{{cite journal | last1 = Childs | first1 = R.F | last2 = Mulholland | first2 = D.L | last3 = Nixon | first3 = A. | year = 1982 | title = Lewis acid adducts of α,β-unsaturated carbonyl and nitrile compounds. A nuclear magnetic resonance study | journal = Can. J. Chem. | volume = 60 | issue = 6| pages = 801–808 | doi=10.1139/v82-117| doi-access = free }}</ref> method.

The [[ECW model]] is a quantitative model that describes and predicts the strength of Lewis acid base interactions, −ΔH. The model assigned E and C parameters to many Lewis acids and bases. Each acid is characterized by an E<sub>A</sub> and a C<sub>A</sub>. Each base is likewise characterized by its own E<sub>B</sub> and C<sub>B</sub>. The E and C parameters refer, respectively, to the electrostatic and covalent contributions to the strength of the bonds that the acid and base will form. The equation is

: −ΔH = E<sub>A</sub>E<sub>B</sub> + C<sub>A</sub>C<sub>B</sub> + W

The W term represents a constant energy contribution for acid–base reaction such as the cleavage of a dimeric acid or base. The equation predicts reversal of acids and base strengths. The graphical presentations of the equation show that there is no single order of Lewis base strengths or Lewis acid strengths.<ref>{{cite journal |doi=10.1021/ed073p701 |title=The ECW Model |year=1996 |last1=Vogel |first1=Glenn C. |last2=Drago |first2=Russell S. |journal=Journal of Chemical Education |volume=73 |issue=8 |page=701 |bibcode=1996JChEd..73..701V }}</ref><ref>{{cite journal |doi=10.1021/ed054p612 |title=Great e and C plot. Graphical display of the enthalpies of adduct formation for Lewis acids and bases |year=1977 |last1=Cramer |first1=Roger E. |last2=Bopp |first2=Thomas T. |journal=Journal of Chemical Education |volume=54 |issue=10 |page=612 |bibcode=1977JChEd..54..612C }}</ref> and that single property scales are limited to a smaller range of acids or bases.