ECW model
This quantitative model is often discussed with the qualitative HSAB theory, which also seeks to rationalize the behavior of diverse acids and bases.
[1] No single rank order of acid or base strength can predict the energetics of the cross reaction.
In 1965 Russell S. Drago and Bradford Wayland published the two term equation such that each acid and each base is described by two parameters.
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.
ΔH is a measure of strength of the bond between the acid and the base, both in the gas phase and in weakly solvating media.
In other cases, W is the enthalpy needed to cleave the internal hydrogen bonding of the H-bonding acid (CF3)3COH.
The plot shown here allows comparison of three chosen Lewis bases: acetonitrile, ammonia, and dimethyl sulfide.
References 5, 8, 12, and 14 contain graphical presentations that define the ranking order of strength of many Lewis acids and bases.
[11] For example, the shift of the phenol OH stretching frequency, Δχ, that occurs upon adduct formation has been analyzed using the following equation: where asterisks on the EA and CA for phenol indicate that the acceptor is held constant and the frequency shift is measured as the base is varied.
An analysis like this provides a basis for using EB and CB parameters as a reference scale of donor strengths for frequency shifts.
Any physicochemical property, Δχ, that is dominated by σ donor-acceptor interaction can be correlated with the enthalpy-derived E and C parameters.
The ECW equations enables one to correlate and predict the enthalpies of adduct formation of neutral donor-acceptor interactions for which the electron-transfer is limited.
[12][13] Others have concluded that the ECW model "is generally found helpful in many fields of solution chemistry and biochemistry".
The relative acceptor strength of I2 toward a series of bases, versus other Lewis acids, can be illustrated by C-B plots.
