Background:
The ubiquitous Lewis acid/base interactions are important in solution processes. Analytical chemistry may
benefit of a better understanding of the role of Lewis basicity, at the molecular level or acting through a bulk solvent effect.
Objective:
To clearly delineate (i) the basicity at a molecular level, hereafter referred as solute basicity, and (ii) the solvent basicity,
which is a bulk-liquid property.
Method:
The literature that relates Lewis basicity scales and solvent effects is analyzed. A special focus is placed on two extensive
scales, the Donor Number, DN, and the BF3 affinity scale, BF3A, which were obtained by calorimetric measurement on molecules as
solutes diluted in a quasi-inert solvent, and therefore define a molecular Lewis basicity. We discuss the validity of these solute scales
when regarded as solvent scales, in particular when the basicity of strongly associated liquids is discussed.
Results:
We demonstrate the drawbacks of confusing the Lewis basicity of a solvent molecule, isolated as solute, and that of the bulk
liquid solvent itself.
Conclusion:
Consequently, we recommend a reasoned use of the concept of Lewis basicity taking clearly into account the specificity
of the process for which a Lewis basicity effect may be invoked. In particular, the action of the Lewis base, either as an isolated entity, or as a bulk liquid, must be distinguished.
Cover Feature: Role of Lewis-Base-Coordinated Halogen(I) Intermediates in Organic Synthesis: The Journey from Unstable Intermediates to Versatile Reagents (Eur. J. Org. Chem. 37/2017)