Can the Lewis Basicity of an Isolated Solvent Molecule be Used for Characterizing Solvent Effects?

Background: The ubiquitous Lewis acid/base interactions are important in solution processes. Analytical chemistry may benefit from a better understanding of the role of Lewis basicity at the molecular level or acting through a bulk solvent effect. Objective: To delineate (i) the basicity at a molecular level, hereafter referred to as solute basicity, and (ii) the solvent basicity, which is a bulk-liquid property. Methods: 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 measurements 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 concerned. Results: We demonstrate the drawbacks of confusing the Lewis basicity of a solvent molecule, isolated as a 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.