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, BF(3)A, 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.

Automated summary

This study aims to investigate the role of Lewis basicity at the molecular level and solvent effects. The results demonstrate the importance of distinguishing between the basicity of solute molecules and that of the bulk liquid solvent when considering Lewis basicity effects.