PKa values in the undergraduate curriculum: introducing pKa values measured in DMSO to illustrate solvent effects

Abstract

The concepts of acidity and pKa values often occupy a privileged place in the undergraduate organic chemistry curriculum. However, by neglecting the relationship between solvent and acidity, many students come to understand acidity as a fixed property of a molecule, invariant regardless of reaction conditions. Moreover, the sole use of pKa values measured in water leads to serious analytical faults in several important areas of the introductory organic curriculum. In this paper, we consider the (mis)uses of aqueous pKa values in the organic curriculum and their pedagogical effects, argue for the adoption of pKa values measured in an organic solvent (DMSO) in organic textbooks and reference materials, and explore the thermodynamic origin of the effect of solvent on acidity. Suggestions for class discussions and activities are provided.