Insights into the Trends in the Acidity Strength of Organic and Inorganic Compounds: A Valence-Bond Perspective

Few concepts are more familiar to chemists than the concept of acidity strength. In almost any undergraduate chemistry textbook, one can find lists of factors affecting the acidities of organic and inorganic molecules. The factors, invoked to explain trends in the acidity strength through series of compounds, rely on concepts such as hybridization, delocalization, inductive effects, and electronegativity. Some of these concepts could be considered somewhat fuzzy, whereas others have a rigorous physical definition, yet together they shape the traditional framework used by chemists for the qualitative assessment of acidity strengths. At the same time, a thermodynamic cycle reveals that the acidity of a H-A bond is dependent on only three unequivocally definable quantities: the bond dissociation energy, the electron affinity of A, and the solvent effects. Here we attempt to answer the following questions: "How are the qualitative factors, found in textbooks, related to these quantities?" and "How can we connect this plethora of factors to the nature of the acidic H-A bond being cleaved heterolytically in an acidic dissociation process?" To do so, we turn to valence bond theory and model a generic acidic dissociation process. Within this model, the quantities, determining the acidity strength of an H-A compound (as revealed through the thermodynamic cycling process), arise naturally and lucidly, thus enabling the evaluation of the effects of the different qualitative factors found in the literature on the bonding situation. Our analysis projects surprising and thought-provoking anomalies, which challenge common chemical knowledge.