Acid-base equilibria classification

All nucleophiles are bases (1). In fact, within the definition of Lewis (2), nucleophilicity is basicity. Following Ingold (3), however, physical-organic chemists have normally used the Brpnsted-Lowry definition (4, 5) of bases as affinity for protons. Likewise, nucleophilicity referred to affinity for nuclei of other elements, most often carbon (3). Another classification reserves basicity, and its counterpart acidity, for equilibrium measurements, while nucleophilicity and its counterpart electrophilicity refer to rate measurements (6). The terms carbon basicity and hydrogen nucleophilicity have been employed (7-9). This classification does not seem to have gained much acceptance. 

Solvent effects on acid-base equilibria are naturally most marked when the solvent itself enters into the equilibrium, as is the case for the conventional definition of acid strength by means of the equilibrium A-fSH B4-SH2 (where SH is the solvent). The existence of such an equilibrium implies that the solvent has some basic properties. Similarly, the occurrence of the reaction B + SH A-f S (where S is the anion derived by abstracting a proton from the solvent) implies that the solvent is acidic. The most important factor determining qualitative behaviour in a wide range of solvents is the acidic or basic nature of the solvent, as determined by its chemical nature. In a preliminary classification we can neglect other factors, notably the effect of dielectric constant on the association of ions or the forces between them. 

To classify redox couples with respect to their strength, a trick is used that is similar to that used with acid-base equilibria. Consider what would happen if the redox couple studied was brought to react with a standard partner . Long ago, the couple hydrogen gas/hydrogen ion (H2/H30 ) was chosen to act as the standard partner. To solve the problem of comparing the strength of redox couples, we simply initiate the reaction and, after equilibrium has been established, determine the actual equilibrium constant (as with acid-base equilibria). This time, however, the classification is done in a somewhat different way. 

A key perspective on leveling is that the classification of a substance as weak or strong is stringently tied to the solvent. A weak aqueous base may be strong in a more acidic solvent. Weak aqueous acids appear strong in basic solvents. For example, the equilibrium position of... 

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