Properties of Acid-Base Reactions

As well as using the hard and soft acid and base description to discuss the interactions, we can look at the thermodynamic properties of the acid-base reaction. The aim is to include electronic, structural rearrangement, and steric effects in a small set of parameters.

The standard enthalpy of formation of the product of the reaction between the acid, A, and the base :B, can be predicted by the Drago-Wayland relationship

The parameters Ei and Ci roughly reflect the ability of the species to participate in electrostatic or covalent interactions respectively. However, in fact as they are empirical parameters, they include all effects apart from solvation, as the reaction has been studied in the gas phase to derive the parameters. Large enthalpies of complex formation result from combinations of acids and bases which both have either large E or large C values.This acid-base interaction has been studied for a wide range of examples, though only in the gas phase.

It is a very successful relationship, being able to predict the enthalpies of formation of many complexes, and is also transferable to reactions in non-polar, non-donor solvents, as well as the gas phase. That the reaction is studied in the gas phase, however, is its major limitation: this means that the relationship is largely limited to neutral species which can exist in the gas phase.

Acid-Base properties of Solvents

Most solvents are either Lewis acids or Lewis bases. Therefore, when a solute dissolves in a solvent, a displacement reaction often occurs.

When SbF5 (the solute) dissolves in BrF3 (the solvent), the strong Lewis acid SbF5 displaces the weaker Lewis acid BrF2+ from its complex with the strong Lewis base F.

In the Bronsted definition of acids, we see that the solvent plays a very important role. The H+ ion never exists on its own, and always forms a complex with the solvent water molecule such that the acid exists as H3O+. The reactions of a Bronsted acid are therefore the transfer of the acidic proton from a basic solvent molecule, the H2O, to another base.

The ability of a solvent to act as a base or an acid can be discussed quantitatively in terms of the solvent parameters, the Donor number and the Acceptor number.

The Donor Number: The reaction enthalpy for the formation of a complex of the base, B, with Antimony pentachloride, gives us the donor number, DN. The higher the donor number, then the stronger B is as a Lewis base.

DN = -ΔHr

The Acceptor Number: The strength of an acid can be expressed in terms of the acceptor number. The chemical shift of 31P in the basic molecule (C2H5)3PO when dissolved in the pure acid solvent is measured, and this is the acceptor number. The chemical shift is scaled such that the solution in hexane has a chemical shift of 0 and the solution in SbCl5 has a chemical shift of 100. This gives acceptor numbers similar in magnitude to the donor numbers. The higher the acceptor number, then the stronger the acid is as a Lewis base.