If we are going to make use of the acid–base properties of compounds as we have just described, we are going to need a way of measuring how acidic or how basic they are. Raising the pH leads to deprotonation of aspirin and lowering the pH leads to protonation of codeine, but how far do we have to raise or lower the pH to do this? The measure of acidity or basicity we need is called pKa. The value of pKa tells us how acidic (or not) a given hydrogen atom in a compound is. Knowing about pKa tells us, for example, that the amine product from the reaction just above will be protonated at weakly acidic pH 5, or that only a weak base (sodium hydrogen carbonate) is needed to deprotonate a carboxylic acid such as aspirin. It is also useful because many reactions proceed through protonation or deprotonation of one of the reactants (you met some examples in Chapter 6), and it is obviously useful to know what strength acid or base is needed. It would be futile to use too weak a base to deprotonate a compound but, equally, using a very strong base where a weak one would do risks the result of cracking open a walnut with a sledge hammer.

The aim of this chapter is to help you to understand why a given compound has the pKa that it does. Once you understand the trends involved, you should have a good feel for the pKa values of commonly encountered compounds and also be able to predict roughly the values for unfamiliar compounds.

Benzoic acid preserves soft drinks Benzoic acid is used as a preservative in foods and soft drinks (E210). Like acetic acid, it is only the acid form that is effective as a bactericide. Consequently, benzoic acid can be used as a preservative only in foodstuffs with a relatively low pH, ideally less than its pKa of 4.2. This isn’t usually a problem: soft drinks, for example, typically have a pH of 2–3. Benzoic acid is often added as the sodium salt (E211), perhaps because this can be added to the recipe as a concentrated solution in water. At the low pH in the final drink, most of the salt will be protonated to give benzoic acid proper, which presumably remains in solution because it is so dilute.