Zippers: Base Catalysis

A base, you should recall from Reaction 2, is the second hand clapping to the acid’s first. That is, whereas an acid is a proton donor, a base is its beneficiary as a proton acceptor. The paradigm base is a hydroxide ion, OH–, which can accept a proton and thereby become H2O. However, in the context of catalysis, the topic of this section, its role is rather different: instead of using its electrons to accept the proton, it uses them to behave as a nucleophile (Reaction 15), a searcher out of positive charge. Instead of forming a hydrogen–oxygen bond with an incoming proton, it sets the electronic fox among the electronic geese of a molecule by forming a new carbon–oxygen bond and thereby loosening the bonds to neighbouring atoms so that they can undergo rearrangement. The OH– ion in effect unzips the molecule and renders it open to further attack. Base catalysis has a lot of important applications. An ancient one is the production of soap from animal fat. To set that scene, I shall consider a simple model system, the ‘hydrolysis’ (severing apart by water) of the two components of an ester, 1 (the same compound I used in Reaction 17, a combination of acetic acid and ethanol), and then turn to soap-making itself. You saw in Reaction 17 how esters can be broken down into their components, a carboxylic acid and an alcohol, by an acid; here we see the analogous reaction in the presence of a base. To be specific, the reagent is a solution of sodium hydroxide, which provides the OH– ions that catalyse the reaction. We watch what happens when a solution of sodium hydroxide is added to an ester and the mixture is boiled. The O oxygen atoms of the ester have already ripened the molecule for nucleophilic attack by drawing some of the electron cloud away from the C atom to which they are both attached, leaving it with a partial positive charge, 2. The negatively charged OH– ion sniffs out that positive charge and jostles in to do its business.