Fasteners: Acid Catalysis

Abstract

Abstract I explained the general basis of catalysis in Reaction 11, where I showed that it accelerated a reaction by opening a new, faster route from reactants to products. One of the ways to achieve catalysis in organic chemistry is to carry out a reaction in an acidic or basic (alkaline) environment, and that is what I explore here. In Reaction 27 you will see the enormous importance of processes like this, not just for keeping organic chemists productive but also for keeping us all alive; I give a first glimpse of that later in this section too. Various kinds of acid and base catalysis, sometimes both simultaneously, are going on throughout the cells of our body and ensuring that all the processes of life are maintained; in fact they are the very processes of life. I deal with acid catalysis in this section and base catalysis in the next. The point to remember throughout this section is that an acid is a proton donor (Reaction 2) and a proton is an aggressive, nutty little centre of positive charge. If a proton gets itself attached to a molecule, it can draw electrons towards itself and so expose the nuclei that they formerly surrounded. That is, a proton can cause the appearance of positive charge elsewhere in the molecule where the nuclei shine through the depleted fog of electrons. Because positive charge is attracted to negative charge, one outcome is that a molecule may be converted into a powerful electron-sniffing electrophile (Reaction 16). Another way of looking at the outcome of adding a proton is to note that a C atom with a positive charge is a target for nucleophilic missile attack (Reaction 15). Therefore, if a proton draws the electron cloud away from a nearby atom, then its presence is like a fifth-column agent preparing a target for later attack. Let’s shrink and watch as some acid is added to a molecule that contains a –CO– group, such as acetic acid. The protons provided by the added acid are riding on water molecules, as H3O+ ions, and arrive in the vicinity of the acetic acid molecule.