Abstract:
(a) The reactions of aldehydes and ketones are particularly important, as they are widely used in synthesis and biosynthesis: in particular, new C-C bonds in biological systems are very often formed by a carbonyl addition reaction.
The main feature that determines the reactivity of the carbonyl group is its polarisation, represented thus: C = 0 . δ + δ- There are two points of attack here: a nucleophile will attack the electrophilic carbon atom, and an electrophile will attack the nucleophilic oxygen atom. In many cases (possibly including many enzyme catalysed reactions) both processes occur. The electrophilic species may be a proton, which reacts with the unpaired electrons on the oxygen atom of the carbonyl group, to enhance the electrophilic reactivity of the carbon atom. Acid catalysis is therefore common in carbonyl addition reactions.
Catalysis by bases is also common, but for a different reason: the function of the base is to generate the nucleophilic species which reacts with the carbon of the carbonyl group.
