This final section of the chapter deals with a few named reactions in which enolate anions react with other carbonyl compounds as nucleophiles – so these could realistically have been placed under the nucleophilic addition chapter.
These are just meant to be representative of the huge number of possibilities present for carbonyls reacting with other carbonyls – by noting the patterns and trends here, the reader will hopefully be adequately equipped to deal with different reaction mechanisms which arise.
Claisen Ester condensation
Perkin
NB. This reaction only works with aryl aldehydes – which have no α protons and therefore cannot become enolates themselves and confuse the reaction.
Knoevenagel
Essential points to take away from these reactions:
i. The steps are almost always equilibria, so the reactions are reversible from start to finish and the direction they go in has to be influenced by the temperature, pressure, concentrations etc.
ii. See how the anion ‘snaps’ back and forth around the molecule reversibly – often there can be more than one place the anion can go, but they will all be reached reversibly, so only the one which produces further reaction steps will appear in the actual mechanism.
iii. See also how moieties such as MeO– can leave the molecule, yet be involved in later stages of the reaction, say as a base for example.