Reactions of enolates with carbonyl compounds: the aldol and Claisen reactions

Abstract

This chapter highlights reactions of enolates with carbonyl compounds: the aldol and Claisen reactions. The simplest enolizable aldehyde is acetaldehyde. What happens if we add a small amount of base, say NaOH, to this aldehyde? Some of it will form the enolate ion. Each molecule of enolate is surrounded by molecules of the aldehyde that are not enolized and so still have the electrophilic carbonyl group intact. The enolate ion will attack one of these aldehydes to form an alkoxide ion, which will be protonated by the water molecule formed in the first step. The product is an aldehyde with a hydroxy (ol) group and it has the trivial name aldol. This reaction is so important because of the carbon–carbon bond formed when the nucleophilic enolate attacks the electrophilic aldehyde.