In this chapter, we consider the fourth and final general type of reaction that carbonyl compounds undergo—the carbonyl condensation reaction. Carbonyl condensation reactions take place between two carbonyl‑containing reactants, one of which must possess an alpha‑hydrogen atom. The first step of the reaction involves the removal of an alpha‑hydrogen atom by a base. In the second step, the enolate anion that results from this removal attacks the carbonyl‑carbon of the second reacting molecule. In the final step of the reaction, a proton is transferred to the tetrahedral intermediate formed in the second step, although in some cases the product that results may subsequently be dehydrated.
The importance of carbonyl condensation reactions to synthetic organic chemistry arises from the large number of combinations of carbonyl compounds that can be used in such reactions. Aldehydes or ketones can be used in a simple aldol condensation to produce β‑hydroxy aldehydes, β‑hydroxy ketones, or their dehydration products. Mixtures of aldehydes, ketones, or both, can be used in a mixed aldol condensation. Internal aldol condensations can occur in compounds containing two suitable carbonyl groups. Aldol‑like condensations can be brought about between aldehydes and a variety of compounds containing acidic alpha‑hydrogen atoms, including diethyl malonate, acetic anhydride, nitriles and nitro compounds. Esters can be used in Claisen condensations and 1,6‑ and 1,7‑diesters can give rise to internal condensations, called Dieckmann cyclizations. Related reactions include the Michael reaction, in which an α,β‑unsaturated carbonyl compound is reacted with an enolate anion; and the Stork enamine reaction, where an enamine adds to an α,β‑unsaturated ketone.
The chapter concludes with a look at how condensation reactions can be used in the synthesis of complex ring‑containing organic compounds, and at the role played by carbonyl condensation reactions in biological systems.