Illustrated Glossary of Organic Chemistry

Darzens condensation (Darzens reaction; glycidic ester condensation): A condensation reaction in which an α-haloester enolate reacts with an aldehyde or ketone to form an epoxide adjacent to an ester.

In this Darzens condensation, methyl 2-chloropropanate (a methyl ester) is deprotonated by sodium methoxide (a strong base) to form an enolate. This enolate (a nucleophile) adds to the carbonyl group of acetone (a ketone; shown in red) to form an oxyanionic tetrahedral intermediate. A subsequent intramolecular S_N2 displacement of chloride ion (a leaving group) forms the epoxy ester product.