E1cb

E1cb, short for elimination unimolecular conjugate base, is a mechanism of elimination in organic chemistry in which a carbanion intermediate is formed before the leaving group exits. It is a stepwise process in which a base removes an acidic hydrogen to generate a resonance-stabilized carbanion (often an enolate or related stabilized anion), and then the leaving group leaves, producing a double bond.

In the typical E1cb sequence, the first step is deprotonation by a base to form the carbanion. The second step is the unimolecular departure of the leaving group from the adjacent carbon, yielding the alkene or conjugated system. The rate-determining step is usually the deprotonation, making the overall rate depend on both the substrate and the base: rate ≈ k[base][substrate]. The presence of electron-withdrawing groups or adjacent carbonyls greatly stabilizes the carbanion, facilitating this pathway.

E1cb is often favored when the leaving group is poor (for example, hydroxide) and when the substrate contains acidic protons adjacent to carbonyls, esters, nitriles, or other strongly electron-withdrawing substituents. Substrates such as malonates, β-dicarbonyl compounds, and enolizable ketones are typical: deprotonation forms an enolate-like anion that then eliminates the leaving group.

Compared with E2 (concerted proton abstraction and leaving-group departure) and E1 (formation of a carbocation), E1cb proceeds via carbanion intermediates and a two-step sequence. Its occurrence is thus closely tied to the stabilization of the carbanion and the leaving group's ability, and it is most common when a stabilized carbanion forms before elimination.