Elimination reaction

An elimination reaction removes groups from a substrate while forming a new multiple bond. Removing a hydrogen and a leaving group from neighboring carbons can produce an alkene.

An E2 elimination combines bond cleavage and bond formation in one elementary step; an E1 elimination passes through an intermediate, usually a carbocation. These alternatives predict different rate laws and stereochemical requirements. In a concerted elimination, orbital alignment can make an antiperiplanar conformation decisive, linking product formation to molecular geometry.

Beta-elimination, also called 1,2-elimination, removes groups from adjacent atoms while forming a new multiple bond between those atoms. The IUPAC definition allows the two central atoms to be elements other than carbon, although carbon–carbon examples are common. It describes the overall connectivity change, rather than specifying one unique reaction mechanism.^[ref]

The net dehydration of ethanol illustrates the topology:One carbon loses a hydrogen and its neighbor loses the oxygen-containing group; the carbon pair becomes the double bond of ethene. Both sides contain two carbons, six hydrogens and one oxygen. The balanced equation describes the net conversion, without asserting that unactivated ethanol changes spontaneously or that all bond changes occur in one step.

The positional label beta must be separated from the mechanistic label in E2 elimination. Both an E1 elimination and an E2 elimination can produce a beta-elimination transformation. Their differing intermediates, rate laws and geometric requirements are further information beyond the atom-connectivity pattern.

Loss of a hydrogen halide to form a carbon–carbon multiple bond. Along a chlorinated polymer backbone, repeated elimination can create unsaturated or conjugated segments rather than exclusively changing ester side groups.

A concerted elimination in which a base removes a beta hydrogen as a leaving group departs and a pi bond forms. Orbital alignment and substrate structure affect the allowed pathway.

antiperiplanar conformation elimination in a cyclohexane chair with axial leaving group and axial beta hydrogen.

An elimination mechanism involving ionization to a carbocation before beta deprotonation forms the alkene. Its limiting unimolecular rate law assumes ionization controls the rate.

In an E1cb elimination, deprotonation first produces a conjugate-base intermediate, followed by leaving-group loss. Dehydration of an appropriate aldol product can form a conjugated enone.