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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.

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.

Methyl addition gives 1-methylcyclohexanol, whose acid-catalysed dehydration yields the shown 1-methylcyclohexene. The methyl end of methyllithium attacks the cyclohexanone carbonyl carbon, and the C=O pi electrons move onto oxygen, giving a lithium alkoxide. Protonation gives the tertiary alcohol.

Protonate OH to make $\mathrm{O}{\mathrm{H}}_2^{+}$, then cleave its C–O bond with both electrons going to departing water. A base removes a beta hydrogen from either adjacent ring methylene, and that C–H pair forms the C=C bond to the tertiary cationic centre. This E1 elimination regenerates the acid catalyst. The two adjacent ring positions are equivalent, and the displayed endocyclic alkene is more substituted than the possible exocyclic methylene alternative.