E2Mechanismen

E2 mechanisms, or E2 eliminations, describe a concerted, bimolecular elimination reaction in organic chemistry in which a base abstracts a β-hydrogen while the leaving group departs from the adjacent carbon in a single transition state. The reaction rate is second order, depending on both substrate and base concentrations (rate = k[substrate][base]). The process yields an alkene and a leaving group-containing byproduct.

A hallmark of E2 is its concerted nature and stereochemical requirements. In most cases, the β-hydrogen and the leaving group must adopt an anti-periplanar arrangement in the transition state, which strongly influences which hydrogens can be abstracted. In cyclic systems, the anti arrangement is achieved by specific conformations, such as axial leaving groups in cyclohexanes, and this can govern product stereochemistry. Although anti-periplanar alignment is typical, syn-E2 pathways have been observed under certain rigid constraints.

Substrate and base effects govern E2 selectivity. E2 is favored by strong, nonnucleophilic bases and by substrates where SN2 is disfavored (e.g., tertiary halides). Primary halides often react by competing SN2, while tertiary halides predominantly give E2 products. Bulky bases (for example, tert-butoxide) tend to give Hofmann products (less-substituted alkenes) due to steric effects, whereas smaller bases may yield Zaitsev products.

Leaving group quality and solvent also matter. Good leaving groups (I–, Br–, tosylates) and polar aprotic solvents generally promote E2. Typical bases include alkoxides and hindered amines; common solvents include DMSO, DMF, or tert-butanol mixtures.

E2 competes with E1 and SN2 pathways, with the dominant route determined by substrate structure, base, and solvent. The reaction is widely used to form alkenes under controlled, concerted conditions.