dehydrohalogenation

Dehydrohalogenation is a chemical reaction in which a hydrogen halide (HX) is eliminated from an organic halide, typically an alkyl halide, to form an alkene. The process is generally promoted by a base and can proceed by two main mechanisms: E2 (bimolecular elimination) and E1 (unimolecular elimination).

In E2, a base abstracts a beta-hydrogen at the same time as the leaving group departs, in a concerted step. The reaction often requires an anti-periplanar arrangement of the C–H and C–X bonds on adjacent carbons. E2 commonly occurs with secondary and tertiary halides using strong bases (for example alkoxides or tert-butoxide). It tends to favor the more substituted alkene (the Zaitsev rule), although bulky bases can give the less substituted Hofmann product.

In E1, the leaving group departs first, generating a carbocation, followed by deprotonation to form the alkene. This pathway is favored by tertiary halides, weak bases, and polar protic solvents. E1 often proceeds with ionization and may involve carbocation rearrangements. It is generally slower and can compete with substitution (SN1) pathways.

Key factors influencing outcome include substrate structure (primary, secondary, tertiary), leaving-group ability (I−/Br−/Cl− versus F−), base strength and sterics, solvent, and temperature. Higher temperature typically favors elimination over substitution.

Examples include the dehydrohalogenation of 2-bromobutane with a strong base to give butenes, and similar eliminations from other alkyl halides. Dehydrohalogenation is a fundamental method for preparing alkenes in organic synthesis.