dehydrohalogenointi

Dehydrohalogenation, also known as dehydrohalogenointi in some languages, is an elimination reaction in organic chemistry in which a hydrogen halide (HX) is removed from a molecule containing a carbon–halogen bond to give an alkene. The general outcome is the formation of a carbon–carbon double bond and the release of HX.

Two main mechanisms are involved. The bimolecular E2 mechanism dominates for many alkyl halides treated with strong bases, especially primary and some secondary substrates. E2 is a concerted process that requires a base to abstract a proton anti to the leaving group, leading to simultaneous C–H and C–X bond breaking. E1 elimination can occur with tertiary halides or very weak bases, proceeding via a carbocation intermediate followed by loss of HX. For vicinal or geminal dihalides, a second dehydrohalogenation can occur, ultimately producing alkynes under strongly basic conditions.

Key factors that influence the reaction include the leaving group quality (I > Br > Cl; F− is usually too unreactive for typical conditions), the strength and steric bulk of the base (bulky bases favor Hofmann over Zaitsev products), and the solvent and temperature (higher temperatures and polar aprotic solvents often promote elimination). Zaitsev’s rule tends to give the more substituted, thermodynamically stable alkene, while bulky bases can give Hofmann products.

Typical examples: ethyl chloride with NaOH yields ethene, while 1,2-dibromopropane with NaNH2 can yield propyne after two eliminations. Dehydrohalogenation is a fundamental method for alkene synthesis and for preparing alkynes from dihalides.