dehydrohalogenated

Dehydrohalogenation is a chemical reaction in which a hydrogen halide (HX) is removed from an organic substrate, most commonly a halogenated alkane, to form a carbon–carbon multiple bond. When used as an adjective, dehydrohalogenated describes a substrate or product that has undergone this transformation, typically bearing an alkene or alkyne moiety instead of the original halide.

The reaction proceeds mainly by elimination mechanisms. In many cases, especially for primary and secondary halides, an E2 mechanism operates, in which a base abstracts a β-hydrogen while the leaving halide departs in a concerted step, giving an alkene. If the substrate is tertiary or the base is weak, an E1 pathway may occur via a carbocation and subsequent loss of a halide. Stereochemical requirements include anti-periplanar arrangement for simple E2 eliminations; regioselectivity is often governed by Zaitsev’s rule, though bulky bases can give Hofmann products.

Conditions typically involve a strong base and heat. Common bases include hydroxide, alkoxides, or amide bases such as NaNH2 in liquid ammonia for forcing double eliminations to alkynes. Solvents vary from alcoholic media to polar aprotic media, depending on substrate and desired product. Dihalides or dihalogenated substrates can undergo two successive eliminations to yield alkynes.

Examples include the elimination of HBr from 2-brominated alkanes to give alkenes, with Saytzeff major products, and the double elimination of 1,2-dibromoethane with NaNH2 to form ethyne. Dehydrohalogenated intermediates are common in organic synthesis and polymer chemistry.