Dehydrohalogenation

Dehydrohalogenation is an organic chemistry reaction from which an alkene is obtained from an alkyl halide. It is also called the β-Elimination reaction. Ethanolic potassium hydroxide when reacted with alkyl halide gives alkene.

In this reaction, the halogen on the carbon in the alkyl halide reacts with hydrogen present on the β-carbon releasing hydrogen chloride. Hence an alkene is formed due to formation of a double bond between the α- and β-carbon.

Reactions


Here ethyl chloride reacts with potassium hydroxide dissolved in ethanol, giving ethene. Similarly 1-chloropropane and 2-chloropropane give propene.

Chlorobenzene does not react with potassium hydroxide due to the presence of the benzene ring which provides too strong of a substrate for conventional elimination.

Promoting elimination
In general, the above reaction of haloalkane with potassium hydroxide would compete with an Sn2 nucleophilic substitution reaction (minor product) due to the fact that OH- is a strong, sterically unhindered nucleophile. Perhaps a better reagent would be a deprotonated alcohol such as potassium tert-Butoxide ([CH3]3CO+ K-) or due to the fact that it is an oxide anion and thus a strong base and nucleophile, potassium ethoxide (CH3CH2O+ K-) and because of steric hinderance therefore promoting an elimination mechanism instead of a substitution mechanism. Also, providing heat for the reaction to progress would further promote elimination.