Dehydrohalogenation is an organic reaction in which alkyl halide when boiled with alcoholic alkali gives corresponding alkene. It is also called a β-Elimination reaction and is a type of elimination reaction.
In this reaction, the halogen on the carbon in the alkyl halide reacts with hydrogen present on the β-carbon releasing hydrogen halide. Hence an alkene is formed due to formation of a double bond between the α- and β-carbon. Zaitsev's rule helps to explain regioselectivity for this reaction type. The opposite transformation is called hydrohalogenation.
Chlorobenzene does not react with potassium hydroxide due to the presence of the benzene ring, which, due to stabilization as a result of aromaticity, does not give conventional elimination, as it would lead to a very unstable benzyne intermediate.
In general, the above reaction of haloalkane with potassium hydroxide would compete with an Sn2 nucleophilic substitution reaction (minor product) because 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, because it is an oxide anion and thus a strong base and nucleophile, potassium ethoxide (CH3CH2O− K+), and because of steric hindrance, thereby promoting an elimination mechanism instead of a substitution mechanism.