Dehydrohalogenation
Dehydrohalogenation is a chemical reaction that involves removal of (elimination of) a hydrogen halide from a substrate. The reaction is usually associated with the synthesis of alkenes, but it has wider applications.
Traditionally, alkyl halides are substrates for this dehydrohalogenations. The alkyl halide must be able to form an alkene, thus methyl and benzy halides are not suitable substrates. Aryl halides are also unsuitable. Upon treatment with strong base, chlorobenzene dehydrohalogenates to give phenol via a benzyne intermediate.
When treated with a strong base many alkyl chlorides convert to corresponding alkene. It is also called a β-Elimination reaction and is a type of elimination reaction. Zaitsev's rule helps to predict regioselectivity for this reaction type. Some examples are shown below:
Here ethyl chloride reacts with potassium hydroxide dissolved in ethanol, giving ethene. Likewise, 1-chloropropane and 2-chloropropane give propene.
In general, the reaction of haloalkane with potassium hydroxide can compete with an SN2 nucleophilic substitution reaction by OH− a strong, unhindered nucleophile. Alcohols are however generally minor products. Often dehydrohalogenations employ strong bases such as potassium tert-butoxide ([CH3]3CO− K+).
On an industrial scale, base-promoted dehydrohalogenations as described above are disfavored. The disposal of the alkali halide salt is problematic. Instead thermally-induced dehydrohalogenations are preferred. One example is provided by the production of vinyl chloride by heating 1,2-dichloroethane:
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