Dehydrohalogenation is an 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.
Dehydrohalogenation from alkyl halides
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:
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 are disfavored since the disposal and low value of the halide salt are problematic. Instead thermally-induced dehydrohalogenations (invariably dehydrochlorinations) are preferred, which yields a more useful HCl coproduct. One example is provided by the production of vinyl chloride by heating 1,2-dichloroethane:
- CH2Cl-CH2Cl → CH2=CHCl + HCl
Formation of epoxides
Chlorohydrins, compounds with the connectivity R(HO)CH-CH(Cl)R', undergo dehydrochlorination to give epoxides. This reaction is employed industrially to produce millions of tons of propylene oxide annually from propylene chlorohydrin:
- CH3CH(OH)CH2Cl + KOH → CH3CH(O)CH2 + H2O + KCl
- March, Jerry (1985), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (3rd ed.), New York: Wiley, ISBN 0-471-85472-7
- M. Rossberg et al. "Chlorinated Hydrocarbons" in Ullmann’s Encyclopedia of Industrial Chemistry, 2006, Wiley-VCH, Weinheim. doi:10.1002/14356007.a06_233.pub2
- Nijhuis, T. Alexander; Makkee, Michiel; Moulijn, Jacob A.; Weckhuysen, Bert M. "The Production of Propene Oxide: Catalytic Processes and Recent Developments" Industrial & Engineering Chemistry Research 2006, volume 45, 3447-3459. doi:10.1021/ie0513090