Inchemistry,dehydrohalogenation is anelimination reaction which removes ahydrogen halide from asubstrate. The reaction is usually associated with the synthesis ofalkenes, but it has wider applications.
Traditionally, alkylhalides are substrates for dehydrohalogenations. The alkyl halide must be able to form an alkene, thus halides having no C–H bond on an adjacent carbon are not suitable substrates. Aryl halides are also unsuitable. Upon treatment with strong base,chlorobenzene dehydrohalogenates to givephenol via abenzyne intermediate.
When treated with a strong base many alkyl chlorides convert to corresponding alkene.[1] It is also called aβ-elimination reaction and is a type ofelimination reaction. Some prototypes are shown below:
Hereethyl chloride reacts with potassium hydroxide, typically in a solvent such asethanol, givingethylene. Likewise,1-chloropropane and2-chloropropane givepropene.
Zaitsev's rule helps to predictregioselectivity for this reaction type.
In general, the reaction of ahaloalkane with potassium hydroxide can compete with an SN2nucleophilic substitution reaction by OH− a strong, unhinderednucleophile. Alcohols are however generally minor products. Dehydrohalogenations often employ strong bases such aspotassiumtert-butoxide (K+ [CH3]3CO−).
Upon treatment with strong base,vicinaldihalides convert to alkynes.[2]
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 ofvinyl chloride by heating1,2-dichloroethane:[3]
The resulting HCl can be reused inoxychlorination reaction.
Thermally induced dehydrofluorinations are employed in the production of fluoroolefins andhydrofluoroolefins. One example is the preparation of1,2,3,3,3-pentafluoropropene from 1,1,2,3,3,3-hexafluoropropane:
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 ofpropylene oxide annually frompropylene chlorohydrin:[4]
Thecarbylamine reaction for the synthesis ofisocyanides from the action of chloroform on aprimary amine involves three dehydrohalogenations. The first dehydrohalogenation is the formation ofdichlorocarbene:
Two successive base-mediated dehydrochlorination steps result in formation of the isocyanide.[5]
Dehydrohalogenation is not limited to organic chemistry. Somemetal-organiccoordination compounds can eliminate hydrogen halides,[6] either spontaneously,[7]thermally, or bymechanochemical reaction with a solid base such aspotassium hydroxide.[8]
For example,salts that contain acidic cationshydrogen bonded tohalometallate anions will often undergo dehydrohalogenation reactions reversibly:[6]
whereB is a basicligand such as apyridine, X is a halogen (typically chlorine or bromine), M is a metal such as cobalt, copper, zinc, palladium or platinum, and Ln arespectator ligands.
