Dehydrohalogenation

Dehydrohalogenation from alkyl halides

Traditionally, alkyl halides 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 give phenol via a benzyne intermediate.

Base-promoted reactions to alkenes

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. Some prototypes are shown below:

${\displaystyle \begin{array}{cc}\underset{\text{Ethyl}\text{Chloride}}{{}^{\beta }\text{CH}_3^{}{-}^{\alpha }{\text{CH}}_2^{}\text{Cl}}+\text{KOH}& ⟶\underset{\text{Ethylene}}{{\text{CH}}_2^{}={\text{CH}}_2^{}}+\text{KCl}+{\text{H}}_2^{}\text{O}\\ \underset{1-\text{Chloropropane}}{{\text{CH}}_3^{}-{\text{CH}}_2^{}-{\text{CH}}_2^{}\text{Cl}}+\text{KOH}& ⟶\underset{\text{Propene}}{{\text{CH}}_3^{}-\text{CH}={\text{CH}}_2^{}}+\text{KCl}+{\text{H}}_2^{}\text{O}\\ \underset{2-\text{Chloropropane}}{{\text{CH}}_3^{}-\text{CHCl}-{\text{CH}}_3^{}}+\text{KOH}& ⟶\underset{\text{Propene}}{{\text{CH}}_3^{}-\text{CH}={\text{CH}}_2^{}}+\text{KCl}+{\text{H}}_2^{}\text{O}\end{array}}$

Here ethyl chloride reacts with potassium hydroxide, typically in a solvent such as ethanol, giving ethylene. Likewise, 1-chloropropane and 2-chloropropane give propene.

Zaitsev's rule helps to predict regioselectivity for this reaction type.

In general, the reaction of a haloalkane with potassium hydroxide can compete with an S_N2 nucleophilic substitution reaction by OH⁻ a strong, unhindered nucleophile. Alcohols are however generally minor products. Dehydrohalogenations often employ strong bases such as potassium tert-butoxide (K⁺ [CH₃]₃CO⁻).

Base-promoted reactions to alkynes

Upon treatment with strong base, vicinal dihalides convert to alkynes.

Thermal cracking

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:

CH₂Cl-CH₂Cl → CH₂=CHCl + HCl

The resulting HCl can be reused in oxychlorination reaction.

Thermally induced dehydrofluorinations are employed in the production of fluoroolefins and hydrofluoroolefins. One example is the preparation of 1,2,3,3,3-pentafluoropropene from 1,1,2,3,3,3-hexafluoropropane:

CF₂HCH(F)CF₃ → CHF=C(F)CF₃ + HF