Dehydrohalogenation

Dehalogenation to give an alkene.

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.

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.

Base-promoted reactions

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:

${\begin{aligned}{\ce {{\underset {Ethyl Chloride}{^{\beta}CH3-^{\alpha}CH2Cl}}+ KOH}}\ &{\ce {-> {\underset {Ethylene}{CH2=CH2}}+ {KCl}+ H2O}}\\{\ce {{\underset {1-Chloropropane}{CH3-CH2-CH2Cl}}+ KOH}}\ &{\ce {-> {\underset {Propene}{CH3-CH=CH2}}+ {KCl}+ H2O}}\\{\ce {{\underset {2-Chloropropane}{CH3-CHCl-CH3}}+ KOH}}\ &{\ce {-> {\underset {Propene}{CH3-CH=CH2}}+ {KCl}+ H2O}}\end{aligned}}$

Here ethyl chloride reacts with potassium hydroxide dissolved in ethanol, giving ethylene. Likewise, 1-chloropropane and 2-chloropropane give propene.

In general, the reaction of 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. Often dehydrohalogenations employ strong bases such as potassium tert-butoxide ([CH₃]₃CO⁻ K⁺).

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

Other dehydrohalogenations

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

Isocyanides

The carbylamine reaction for the synthesis of isocyanides from the action of chloroform on a primary amines involves three dehydrohalogenation. The first dehydrohalogenation is the formation of dichlorocarbene:

KOH + CHCl₃ → KCl + H₂O + CCl₂

Two successive base-mediated dehydrochlorination steps result in formation of the isocyanide.

External links

• Dehydrohalogenation