Dehydrohalogenation

When treated with a strong base many alkyl chlorides convert to corresponding alkene.[1] It is also called a β-elimination reaction and is a type of elimination reaction. Some prototypes are shown below:

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

Here ethyl chloride reacts with potassium hydroxide, 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⁻).

Upon treatment with strong base, vicinal dihalides 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 of vinyl chloride by heating 1,2-dichloroethane:[3]

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

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:[4]

CH₃CH(OH)CH₂Cl + KOH → CH₃CH(O)CH₂ + H₂O + KCl

The carbylamine reaction for the synthesis of isocyanides from the action of chloroform on a primary amine involves three dehydrohalogenations. 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.[5]

Dehydrohalogenation is not limited to organic chemistry. Some metal-organic coordination compounds can eliminate hydrogen halides,[6] either spontaneously,[7] thermally, or by mechanochemical reaction with a solid base such as potassium hydroxide.[8]

For example, salts that contain acidic cations hydrogen bonded to halometallate anions will often undergo dehydrohalogenation reactions reversibly:[6]

[B–H]⁺···[X–ML_n]⁻ ⇌ [B–ML_n] + HX

where B is a basic ligand such as a pyridine, X is a halogen (typically chlorine or bromine), M is a metal such as cobalt, copper, zinc, palladium or platinum, and L_n are spectator ligands.