Question

mechanism of dehydrohalogenation of alkyle hallides

Answer 1

Answer:

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. Zaitsev's rule helps to predict regioselectivity for this reaction type. Some examples 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}}}{\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 dissolved in ethanol, giving ethylene. Likewise, 1-chloropropane and 2-chloropropane give propene.

In general, the reaction of a haloalkane with potassium hydroxide can compete with an SN2 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−).

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:[2]

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:[3]

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 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.[4]

Explanation: