Question

Write a method for the preparation of alkyl halides.

Answer 1

Hint: Alkyl halides are the organic compounds in which one or more H-atom is replaced by halogen atoms like F, Cl, Br, etc. They are also known as haloalkanes. Their general formula is $ RX $ , where $ R $ is the alkyl group and $ X $ is the halogen. Alkyl halides can be obtained from alkenes via Markovnikov’s rule and from alcohols on reaction with thionyl chloride.

Complete step by step answer:
PREPARATION OF ALKYL HALIDES FROM ALCOHOLS:
Alcohols can be transformed to alkyl halides on reaction with thionyl chloride $ SOCl{_2} $ phosphorus trichloride $ PCl{_3} $ phosphorus pentachloride $ PCl{_5} $ , or phosphorus tribromide $ PBr{_3} $ . Thionyl Chloride is the most used agent for the production of alkyl halides because in this reaction all the by-products are gas, hence, they flow into the atmosphere leaving the pure alkyl halide. These reactions are used mostly for primary and secondary alcohols.
 $ C{H_3}C{H_2}OH\, + SOCl{_2}\underrightarrow \Delta \,\,C{H_3}C{H_2}Cl\, + S{O_2}\, + HCl $
 $ C{H_3}C{H_2}OH\, + PCl{_2}\underrightarrow \Delta \,\,C{H_3}C{H_2}Cl\, + P{(OH)_3}\, + HCl $
 $ C{H_3}C{H_2}OH\, + PBr{_3}\,\underrightarrow \Delta \,\,C{H_3}C{H_2}Br\, + P{(OH)_3}\, + HBr $
Alcohol can be transformed to alkyl halides on reaction with Hydrogen Halides $ HI,HBr,HCl $ . The main intermediate in the reaction of $ {2^0} $ alcohols and $ {3^0} $ alcohols with hydrogen halides is the carbocation. The mechanism involves three steps; In step one Protonation of the alcoholic oxygen is done. It is a very fast and reversible process. The second step leads to the formation of carbocation The third step is the conversion of this carbocation to the alkyl halide.
 $ {\left( {C{H_3}{{ }}} \right)_3}COH{{ }} + {{ }}HCl \to {{ }}{\left( {C{H_3}{{ }}} \right)_3}CCl{{ }} + {{ }}{H_2}O $ .

Note:
When alcohols are reacted with Hydrogen halides they typically undergo a nucleophilic substitution reaction. The reactivity order is $ {3^o} > {2^o} > {1^o} > $ . The reactivity order of Hydrogen halide is $ HI > HBr > HCl > HF $ . The Reaction generally proceeds via $ {S_N}1 $ mechanism which proceeds via the formation of a carbocation intermediate, which also undergoes rearrangement. Methanol and primary alcohols proceed via $ {S_N}2 $ mechanism since these have highly preferable carbocations.