Question

Write a method for the preparation of alkyl halides.

Answer 1

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.