Write down the preparation of alkyl halides from alcohols. - Vidyadip

Question

Write down the preparation of alkyl halides from alcohols.

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Answer

$\rightarrow$ The hydroxyl group of an alcohol is replaced by halogen on reaction with concentrated halogen acids, phosphorus halides or thionyl chloride etc.
$(1)$ Reaction with thionyl chloride $\ce{(SOCI_2)}$ :
$\rightarrow$ Thionyl chloride is first preferred because in this reaction alkyl halide is formed along with gases $SO_2$ and $\text{HCl}$. The two gaseous products are escapable, hence, the reaction gives pure alkyl halides.
$\ce{R-OH + SOCI_2 \rightarrow R-C_1 + SO_2 + HCI}$
$(2)$ Reaction with halogen acids $OR$ Lucas Test:
$\rightarrow$ The reactions of primary and secondary alcohols with $\text{HCl}$ require the presence of a catalyst $, \ce{ZnCl_2}$.
With tertiary alcohols, the reaction is conducted by simply shaking the alcohol with concentrated $\text{HCl}$ at room temperature.
$\rightarrow$ Constant boiling with $\text{HBr}\ (48\%)$ is used for preparing alkyl bromide.
$\rightarrow$ Good yields of $R-I$ may be obtained by heating alcohols with sodium or potassium iodide in $95\%$ orthophosphoric acid.
$\rightarrow$ The order of reactivity of alcohols with a given haloacid is $3^\circ >2^\circ >1^\circ .$
$\ce{R - OH + HCl \xrightarrow{ \ce{ZnCl _2}} R - Cl + H _2 O}$
$(3)$ Reaction with Phosphorus halides:
$\rightarrow$ Phosphorus tribromide and triiodide are usually generated in situ $($produced in the reaction mixture$)$ by the reaction of red phosphorus with bromine and iodine respectively.
$\ce{\rightarrow 3R-OH + PX_3 \rightarrow 3R-X + H_3PO_3(X = Cl, Br)}$
$\ce{\rightarrow R-OH + PCI_5 \rightarrow R-CI + POCI_2 + HCl}$
$(4)$ Reaction with $\ce{NaBr}$ and $\ce{H_2SO_4}:$
$\ce{R - OH + NaBr + H_2SO_4 \rightarrow R-Br + NaHSO_4 + H_2O}$
$(5)$ Reaction with halogen:
$\ce{R - OH \xrightarrow[ X _2= Br _2, I _2]{\operatorname{red} P / X _2} R - X}$
$\rightarrow$ The above methods are not applicable for the preparation of aryl halides because the carbon $-$ oxygen bond in phenols has a partial double bond character and is difficult to break being stronger than a single bond.stronger than a single bond.

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