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 $(SOCI_2):$
$\rightarrow$ Thionyl chloride is first preferred because in this reaction alkyl halide is formed along with gases $SO_2$ and $HCl.$ The two gaseous products are escapable, hence, the reaction gives pure alkyl halides.
$R-OH + SOCI_2$ $\rightarrow$ $R-C_1 + SO_2 + HCI$
$2)$ Reaction with halogen acids $OR$ Lucas Test:
$\rightarrow$ The reactions of primary and secondary alcohols with $HCl$ require the presence of a catalyst, $ZnCl_2.$ With tertiary alcohols, the reaction is conducted by simply shaking the alcohol with concentrated $HCl$ at room temperature.
$\rightarrow$ Constant boiling with $HBr \ (48\%)$ 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 $3^\circ >2^\circ >1^\circ .$
$R - OH + HCl \xrightarrow{ ZnCl _2} R - Cl + H _2 O$
$(3)$ 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.
$\rightarrow$ $3R-OH + PX_3 \rightarrow 3R-X + H_3PO_3(X = Cl, Br)$
$\rightarrow$ $R-OH + PCI_5 \rightarrow R-CI + POCI_2 + HCl$
$(4)$ Reaction with $NaBr$ and $H_2SO_4:$
$R - OH + NaBr + H_2SO_4 \rightarrow R-Br + NaHSO_4 + H_2O$
$(5)$ Reaction with halogen:
$R - OH \xrightarrow[ X _2= Br _2, I _2]{\operatorname{red} P / X _2} R - X$
$\rightarrow$ 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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