Discuss the methods of preparing haloalkanes from alcohols.

(N/A) Alcohols $\rightarrow$ Chlorides / Halides: When alcohols react with concentrated halogen acids,the hydroxyl group is replaced by a halogen atom. The reaction of primary and secondary alcohols with $HCl$ requires the presence of $ZnCl_{2}$ as a catalyst. Tertiary alcohols react with concentrated $HCl$ simply by shaking at room temperature.
$(i)$ $R_{3}C-OH + HCl \rightarrow R_{3}C-Cl + H_{2}O$
$(ii)$ $R-OH + HCl \text{ (conc.)} \xrightarrow{\Delta} RCl + H_{2}O$
$(iii)$ $R-OH + HCl \xrightarrow{ZnCl_{2}} RCl + H_{2}O$
$(iv)$ $R-OH + HCl_{(g)} \rightarrow R-Cl + H_{2}O$
The order of reactivity of halogen acids $(HX)$ with alcohols is $3^{0} > 2^{0} > 1^{0}$. Alkyl chlorides can be prepared by passing dry $HCl$ gas through an alcohol solution or by heating a mixture of concentrated aqueous halogen acid and alcohol.
$(b)$ Reaction with phosphorus halides $(PX_{3} / PX_{5})$: The $-OH$ group is replaced by a halogen atom upon reaction with phosphorus halides.
$(i)$ $3R-OH + PX_{3} \rightarrow 3R-X + H_{3}PO_{3} \text{ (where } X = Cl, Br)$
$(ii)$ $R-OH + PCl_{5} \rightarrow R-Cl + POCl_{3} + HCl$
Generally,red phosphorus reacts with bromine and iodine to form $PBr_{3}$ and $PI_{3}$ respectively.
$(c)$ Reaction with thionyl chloride $(SOCl_{2})$: Alcohols react with thionyl chloride to form alkyl chlorides.
$R-OH + SOCl_{2} \rightarrow R-Cl + SO_{2(g)} + HCl_{(g)}$
This method is preferred for preparing alkyl chlorides because the by-products $SO_{2}$ and $HCl$ are gases,leaving behind pure alkyl chloride.
$(d)$ Preparation of bromides: Alcohols are heated with $48\% HBr$ to prepare alkyl bromides.
$ROH + HBr \text{ (48\%)} \xrightarrow{\text{Boiling}} R-Br + H_{2}O$