Describe the preparation of alkyl halides from alcohols.

(N/A) The hydroxyl group of an alcohol is replaced by a halogen atom upon reaction with concentrated halogen acids,phosphorus halides,or thionyl chloride.
$(i)$ By reaction of alcohols with halogen acids $(HX)$: When an alcohol is reacted with concentrated halogen acids,the $-OH$ group is replaced by $-X$ to form haloalkanes.
Primary $(1^{\circ})$ and secondary $(2^{\circ})$ alcohols require a catalyst,such as anhydrous $ZnCl_{2}$,when reacted with $HCl$.
Example: $CH_{3}CH_{2}OH + HCl \xrightarrow{ZnCl_{2}} CH_{3}CH_{2}Cl + H_{2}O$
The reaction of tertiary $(3^{\circ})$ alcohols with concentrated $HCl$ is carried out simply by shaking at room temperature.
Example: $(CH_{3})_{3}C-OH + HCl \rightarrow (CH_{3})_{3}C-Cl + H_{2}O$
Bromoalkanes are prepared by constant boiling of the suitable alcohol with concentrated $HBr$ $(48\%)$ or by the reaction of alcohol with $NaBr$ in the presence of $H_{2}SO_{4}$.
$CH_{3}CH_{2}OH + HBr \rightarrow CH_{3}CH_{2}Br + H_{2}O$
$CH_{3}CH_{2}OH + NaBr + H_{2}SO_{4} \rightarrow CH_{3}CH_{2}Br + NaHSO_{4} + H_{2}O$