Given preparation of alkyl halides from alcohols.

Explanation

The hydroxyl group of an alcohol is replaced by halogen on reaction with concentrated halogen acids, phosphorus halides or thionyl chloride.
(i) By reaction of alcohols with halogen acids (HX) : When alcohol is reacted with halogen acids (concentrated), the - OH group is replaced by - X and the haloalkanes are obtained.
The `1^(@)` and `2^(@)` alcohols requires a catalyst `ZnCl_(2)` when reacted with HCl.
(i) `R-OH+HCl overset(ZnCl_(2))rarr R-Cl+H_(2)O`
(ii) `underset("Ethanol")(CH_(3))-CH_(2)-OH+HCl overset(ZnCl_(2))rarr underset("Chloroethane")(CH_(3)CH_(2))-Cl+H_(2)O`
(iii) `underset("Propan-2-ol")(CH_(3)-underset("OH")underset("| ")("CH")-CH_(3))+HCl overset(ZnCl_(2))rarr underset("2-chloropropane")(CH_(3)-underset("Cl")underset("|")("CH")-CH_(3))+H_(2)O`
The reaction of tertiary alcohols with conducted HCl is carried out simply by shaking at a room temperature.
`underset("2-methylpropane-2-ol")(CH_(3)-overset(CH_(3))overset("| ")underset(CH_(3))underset("| ")("C ")-OH+HCl) to underset("2-chloro-2-methylpropane")(CH_(3)-overset(CH_(3))overset("| ")underset(CH_(3))underset("| ")("C ")-Cl+H_(2)O)`
Bromoalkanes are prepared by constant boiling of suitable alcohol with concentrated HBr (48%) or by reaction of alcohol with NaBr in presence of `H_(2)SO_(4)`.
`underset("Ethanol")(CH_(3))-CH_(2)-OH+underset(("Conc."))(HBr)to underset("Bromoethane")(CH_(3)CH_(2)Br)+H_(2)O`
`underset("Ethanol")(CH_(3)CH_(2)OH)+NaBr+H_(2)SO_(4)to underset("Bromoethane")(CH_(3)CH_(2)Br)+NaHSO_(4)+H_(2)O`
Good yields of iodoalkanes R - I may be obtained by heating alcohols with sodium or potassium iodide in 95% orthophosphoric acid.
`KI+H_(3)PO_(4)to KH_(2)PO_(4)+HI`
`underset("Ethanol")(CH_(3)CH_(2))-OH+HI to underset("Iodoethane")(CH_(3)CH_(2))-I+H_(2)O`
The order of reactivity of alcohols with a given haloacid is `3^(@)gt 2^(@)gt 1^(@)` and that of haloacid is `HI gt HBr gt HCl`
The preparation of alkyl chloride is carried out either by passing dry hydrogen chloride gas through a solution of alcohol or by heating a mixture of alcohol and concentrated aqueous halogen acid.
(ii) By reaction of alchols with phosphorus halides : Chloroalkanes are prepared by the action of phosphorus trichloride `(PCl_(3))` or phosphorus pentachloride `(PCl_(5))` on alcohols.
`underset("Ethanol")(3CH_(3)CH_(2)OH)+PCl_(3) to underset("Chloroethane")(3CH_(3)CH_(2)Cl)+H_(3)PO_(3)`
`underset("Ethanol")(CH_(3)CH_(2)OH)+PCl_(5)to underset("Chloroethane")(CH_(3)CH_(2)Cl)+POCl_(3)+HCl`
Bromoalkanes and iodoalkanes are prepared by the action of mixture of red phosphorus and `Br_(2)` and `I_(2)` on alcohols. The `PBr_(2)` and `PI_(3)` are produced in situ (Produced in the reaction mixture).
`underset("(Red)")(P_(4))+6Br_(2)to 4PBr_(3)`
`underset("Ethanol")(3CH_(3)CH_(2)OH)+PBr_(3)to underset("Bromoethane")(3CH_(3)CH_(2)Br)+H_(3)PO_(3)`
(iii) By reactions of alcohol with thionyl chloride `(SOCl_(2))` : Thionly chloride is preferred by cause in this reaction alkyl halide is formed along with gases `SO_(2)` and HCl. The two gaseous product are escapable, hence, the reaction gives pure alkyl halides.
`underset("Ethanol")(CH_(3)CH_(2)-OH)+SOCl_(2)to underset("Chloroethane")(CH_(3)-CH_(2)-Cl)+HCl_((g))+SO_(2(g))`
These methods are not suitable 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.