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Given preparation of alkyl halides from alcohols. |
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Answer» Solution :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. |
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