Haloalkanes and Haloarenes — Chemistry STD 12 Science — Question
Bihar BoardEnglish MediumSTD 12 ScienceChemistryHaloalkanes and Haloarenes4 Marks
Question
Write down the preparation of alkyl halides from alcohols.
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Answer
→ 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 (SOCI2): → Thionyl chloride is first preferred because in this reaction alkyl halide is formed along with gases SO2 and HCl. The two gaseous products are escapable, hence, the reaction gives pure alkyl halides. R-OH + SOCI2 → R-C1 + SO2 + HCI 2) Reaction with halogen acids OR Lucas Test: → The reactions of primary and secondary alcohols with HCl require the presence of a catalyst, ZnCl2. With tertiary alcohols, the reaction is conducted by simply shaking the alcohol with concentrated HCl at room temperature. → Constant boiling with HBr (48%) is used for preparing alkyl bromide. → Good yields of R-I may be obtained by heating alcohols with sodium or potassium iodide in 95% orthophosphoric acid. → The order of reactivity of alcohols with a given haloacid is 3°>2°>1°. $R - OH + HCl \xrightarrow{ ZnCl _2} R - Cl + H _2 O$ (3) Reaction with Phosphorus halides: → 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. → 3R-OH + PX3 → 3R-X + H3PO3 (X = Cl, Br) → R-OH + PCI5 → R-CI + POCI2 + HCl (4) Reaction with NaBr and H2SO4: R - OH + NaBr + H2SO4 → R-Br + NaHSO4 + H2O (5) Reaction with halogen: $R - OH \xrightarrow[ X _2= Br _2, I _2]{\operatorname{red} P / X _2} R - X$ → 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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