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Preparation of Alkyl Halides from Alcohols and Hydrogen Halides

Preparation of Alkyl Halides from Alcohols and Hydrogen Halides. ROH + HX  RX + H 2 O. Reaction of Alcohols with Hydrogen Halides. Hydrogen halide reactivity HI HBr HCl HF most reactive least reactive. R OH + H X  RX + H OH.

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Preparation of Alkyl Halides from Alcohols and Hydrogen Halides

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  1. Preparation of Alkyl Halides fromAlcohols and Hydrogen Halides ROH + HX  RX + H2O

  2. Reaction of Alcohols with Hydrogen Halides Hydrogen halide reactivity HI HBr HCl HFmost reactiveleast reactive ROH + HX  RX + HOH

  3. Reaction of Alcohols with Hydrogen Halides Alcohol reactivityR3COH R2CHOH RCH2OH CH3OHTertiary Secondary Primary Methanolmost reactiveleast reactive ROH + HX  RX + HOH

  4. OH + HBr Br Preparation of Alkyl Halides 25°C (CH3)3CCl + H2O (CH3)3COH + HCl 78-88% 80-100°C + H2O 73% 120°C CH3(CH2)5CH2OH + HBr CH3(CH2)5CH2Br + H2O 87-90%

  5. Preparation of Alkyl Halides A mixture of sodium bromide and sulfuric a cid may be used in place of HBr. NaBrH2SO4 CH3CH2CH2CH2Br CH3CH2CH2CH2OH heat 70-83%

  6. Mechanism of the Reaction of Alcohols with Hydrogen Halides

  7. + R R C R Carbocation The key intermediate in reaction of secondary and tertiary alcohols with hydrogen halides is a carbocation. A carbocation is a cation in which carbon has6 valence electrons and a positive charge.

  8. + R R C R Carbocation The key intermediate in reaction of secondary and tertiary alcohols with hydrogen halides is a carbocation. The overall reaction mechanism involves threeelementary steps; the first two steps lead to thecarbocation intermediate, the third step is the conversion of this carbocation to the alkyl halide.

  9. + H3C CH3 C CH3 Example 25°C (CH3)3COH + HCl (CH3)3CCl + H2O tert-Butyl alcohol tert-Butyl chloride Carbocation intermediate is: tert-Butyl cation

  10. .. + : H Cl .. H .. – + + : : : O Cl (CH3)3C .. H Mechanism Step 1: Proton transfer from HCl to tert-butyl alcohol (CH3)3C .. : O H fast, bimolecular tert-Butyloxonium ion

  11. H + : O (CH3)3C H H : : O H Mechanism Step 2: Dissociation of tert-butyloxonium ion slow, unimolecular + + (CH3)3C tert-Butyl cation

  12. .. – : : Cl .. .. : Cl (CH3)3C .. Mechanism Step 3: Capture of tert-butyl cation by chloride ion. + + (CH3)3C fast, bimolecular tert-Butyl chloride

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