Preparation of Alkyl Halides from Alcohols and Hydrogen Halides

1
Preparation of Alkyl Halides fromAlcohols and
Hydrogen Halides

• ROH HX ? RX H2O

2
Reaction of Alcohols with Hydrogen Halides
ROH HX ? RX HOH

• Hydrogen halide reactivity HI HBr HCl
  HFmost reactive least reactive

3
Reaction of Alcohols with Hydrogen Halides
ROH HX ? RX HOH

• Alcohol reactivityR3COH R2CHOH RCH2OH
  CH3OHTertiary Secondary Primary Methanolmost
  reactive least reactive

4
Preparation of Alkyl Halides
25C
(CH3)3CCl H2O

• (CH3)3COH HCl

78-88
80-100C
H2O
73
120C
CH3(CH2)5CH2OH HBr
CH3(CH2)5CH2Br H2O
87-90
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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
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Mechanism of the Reaction of Alcohols with
Hydrogen Halides
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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.

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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.

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Example
25C
(CH3)3CCl H2O

• (CH3)3COH HCl

tert-Butyl alcohol
tert-Butyl chloride
Carbocation intermediate is
tert-Butyl cation
10
Mechanism
Step 1 Proton transfer from HCl to tert-butyl
alcohol

• (CH3)3C

..

O
H
fast, bimolecular
tert-Butyloxonium ion
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Mechanism
Step 2 Dissociation of tert-butyloxonium ion
slow, unimolecular


(CH3)3C
tert-Butyl cation
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Mechanism
Step 3 Capture of tert-butyl cation by
chloride ion.


(CH3)3C
fast, bimolecular
tert-Butyl chloride