Transcript Preparation of Alkyl Halides from Alcohols and Hydrogen Halides

Preparation of Alkyl Halides from
Alcohols and Hydrogen Halides
ROH + HX  RX + H2O
Reaction of Alcohols with Hydrogen Halides
ROH +
HX  RX + HOH
Hydrogen halide reactivity
HF
least reactive
HCl
HBr
HI
most reactive
Reaction of Alcohols with Hydrogen Halides
ROH +
HX  RX + HOH
Alcohol reactivity
CH3OH RCH2OH R2CHOH R3COH
Methanol PrimarySecondaryTertiary
least reactive
most reactive
Preparation of Alkyl Halides
(CH3)3COH + HCl
25°C
(CH3)3CCl + H2O
78-88%
OH + HBr
80-100°C
Br + H2O
73%
CH3(CH2)5CH2OH + HBr
120°C
CH3(CH2)5CH2Br + H2O
87-90%
Preparation of Alkyl Halides
A mixture of sodium bromide and sulfuric a
cid may be used in place of HBr.
CH3CH2CH2CH2OH
NaBr
H2SO4
heat
CH3CH2CH2CH2Br
70-83%
Mechanism of the Reaction of
Alcohols with Hydrogen
Halides
Carbocation
R
+
C
R
R
The key intermediate in reaction of secondary
and tertiary alcohols with hydrogen halides is
a carbocation.
A carbocation is a cation in which carbon has
6 valence electrons and a positive charge.
Carbocation
R
+
C
R
R
The key intermediate in reaction of secondary
and tertiary alcohols with hydrogen halides is
a carbocation.
The overall reaction mechanism involves three
elementary steps; the first two steps lead to the
carbocation intermediate, the third step is the
conversion of this carbocation to the alkyl halide.
Example
(CH3)3COH + HCl
25°C
tert-Butyl alcohol
(CH3)3CCl + H2O
tert-Butyl chloride
Carbocation intermediate is:
H3C
+
C
CH3
CH3
tert-Butyl cation
Mechanism
Step 1:
Proton transfer from HCl to tert-butyl alcohol
.. (CH3)3C ..
O : + H Cl:
..
H
fast, bimolecular
H
+
(CH3)3C O :
+
H
tert-Butyloxonium ion
.. –
: Cl:
..
Mechanism
Step 2:
Dissociation of tert-butyloxonium ion
H
+
(CH3)3C O :
H
slow, unimolecular
H
+
(CH3)3C
+
:O:
tert-Butyl cation
H
Mechanism
Step 3:
Capture of tert-butyl cation by chloride ion.
(CH3)3C
+
+
.. –
: Cl:
..
fast, bimolecular
(CH3)3C
..
Cl
.. :
tert-Butyl chloride