Transcript 06_08_09.html

6.8
Free-radical Addition of HBr to
Alkenes
The "peroxide effect"
Markovnikov's Rule
CH3CH2CH
CH2
HBr
acetic acid
CH3CH2CHCH3
Br
(80%)
Addition of HBr to 1-Butene
CH3CH2CH
CH2
HBr
CH3CH2CHCH3
Br
only product in
absence of peroxides
CH3CH2CH2CH2Br
only product when
peroxides added to
reaction mixture
Addition of HBr to 1-Butene
CH3CH2CH
CH2
HBr
addition opposite to
Markovnikov's rule
occurs with HBr (not
HCl or HI)
CH3CH2CH2CH2Br
only product when
peroxides added to
reaction mixture
Photochemical Addition of HBr
CH2 + HBr
hn
CH2Br
H
(60%)
Addition of HBr with a regiochemistry opposite
to Markovnikov's rule can also occur when
initiated with light with or without added peroxides.
Mechanism
Addition of HBr opposite to Markovnikov's rule
proceeds by a free-radical chain mechanism.
Initiation steps:
R
..
O
..
..
O
..
R
R
..
..
. + .O
O
..
..
R
Mechanism
Addition of HBr opposite to Markovnikov's rule
proceeds by a free-radical chain mechanism.
Initiation steps:
R
R
..
O
..
..
O
..
..
. + H
O
..
R
..
:
Br
..
R
R
..
..
. + .O
O
..
..
R
..
..
.
+
:
O
H
Br
..
..
Propagation steps:
CH3CH2CH
..
CH2 + . Br :
..
Mechanism
.
CH3CH2CH
CH2
..
Br:
..
Propagation steps:
..
CH2 + . Br :
..
CH3CH2CH
CH3CH2CH
.
H
CH2
..
Br
.. :
..
Br
.. :
Mechanism
.
CH3CH2CH
CH2
..
Br:
..
..
CH3CH2CH2CH2 Br:
..
+
..
. Br :
..
6.9
Addition of Sulfuric Acid to Alkenes
Addition of H2SO4
CH3CH
CH2
HOSO2OH
CH3CHCH3
OSO2OH
Isopropyl
hydrogen sulfate
follows Markovnikov's rule:
yields an alkyl hydrogen sulfate
Mechanism
CH3CH
CH2 + H
..
O
..
SO2OH
slow
+
CH3CH
CH3
..–
+ :O
..
fast
SO2OH
CH3CHCH3
: OSO
.. 2OH
Alkyl hydrogen sulfates undergo
hydrolysis in hot water
CH3CHCH3
O
+
H—OH
SO2OH
heat
CH3CHCH3
O
H
+
HO—SO2OH
Application: Conversion of alkenes to alcohols
1. H2SO4
OH
2. H2O, heat
(75%)
But...
not all alkenes yield alkyl hydrogen sulfates
on reaction with sulfuric acid
these do:
H2C=CH2, RCH=CH2, and RCH=CHR'
these don't:
R2C=CH2, R2C=CH2, and R2C=CR2