The%20"peroxide%20effect"

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6.8Free-radical Addition of HBr to Alkenes

• The "peroxide effect"

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Markovnikov's Rule
HBr
acetic acid
(80)
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Addition of HBr to 1-Butene
HBr
CH3CH2CH2CH2Br
only product when peroxides added to reaction
mixture
only product inabsence of peroxides
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Addition of HBr to 1-Butene
HBr
addition opposite to Markovnikov's rule occurs
with HBr (not HCl or HI)
CH3CH2CH2CH2Br
only product when peroxides added to reaction
mixture
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Photochemical Addition of HBr
hn
HBr
(60)

• Addition of HBr with a regiochemistry oppositeto
  Markovnikov's rule can also occur wheninitiated
  with light with or without added peroxides.

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Mechanism

• Addition of HBr opposite to Markovnikov's rule
  proceeds by a free-radical chain mechanism.
• Initiation steps

..

R
O
R
O
..
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Mechanism

• Addition of HBr opposite to Markovnikov's rule
  proceeds by a free-radical chain mechanism.
• Initiation steps

..

R
O
R
O
..
..
..


Br
H

O
H
R
..
..
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Mechanism
Propagation steps

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Mechanism
Propagation steps

..

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6.9Addition of Sulfuric Acid to Alkenes
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Addition of H2SO4
HOSO2OH
Isopropylhydrogen sulfate

• follows Markovnikov's rule
• yields an alkyl hydrogen sulfate

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Mechanism
..

SO2OH
O
H
..
slow

..


SO2OH
O
..
fast

..
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Alkyl hydrogen sulfates undergo hydrolysis in hot
water
HOH


HOSO2OH
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Application Conversion of alkenes to alcohols
1. H2SO4
2. H2O, heat
(75)
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But...

• not all alkenes yield alkyl hydrogen sulfateson
  reaction with sulfuric acid
• these do
• H2CCH2, RCHCH2, and RCHCHR'
• these don't
• R2CCH2, R2CCH2, and R2CCR2