6.4 Electrophilic Addition of Hydrogen Halides to Alkenes

1
6.4Electrophilic Addition of Hydrogen Halides
to Alkenes
2
General equation for electrophilic addition
??
?
EY
3
When EY is a hydrogen halide
??
?
HX
4
Example
CH3CH2
CH2CH3
HBr
CHCl3, -30C
H
H
CH3CH2CH2CHCH2CH3
Br
(76)
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Mechanism

• Electrophilic addition of hydrogen halides to
  alkenes proceeds by rate-determining formation
  of a carbocation intermediate.

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Mechanism

• Electrons flow fromthe ? system of thealkene
  (electron rich) toward the positivelypolarized
  proton of the hydrogen halide.

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Mechanism

C
C
H

X
H
8
Mechanism

C
C
H

X
H
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6.5Regioselectivity ofHydrogen Halide
AdditionMarkovnikov's Rule
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Markovnikov's Rule

• When an unsymmetrically substituted alkene
  reacts with a hydrogen halide, the hydrogen adds
  to the carbon that has the greater number of
  hydrogen substituents, and the halogen adds to
  the carbon that has the fewer hydrogen
  substituents.

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Markovnikov's Rule
HBr
acetic acid
(80)
Example 1
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Markovnikov's Rule
CH3
H
HBr
acetic acid
CH3
H
(90)
Example 2
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Markovnikov's Rule
HCl
0C
(100)
Example 3
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6.6Mechanistic BasisforMarkovnikov's Rule

• Protonation of double bond occurs in direction
  that gives more stable of two possible
  carbocations.

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Mechanistic Basis for Markovnikov's RuleExample
1
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Mechanistic Basis for Markovnikov's RuleExample
1
HBr
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Mechanistic Basis for Markovnikov's RuleExample
1

CH3CH2CH2CH2
primary carbocation is less stable not formed
HBr
18
Mechanistic Basis for Markovnikov's RuleExample
3
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Mechanistic Basis for Markovnikov's RuleExample
3
Cl
HCl
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Mechanistic Basis for Markovnikov's RuleExample
3
secondary carbocation is less stable not formed
H
H
CH3
Cl

HCl
21
6.7Carbocation Rearrangements in Hydrogen
Halide Addition to Alkenes
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Rearrangements sometimes occur
HCl, 0C
H

CH3CHC(CH3)2
(40)
(60)