Reactions of Alkenes: Addition Reactions

1
Chapter 6

• Reactions of AlkenesAddition Reactions

2
Reactions of Alkenes

• The characteristic reaction of alkenes is
  addition to the double bond.

AB
3
6.1Hydrogenation of Alkenes
4
Hydrogenation of ethylene
?
?
?
?
HH

• exothermic ?H 136 kJ/mol
• catalyzed by finely divided Pt, Pd, Rh, Ni

5
Example
H2, Pt
(73)
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Problem 6.1

• What three alkenes yield 2-methylbutane on
  catalytic hydrogenation?

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Problem 6.1

• What three alkenes yield 2-methylbutane on
  catalytic hydrogenation?

H2, Pt
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Mechanism of catalytic hydrogenation.Figure 6.1
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Mechanism of catalytic hydrogenation.Figure 6.1
B
Y
C
C
A
X
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Mechanism of catalytic hydrogenation.Figure 6.1
11
Mechanism of catalytic hydrogenation.Figure 6.1
B
Y
A
X
C
C
12
Mechanism of catalytic hydrogenation.Figure 6.1
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Mechanism of catalytic hydrogenation.Figure 6.1
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6.2Heats of Hydrogenation

• can be used to measure relative stability of
  isomeric alkenes
• correlation with structure is same as when heats
  of combustion are measured

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126
119
115
CH3CH2CH2CH3
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Heats of Hydrogenation (kJ/mol)

• Ethylene 136
• Monosubstituted 125-126
• cis-Disubstituted 117-119
• trans-Disubstituted 114-115
• Terminally disubstituted 116-117
• Trisubstituted 112
• Tetrasubstituted 110

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Problem 6.2
Match each alkene of Problem 6.1 with its
correctheat of hydrogenation.
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Problem 6.2
Match each alkene of Problem 6.1 with its
correctheat of hydrogenation.
highest heat ofhydrogenationleast stable isomer
lowest heat ofhydrogenationmost stable isomer
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6.3Stereochemistry of Alkene Hydrogenation
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Two spatial (stereochemical) aspects ofalkene
hydrogenation

• syn addition of both H atoms to double bond
• hydrogenation is stereoselective, corresponding
  to addition to less crowded face of double bond

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syn-Addition versus anti-Addition
syn addition
anti addition
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Example of syn-Addition
H
CO2CH3
CO2CH3
H
(100)
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Stereoselectivity

• A reaction in which a single starting
  materialcan give two or more stereoisomeric
  productsbut yields one of them in greater
  amounts thanthe other (or even to the exclusion
  of the other)is said to be stereoselective.

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Example ofstereoselective reaction
H2, cat
Both productscorrespond tosyn additionof H2.
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Example ofstereoselective reaction
H2, cat
But only thisone is formed.
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Example ofstereoselective reaction
H2, cat
Top face of doublebond blocked bythis methyl
group
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Example ofstereoselective reaction
H2, cat
H2 adds to bottom face of double bond.
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6.4Electrophilic Addition of Hydrogen Halides
to Alkenes
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General equation for electrophilic addition
??
?
EY
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When EY is a hydrogen halide
??
?
HX
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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.
• Electrons flow from the ? system of thealkene
  (electron rich) toward the positivelypolarized
  proton of the hydrogen halide.

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Mechanism

C
C
H

X
H
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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
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Mechanistic Basis for Markovnikov's RuleExample
3
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Mechanistic Basis for Markovnikov's RuleExample
3
H
H
Cl
CH3

HCl
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Mechanistic Basis for Markovnikov's RuleExample
3
secondary carbocation is less stable not formed
H
H
H
CH3

Cl
CH3

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

CH3CHC(CH3)2
(40)
(60)
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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)
Example 1
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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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h?
(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

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Mechanism

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

..

Br

H
O
R
..
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Propagation steps

Gives most stable free radical
..
.
..


Br
H
..