10.9 Preparation of Dienes

1
10.9Preparation of Dienes
2
1,3-Butadiene
590-675C
CH3CH2CH2CH3
chromia- alumina

2H2

• More than 4 billion pounds of 1,3-butadiene
  prepared by this method in U.S. each year
• used to prepare synthetic rubber (See "Diene
  Polymers" box)

3
Dehydration of Alcohols
KHSO4
heat
4
Dehydration of Alcohols
KHSO4
heat

• major product 88 yield

5
Dehydrohalogenation of Alkyl Halides
KOH
heat
6
Dehydrohalogenation of Alkyl Halides
KOH
heat

• major product 78 yield

7
Reactions of Dienes

• isolated dienes double bonds react
  independently of one another
• cumulated dienes specialized topic
• conjugated dienes reactivity pattern requires
  us to think of conjugated diene system as a
  functional group of its own

8
10.10Addition of Hydrogen HalidestoConjugated
Dienes
9
Electrophilic Addition to Conjugated Dienes

H
X
H

• Proton adds to end of diene system
• Carbocation formed is allylic

10
Example
HCl
?
?
11
Example
HCl
12
via
H
X
13
and
Cl
3-Chlorocyclopentene
H
H
Cl
H
H
H
H
H
14
1,2-Addition versus 1,4-Addition
1,2-addition of XY
15
1,2-Addition versus 1,4-Addition
1,2-addition of XY
1,4-addition of XY
16
1,2-Addition versus 1,4-Addition
1,2-addition of XY
1,4-addition of XY
via
17
HBr Addition to 1,3-Butadiene
HBr

• electrophilic addition
• 1,2 and 1,4-addition both observed
• product ratio depends on temperature

18
Rationale

• 3-Bromo-1-butene is formed faster
  than1-bromo-2-butene because allylic
  carbocations react with nucleophiles
  preferentially at the carbon that bears the
  greater share of positive charge.

via


19
Rationale

• 3-Bromo-1-butene is formed faster
  than1-bromo-2-butene because allylic
  carbocations react with nucleophiles
  preferentially at the carbon that bears the
  greater share of positive charge.

formed faster
20
Rationale
1-Bromo-2-butene is more stable
than3-bromo-1-butene because it has amore
highly substituted double bond.

• more stable

21
Rationale
The two products equilibrate at 25C.Once
equilibrium is established, the morestable
isomer predominates.
major product at -80C
major product at 25C
(formed faster)
(more stable)
22
Kinetic ControlversusThermodynamic Control

• Kinetic control major product is the one formed
  at the fastest rate
• Thermodynamic control major product is the one
  that is the most stable

23
HBr
24

higher activation energy
CH3CHCH
CH2

CH3CH
CHCH2
formed more slowly
25
Problem 10.10(page 382)

• Addition of hydrogen chloride to
  2-methyl-1,3-butadiene is a kinetically
  controlled reaction and gives one product in
  much greater amounts than any isomers. What is
  this product?

HCl
?
26
Problem 10.10(page 382)

• Think mechanistically.
• Protonation occurs at end of diene system in
  direction that gives most stable carbocation
• Kinetically controlled product corresponds to
  attack by chloride ion at carbon that has the
  greatest share of positive charge in the
  carbocation

HCl
27
Problem 10.10(page 382)
Think mechanistically

• one resonance form is tertiary carbocation
  other is primary

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Problem 10.10(page 382)
Think mechanistically
Cl
H

• one resonance form is secondary carbocation
  other is primary

one resonance form is tertiary carbocation
other is primary
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Problem 10.10(page 382)
Think mechanistically

• More stable carbocation
• Is attacked by chloride ion at carbon that bears
  greater share of positive charge

one resonance form is tertiary carbocation
other is primary
30
Problem 10.10(page 382)
Think mechanistically
Cl
Cl


one resonance form is tertiary carbocation
other is primary
majorproduct
31
10.11Halogen Addition to Dienes

• gives mixtures of 1,2 and 1,4-addition products

32
Example
Br2

(37)
(63)