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Carey Chapter 9 Alkynes

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Alkynes could be used to prepare alkenes if a. catalyst were ... poisoned with lead acetate and quinoline. syn-Hydrogenation occurs; cis alkenes are formed. ... – PowerPoint PPT presentation

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Title: Carey Chapter 9 Alkynes


1
9.9 Hydrogenation of Alkynes
2
Hydrogenation of Alkynes
cat
RCH2CH2R'
catalyst Pt, Pd, Ni, or Rh
  • alkene is an intermediate

3
Heats of hydrogenation
292 kJ/mol
275 kJ/mol
Alkyl groups stabilize triple bonds in the same
way that they stabilize double bonds. Internal
triple bonds are more stable than terminal ones.
4
Partial Hydrogenation
RCH2CH2R'
  • Alkynes could be used to prepare alkenes if
    a catalyst were available that is active enough
    to catalyze the hydrogenation of alkynes, but
    not active enough for the hydrogenation of
    alkenes.

5
Lindlar Palladium
RCH2CH2R'
  • There is a catalyst that will catalyze the
    hydrogenation of alkynes to alkenes, but not that
    of alkenes to alkanes.
  • It is called the Lindlar catalyst and consists
    of palladium supported on CaCO3, which has been
    poisoned with lead acetate and quinoline.
  • syn-Hydrogenation occurs cis alkenes are formed.

6
Example
H2
Lindlar Pd
CH3(CH2)3
(CH2)3CH3
H
H
(87)
7
9.10 Metal-Ammonia Reduction of Alkynes
  • Alkynes trans-Alkenes

8
Partial Reduction
RCH2CH2R'
RC
CR'
RCH
CHR'
  • Another way to convert alkynes to alkenes is by
    reduction with sodium (or lithium or
    potassium) in ammonia.
  • trans-Alkenes are formed.

9
Example
Na, NH3
CH3CH2
H
CH2CH3
H
(82)
10
Mechanism
Metal (Li, Na, K) is reducing agent H2 is not
involved
  • four steps
  • (1) electron transfer
  • (2) proton transfer
  • (3) electron transfer
  • (4) proton transfer

11
Mechanism
  • Step (1) Transfer of an electron from the
    metal to the alkyne to give an anion radical.

12
Mechanism
  • Step (2) Transfer of a proton from the solvent
    (liquid ammonia) to the anion radical.

13
Mechanism
  • Step (3) Transfer of an electron from the
    metal to the alkenyl radical to give a carbanion.

R
.
.

M
R'
C
C
H
14
Mechanism
  • Step (4) Transfer of a proton from the
    solvent (liquid ammonia) to the carbanion .

R'
15
  • Suggest efficient syntheses of (E)- and
    (Z)-2- heptene from propyne and any necessary
    organic or inorganic reagents.

16
1. NaNH2 2. CH3CH2CH2CH2Br

Na, NH3
H2, Lindlar Pd
17
9.11 Addition of Hydrogen Halides to Alkynes
18
Follows Markovnikov's Rule
HBr
(60)
  • Alkynes are slightly less reactive than alkenes

19
Termolecular transition state
CH
RC
Observed rate law rate kalkyneHX2
20
Reaction with two moles of a hydrogen halide
yields a geminal dihalide
CH3CH2C
CCH2CH3
2 HF
(76)
21
Free-radical addition of HBr occurs
when peroxides are present
HBr
peroxides
(79)
  • regioselectivity opposite to Markovnikov's rule

22
9.12 Hydration of Alkynes
23
Hydration of Alkynes
  • expected reaction

enol
observed reaction
H

H2O
ketone
24
enol
ketone
  • enols are regioisomers of ketones, and exist in
    equilibrium with them
  • keto-enol equilibration is rapid in acidic media
  • ketones are more stable than enols
    and predominate at equilibrium

25
Mechanism of conversion of enol to ketone
..

H


H
O
H
26
Mechanism of conversion of enol to ketone
..

H


H
O
H
27
Mechanism of conversion of enol to ketone
..

O
H
H
H
C
C



O
H
28
Mechanism of conversion of enol to ketone
H
..

O
H
H
H
C
C

29
Mechanism of conversion of enol to ketone
H
..

O
H
H
H
C
C

30
Mechanism of conversion of enol to ketone
H
..

O
H


O
H
H
C
C
31
Key carbocation intermediate is stabilized by
electron delocalization (resonance)
32
Example
via
H2O, H
CH3(CH2)2CH2C(CH2)2CH3
(89)
33
Markovnikov's rule followed in formation of enol
H2O, H2SO4
CH3(CH2)5CCH3
HgSO4
(91)
via
34
9.13 Addition of Halogens to Alkynes
35
Example
2 Cl2
36
Addition is anti
CH3CH2
Br
Br2
CH2CH3
Br
(90)
37
9.14 Ozonolysis of Alkynes
  • gives two carboxylic acids by cleavage of triple
    bond

38
Example

39
End of Chapter 9
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