Carey Chapter 9 Alkynes

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9.9Hydrogenation of Alkynes
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Hydrogenation of Alkynes
cat
RCH2CH2R'
catalyst Pt, Pd, Ni, or Rh

• alkene is an intermediate

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Heats of hydrogenation
292 kJ/mol
275 kJ/mol
Alkyl groups stabilize triple bonds in the same
way that they stabilize doublebonds. Internal
triple bonds are more stable than terminal ones.
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Partial Hydrogenation
RCH2CH2R'

• Alkynes could be used to prepare alkenes if
  acatalyst were available that is active enough
  to catalyze the hydrogenation of alkynes, but
  notactive enough for the hydrogenation of
  alkenes.

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Lindlar Palladium
RCH2CH2R'

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

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Example
H2
Lindlar Pd
CH3(CH2)3
(CH2)3CH3
H
H
(87)
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9.10 Metal-Ammonia Reduction of Alkynes

• Alkynes trans-Alkenes

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Partial Reduction
RCH2CH2R'
RC
CR'
RCH
CHR'

• Another way to convert alkynes to alkenes isby
  reduction with sodium (or lithium or
  potassium)in ammonia.
• trans-Alkenes are formed.

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Example
Na, NH3
CH3CH2
H
CH2CH3
H
(82)
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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

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Mechanism

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

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Mechanism

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

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Mechanism

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

R
.
.

M
R'
C
C
H
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Mechanism

• Step (4) Transfer of a proton from the
  solvent(liquid ammonia) to the carbanion .

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

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1. NaNH2 2. CH3CH2CH2CH2Br

Na, NH3
H2, Lindlar Pd
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9.11Addition of Hydrogen Halides to Alkynes
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Follows Markovnikov's Rule
HBr
(60)

• Alkynes are slightly less reactive than alkenes

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Termolecular transition state
CH
RC
Observed rate law rate kalkyneHX2
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Reaction with two moles of a hydrogenhalide
yields a geminal dihalide
CH3CH2C
CCH2CH3
2 HF
(76)
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Free-radical addition of HBr occurs
whenperoxides are present
HBr
peroxides
(79)

• regioselectivity opposite to Markovnikov's rule

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9.12Hydration of Alkynes
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Hydration of Alkynes

• expected reaction

enol
observed reaction
H

H2O
ketone
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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
  andpredominate at equilibrium

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Mechanism of conversion of enol to ketone
..

H


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

H


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

O
H
H
H
C
C



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

O
H
H
H
C
C

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Mechanism of conversion of enol to ketone
H
..

O
H
H
H
C
C

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Mechanism of conversion of enol to ketone
H
..

O
H


O
H
H
C
C
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Key carbocation intermediate is stabilized by
electron delocalization (resonance)
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Example
via
H2O, H
CH3(CH2)2CH2C(CH2)2CH3
(89)
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Markovnikov's rule followed in formation of enol
H2O, H2SO4
CH3(CH2)5CCH3
HgSO4
(91)
via
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9.13Addition of Halogens to Alkynes
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Example
2 Cl2
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Addition is anti
CH3CH2
Br
Br2
CH2CH3
Br
(90)
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9.14Ozonolysis of Alkynes

• gives two carboxylic acids by cleavage of triple
  bond

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Example

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End of Chapter 9