Alkynes

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Alkynes
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Synthesis of Acetylene

• Heating coke with lime in an electric furnace to
  forms calcium carbide.
• Then drip water on the calcium carbide.

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The Structure of Alkynes
A triple bond is composed of a s bond and two p
bonds
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Acidity of Acetyleneand Terminal Alkynes
H
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Acidity of Hydrocarbons
In general, hydrocarbons are excedingly weak
acids

• Compound pKa
• HF 3.2
• H2O 16
• NH3 36
• 45
• CH4 60

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Acetylene
Acetylene is a weak acid, but not nearlyas weak
as alkanes or alkenes.

• Compound pKa
• HF 3.2
• H2O 16
• NH3 36
• 45
• CH4 60

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Carbon Hybridization and Electronegativity
10-60
sp3

H
C
sp2

10-45
H
C
C
10-26
sp

H
Electrons in an orbital with more s character are
closer to thenucleus and more strongly held.
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The stronger the acid, the weaker its conjugate
base
top 252
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Sodium Acetylide
Solution Use a stronger base. Sodium amideis
a stronger base than sodium hydroxide.

..
..





H2N
H
H2N
weaker acidpKa 36
stronger acidpKa 26
Ammonia is a weaker acid than acetylene.The
position of equilibrium lies to the right.
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Preparation of Various Alkynes by alkylation
reactions withAcetylide or Terminal Alkynes
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Synthesis Using Acetylide Ions Formation of CC
Bond
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Alkylation of Acetylene and Terminal Alkynes
HC
CH
RC
CH
CR
RC
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Alkylation of Acetylene and Terminal Alkynes
SN2

• The alkylating agent is an alkyl halide, andthe
  reaction is nucleophilic substitution.
• The nucleophile is sodium acetylide or the
  sodium salt of a terminal (monosubstituted)
  alkyne.

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Example Alkylation of Acetylene
NaNH2
NH3
CH3CH2CH2CH2Br
(70-77)
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Example Alkylation of a Terminal Alkyne
NaNH2, NH3
CH3Br
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Example Dialkylation of Acetylene
(81)
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Limitation

• Effective only with primary alkyl halides
• Secondary and tertiary alkyl halides undergo
  elimination

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Reactions of Alkynes
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Reactions of Alkynes

• Acidity
• Hydrogenation
• Metal-Ammonia Reduction
• Addition of Hydrogen Halides
• Hydration

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Hydrogenation 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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Partial Hydrogenation
RCH2CH2R'

• Alkenes 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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Metal-Ammonia Reductionof 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 proton comes from NH3

• four steps
• (1) electron transfer
• (2) proton transfer
• (3) electron transfer
• (4) proton transfer

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Problem

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

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Problem Strategy
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Problem Strategy
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Problem Synthesis
1. NaNH2 2. CH3CH2CH2CH2Br
Na, NH3
H2, Lindlar Pd
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Addition of Hydrogen Halidesto Alkynes
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Follows Markovnikov's Rule
HBr
(60)

• Alkynes are slightly less reactive than alkenes

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Two Molar Equivalents of Hydrogen Halide
CH3CH2C
CCH2CH3
2 HF
(76)
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Free-radical Addition of HBr
HBr
peroxides
(79)

• regioselectivity opposite to Markovnikov's rule

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Hydration of Alkynes
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Hydration of Alkynes

• expected reaction

enol
observed reaction
H

H2O
ketone
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Enols
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
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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
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Useful for symmetrical starting alkynes to
produce a single product.
Unsymmetrical starting alkynes produce a mixture
of ketones not so useful.
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Aldehyde vs. Ketone
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Can you identify and name the function?
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Example
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