9.5%20Acidity%20of%20Acetylene%20and%20Terminal%20Alkynes

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9.5Acidity of Acetyleneand Terminal Alkynes
H
2
Acidity of Hydrocarbons
In general, hydrocarbons are exceedingly
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

26
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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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Sodium Acetylide
Objective Prepare a solution containing sodium
acetylideWill treatment of acetylene with NaOH
be effective?
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Sodium Acetylide
No. Hydroxide is not a strong enough base to
deprotonate acetylene.





stronger acidpKa 16
weaker acidpKa 26
In acid-base reactions, the equilibrium lies
tothe side of the weaker acid.
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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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9.6Preparation of AlkynesbyAlkylation of
Acetylene and Terminal Alkynes
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Preparation of Alkynes
There are two main methods for the preparationof
alkynes

• Carbon-carbon bond formation alkylation of
  acetylene and terminal alkynes
• Functional-group transformations elimination

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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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Acetylide Ion as a Base
E2 predominates over SN2 when alkyl halide is
secondary or tertiary
E2
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9.7Preparation of Alkynesby Elimination
Reactions
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Preparation of Alkynesby "Double
Dehydrohalogenation"
Geminal dihalide
Vicinal dihalide
The most frequent applications are in preparation
of terminal alkynes.
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Geminal dihalide Æ Alkyne
(CH3)3CCH2CHCl2
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Geminal dihalide Æ Alkyne
(CH3)3CCH2CHCl2
(slow)
NaNH2, NH3
(slow)
NaNH2, NH3
H2O
(fast)
NaNH2, NH3
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Vicinal dihalide Æ Alkyne