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Preparation of Alkynes by Elimination Reactions

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Title: Preparation of Alkynes by Elimination Reactions


1
Preparation of Alkynes by Elimination Reactions
2
Preparation of Alkynes by "Double
Dehydrohalogenation"
Geminal dihalide
Vicinal dihalide
The most frequent applications are in preparation
of terminal alkynes.
3
Geminal dihalide Alkyne
(CH3)3CCH2CHCl2
4
Geminal dihalide Alkyne
(CH3)3CCH2CHCl2
(slow)
NaNH2, NH3
(slow)
NaNH2, NH3
H2O
(fast)
NaNH2, NH3
5
Vicinal dihalide Alkyne
6
Acidity of Acetylene and Terminal Alkynes
H
7
Acidity of Hydrocarbons
In general, hydrocarbons are exceedingly
weak acids
  • Compound pKa
  • HF 3.2
  • H2O 16
  • NH3 36
  • 45
  • CH4 60

8
Acetylene
Acetylene is a weak acid, but not nearly as weak
as alkanes or alkenes.
  • Compound pKa
  • HF 3.2
  • H2O 16
  • NH3 36
  • 45
  • CH4 60

26
9
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 the nucleus and more strongly held.
10
Sodium Acetylide
Objective Prepare a solution containing sodium
acetylide Will treatment of acetylene with NaOH
be effective?
11
Sodium Acetylide
No. Hydroxide is not a strong enough base to
deprotonate acetylene.





stronger acid pKa 16
weaker acid pKa 26
In acid-base reactions, the equilibrium lies
to the side of the weaker acid.
12
Sodium Acetylide
Solution Use a stronger base. Sodium amide is
a stronger base than sodium hydroxide.

..
..





H2N
H
H2N
weaker acid pKa 36
stronger acid pKa 26
Ammonia is a weaker acid than acetylene. The
position of equilibrium lies to the right.
13
Preparation of Alkynes by Alkylation of
Acetylene and Terminal Alkynes
14
Preparation of Alkynes
There are two main methods for the preparation of
alkynes
  • Carbon-carbon bond formation alkylation of
    acetylene and terminal alkynes
  • Functional-group transformations elimination

15
Alkylation of Acetylene and Terminal Alkynes
HC
CH
RC
CH
CR
RC
16
Alkylation of Acetylene and Terminal Alkynes
SN2

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

17
Example Alkylation of Acetylene
NaNH2
NH3
CH3CH2CH2CH2Br
(70-77)
18
Example Alkylation of a Terminal Alkyne
NaNH2, NH3
CH3Br
19
Example Dialkylation of Acetylene
(81)
20
Limitation
  • Effective only with primary alkyl halides
  • Secondary and tertiary alkyl halides undergo
    elimination

21
Acetylide Ion as a Base
E2 predominates over SN2 when alkyl halide is
secondary or tertiary
E2
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