Chapter 9 Alkynes

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Chapter 9Alkynes
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9.1Sources of Alkynes
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Acetylene

• Industrial preparation of acetylene isby
  dehydrogenation of ethylene

800C
CH2
H2C
CH3CH3
1150C
cost of energy makes acetylene a more expensive
industrial chemical than ethylene
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9.2Nomenclature
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Nomenclature
Higher alkynes are named in much the sameway as
alkenes except using an -yne suffixinstead of
-ene.
1-Butyne
4,4-Dimethyl-2-pentyne
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9.3Physical Properties of Alkynes
The physical properties of alkynes are similar
to those of alkanes and alkenes.
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9.4Structure and Bonding in Alkynessp
Hybridization
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Structure

• linear geometry for acetylene

121 pm
C
C
CH3
H
146 pm
106 pm
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• Cyclononyne is the smallest cycloalkyne stable
  enough to be stored at room temperaturefor a
  reasonable length of time.
• Cyclooctyne polymerizeson standing.

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Bonding in acetylene is based on
sp-hybridizationfor each carbon
Mix together (hybridize) the 2s orbital and one
of the three 2p orbitals
2p
2p
2sp
2s
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Bonding in acetylene is based on
sp-hybridizationfor each carbon
Mix together (hybridize) the 2s orbital and one
of the three 2p orbitals
2p
Each carbon has two half-filled sp
orbitalsavailable to form s bonds.
2sp
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s Bonds in Acetylene
Each carbon isconnected to ahydrogen by as
bond. The twocarbons are connectedto each
other by as bond and two p bonds.
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p Bonds in Acetylene
One of the twop bonds in acetylene isshown
here.The second pbond is at rightangles to the
first.
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p Bonds in Acetylene
This is the secondof the twop bonds in
acetylene.
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The region of highest negative charge lies
aboveand below the molecular plane in ethylene.
The region of highest negative charge
encirclesthe molecule around itscenter in
acetylene.
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Table 9.1 Comparison of ethane, ethylene, and
acetylene
Ethane Ethylene Acetylene
CC distance
153 pm
134 pm
120 pm
CH distance
111 pm
110 pm
106 pm
HCC angles
111.0
121.4
180
CC BDE
368 kJ/mol
611 kJ/mol
820 kJ/mol
CH BDE
410 kJ/mol
452 kJ/mol
536 kJ/mol
hybridization of C
sp3
sp2
sp
s character
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33
50
pKa
62
45
26
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9.5Acidity of Acetylene andTerminal Alkynes
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In general, hydrocarbons are exceedingly
weak acids

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

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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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Electronegativity of carbon increases with its s
character
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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Objective Prepare a solution containing sodium
acetylideWill treatment of acetylene with NaOH
be effective?
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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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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 Alkynes byAlkylation 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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E2 predominates over SN2 when alkyl halide is
secondary or tertiary
C
H
C X
E2
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9.7Preparation of Alkynes by Elimination
Reactions
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Preparation of Alkynes by "Double"
Dehydrohalogenation
Geminal dihalide
Vicinal dihalide
The most frequent applications are in preparation
of terminal alkynes.
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Geminal dihalide Alkyne
(CH3)3CCH2CHCl2
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Geminal dihalide Alkyne
(CH3)3CCH2CHCl2
(slow)
NaNH2, NH3
(slow)
NaNH2, NH3
H2O
(fast)
NaNH2, NH3
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Vicinal dihalide Alkyne
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9.8Reactions of Alkynes
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Reactions of Alkynes

• Acidity (Section 9.5)
• Hydrogenation (Section 9.9)
• Metal-Ammonia Reduction (Section 9.10)
• Addition of Hydrogen Halides (Section 9.11)
• Hydration (Section 9.12)
• Addition of Halogens (Section 9.13)
• Ozonolysis (Section 9.14)