Chapter 9 Alkynes

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Chapter 9Alkynes
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Nomenclature
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Nomenclature
Higher alkynes are named in much the sameway as
alkenes except using an -yne suffixinstead of
-ene.
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Nomenclature
The same naming priority applies alkyl halide lt
alkenyl lt alkynyl lt hydroxy
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Structure and Bonding in Alkynessp Hybridization
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Structure

• linear geometry for acetylene

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Cycloalkynes

• 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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? Bonds in Acetylene
Each carbon isconnected to ahydrogen by a?
bond. The twocarbons are connectedto each
other by a? bond and two ? bonds.
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? Bonds in Acetylene
One of the two? bonds in acetylene isshown
here.The second ?bond is at rightangles to the
first.
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? Bonds in Acetylene
This is the secondof the two? bonds in
acetylene.
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The region of highest negative charge lies above
and below the molecular plane in ethylene.
The region of highest negative charge encircles
the molecule around itscenter in acetylene.
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Acidity of Acetyleneand Terminal Alkynes
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Acidity of Hydrocarbons
In general, hydrocarbons are exceedingly
weak acids

• Compound pKa
• HF 3.2
• H2O 15.7
• 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 15.7
• NH3 36
• 45
• CH4 60

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Carbon Hybridization and Electronegativity
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.
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.
Ammonia is a weaker acid than acetylene.The
position of equilibrium lies to the right.
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Preparation 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
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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
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Example Alkylation of a Terminal Alkyne
NaNH2, NH3
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Example Dialkylation of Acetylene
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Limitations

• 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
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Preparation 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 ? Alkyne
(CH3)3CCH2CHCl2
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Geminal dihalide ? Alkyne
(CH3)3CCH2CHCl2
(slow)
NaNH2, NH3
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Vicinal dihalide ? Alkyne