Organic Chemistry
Second Edition
David Klein
Chapter 10
Alkynes
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Klein, Organic Chemistry 2e
10.1 Alkynes
• Alkynes are molecules that incorporate a C≡C triple
bond
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10-2
Klein, Organic Chemistry 2e
10.1 Alkynes
• Given the presence of two pi bonds and their associated
electron density, alkynes are similar to alkenes in their
ability to act as a nucleophile
• Converting pi bonds to sigma bonds generally makes a
molecule more stable. WHY?
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10-3
Klein, Organic Chemistry 2e
10.1 Alkyne Uses
• Acetylene is the simplest alkyne
• It is used in blow torches and as a precursor for the
synthesis of more complex alkynes
• More than 1000 different alkyne natural products have
been isolated
• One example is
histrionicotoxin, which can be
isolated from South American
frogs and is used on poisontipped arrows by South
American tribes
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10-4
Klein, Organic Chemistry 2e
10.1 Alkyne Uses
• An example of a synthetic alkyne is ethynylestradiol
• Ethynylestradiol is the active
ingredient in many birth control pills
• The presence of the triple
bond increases the potency
of the drug compared to
the natural analog
• How do you think a C≡C triple bond affects the
molecules geometry? Its rigidity? Its intermolecular
attractions?
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10-5
Klein, Organic Chemistry 2e
10.2 Alkyne Nomenclature
•
Alkynes are named using the same procedure we used
in Chapter 4 to name alkanes with minor modifications
1. Identify the parent chain, which should include the C≡C triple
bond
2. Identify and Name the substituents
3. Assign a locant (and prefix if necessary) to each substituent
giving the C≡C triple bond the lowest number possible
4. List the numbered substituents before the parent name in
alphabetical order. Ignore prefixes (except iso) when ordering
alphabetically
5. The C≡C triple bond locant is placed either just before the
parent name or just before the -yne suffix
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10-6
Klein, Organic Chemistry 2e
10.2 Alkyne Nomenclature
•
Alkynes are named using the same procedure we used
in Chapter 4 to name alkanes with minor modifications
1. Identify the parent chain, which should include the C≡C triple
bond
2. Identify and name the substituents.
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
10-7
Klein, Organic Chemistry 2e
10.2 Alkyne Nomenclature
•
Alkynes are named using the same procedure we used
in Chapter 4 to name alkanes with minor modifications
3. Assign a locant (and prefix if necessary) to each substituent
giving the C≡C triple bond the lowest number possible
–
The locant is ONE number, NOT two. Although the triple bond
bridges carbons 2 and 3, the locant is the lower of those two
numbers
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
10-8
Klein, Organic Chemistry 2e
10.2 Alkyne Nomenclature
•
Alkynes are named using the same procedure we used
in Chapter 4 to name alkanes with minor modifications
4. List the numbered substituents before the parent name in
alphabetical order. Ignore prefixes (except iso) when ordering
alphabetically
5. The C≡C triple bond locant is placed either just before the
parent name or just before the -yne suffix
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
10-9
Klein, Organic Chemistry 2e
10.2 Alkyne Nomenclature
•
In addition to the IUPAC naming system, chemists often
use common names that are derived from the common
parent name acetylene
•
You should also be aware of the terminology below
•
Practice with SkillBuilder 10.1
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10-10
Klein, Organic Chemistry 2e
10.2 Alkyne Nomenclature
•
Name the molecule below
•
Recall that when triple bonds are drawn their angles are
180°
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
10-11
Klein, Organic Chemistry 2e
10.3 Alkyne Acidity
•
Recall that terminal alkynes have a lower pKa than other
hydrocarbons
•
Acetylene is 19 pKa units more acidic than ethylene,
which is 1019 times stronger
Does that mean that terminal alkynes are strong acids?
•
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10-12
Klein, Organic Chemistry 2e
10.3 Alkyne Acidity
•
Because acetylene (pKa=25) is still much weaker than
water (pKa=15.7), a strong base is needed to make it
react, and water cannot be used as the solvent
•
Recall from chapter 3 we used the acronym, ARIO, to
rationalize differences in acidity strengths
Use ARIO to explain why acetylene is a stronger acid
than ethylene which is stronger than ethane
•
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10-13
Klein, Organic Chemistry 2e
10.3 Alkyne Acidity
•
Use ARIO to rationalize the equilibria below
•
A bases conjugate acid pKa must be greater than 25 for
it to be able to deprotonate a terminal alkyne
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10-14
Klein, Organic Chemistry 2e
10.4 Preparation of Alkynes
•
Like alkenes, alkynes can also be prepared by
elimination
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10-15
Klein, Organic Chemistry 2e
10.4 Preparation of Alkynes
•
•
Such eliminations usually occur via an E2 mechanism
Geminal dihalides can be used
•
Vicinal dihalides can also be used
•
E2 requires anti-periplanar geometry
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10-16
Klein, Organic Chemistry 2e
10.4 Preparation of Alkynes
•
Often, excess equivalents of NaNH2 are used to shift the
equilibrium toward the elimination products
•
NH21- is quite strong, so if a terminal alkyne is produced,
it will be deprotonated
That equilibrium will greatly favor products
•
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10-17
Klein, Organic Chemistry 2e
10.4 Preparation of Alkynes
•
A proton source is needed to produce the alkyne
•
Predict the products in the example below
•
Practice with conceptual checkpoint 10.7
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10-18
Klein, Organic Chemistry 2e
10.5 Reduction of Alkynes
•
Like alkenes, alkynes can readily undergo hydrogenation
•
Two equivalents of H2
are consumed for each
alkynealkane
conversion
The cis alkene is
produced as an
intermediate. WHY cis?
•
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10-19
Klein, Organic Chemistry 2e
10.5 Reduction w/ a Poisoned Catalyst
•
A deactivated or poisoned catalyst can be used to
selectively react with the alkyne
•
Lindlar’s catalyst and P-2 (Ni2B complex) are common
examples of a poisoned catalysts
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10-20
Klein, Organic Chemistry 2e
10.5 Reduction w/ a Poisoned Catalyst
•
Is this a syn or anti addition?
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10-21
•
Practice with
conceptual
checkpoint
10.9
Klein, Organic Chemistry 2e
10.5 Dissolving Metal Reductions
•
•
Reduction with H2 gives syn addition
Dissolving metal conditions can give Anti addition
producing the trans alkene
•
Ammonia has a boiling point = -33°C, so the
temperature for these reactions must remain very low
Why can’t water be used as the solvent?
•
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10-22
Klein, Organic Chemistry 2e
10.5 Dissolving Metal Reductions
•
Mechanism: Step 1
•
Note the single-barbed and double-barbed (fishhook)
arrows.
Why does Na metal so readily give up an electron?
•
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10-23
Klein, Organic Chemistry 2e
10.5 Dissolving Metal Reductions
•
Mechanism: Step 1
•
•
Why is the first intermediate called a radical anion?
The radical anion adopts a trans configuration to reduce
repulsion
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10-24
Klein, Organic Chemistry 2e
10.5 Dissolving Metal Reductions
•
Mechanism: step 2 and 3
•
Draw the product for step 3 of the mechanism
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10-25
Klein, Organic Chemistry 2e
10.5 Dissolving Metal Reductions
•
Mechanism: step 4
•
Do the pKa values for NH3 and the alkene favor the
proton transfer?
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10-26
Klein, Organic Chemistry 2e
10.5 Dissolving Metal Reductions
•
Predict the product(s) for the following reaction
•
Practice with conceptual checkpoint 10.10
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10-27
Klein, Organic Chemistry 2e
10.5 Summary of Reductions
•
Familiarize yourself with the reagents necessary to
manipulate alkynes
•
Practice with conceptual checkpoint 10.11
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10-28
Klein, Organic Chemistry 2e
10.6 Hydrohalogenation of Alkynes
•
Like alkenes, alkynes also undergo hydrohalogenation
•
•
Draw the final product for the reaction above
Do the reactions above exhibit Markovnikov
regioselectivity?
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10-29
Klein, Organic Chemistry 2e
10.6 Hydrohalogenation of Alkynes
•
•
You might expect alkynes to undergo
hydrohalogenation by a mechanism similar to alkenes
Vinylic
carbocation
Yet, the mechanism above does not explain all observed
phenomena
–
A slow reaction rate, 3rd order overall rate law, like 1°
carbocations, vinylic carbocations are especially
unstable
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10-30
Klein, Organic Chemistry 2e