Transcript Slide 1

Organic Chemistry
Second Edition
David Klein
Chapter 10
Alkynes
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
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.
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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
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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
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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°
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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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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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Klein, Organic Chemistry 2e
10.4 Preparation of Alkynes
•
Like alkenes, alkynes can also be prepared by
elimination
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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
10.6 Hydrohalogenation of Alkynes
•
Kinetic studies on the hydrohalogenation of an alkyne
suggest that the rate law is 1st order with respect to the
alkyne and 2nd order with respect to HX
•
What type of collision would result in such a rate law?
Unimolecular, bimolecular, or termolecular?
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10-31
Klein, Organic Chemistry 2e
10.6 Hydrohalogenation of Alkynes
•
•
•
Reaction rate is generally slow for termolecular
collisions. WHY?
Considering the polarizability of the alkyne, does the
mechanism explain the regioselectivity?
May involve multiple competing mechanisms
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10-32
Klein, Organic Chemistry 2e
10.6 Hydrohalogenation of Alkynes
•
Peroxides can be used in the hydrohalogenation of
alkynes to promote anti-Markovnikov addition just like
with alkenes
•
•
Which product is E and which is Z?
The process proceeds through a free radical mechanism
that we will discuss in detail in Chapter 11
Practice with conceptual checkpoint 10.13
•
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10-33
Klein, Organic Chemistry 2e
10.7 Hydration of Alkynes
•
•
Like alkenes, alkynes can also undergo acid catalyzed
Markovnikov hydration
The process is generally catalyzed with HgSO4 to
compensate for the slow reaction rate that results from
the formation of vinylic carbocation
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10-34
Klein, Organic Chemistry 2e
10.7 Hydration of Alkynes
•
•
HgSO4 catalyzed hydration involves the mecury (II) ion
interacting with the alkyne
Can you imagine what that interaction might look like
and how it will increase the rate of reaction for the
process?
•
Why is the intermediate called an enol?
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10-35
Klein, Organic Chemistry 2e
10.7 Hydration of Alkynes
•
The enol/ketone tautomerization generally cannot be
prevented and favors the ketone greatly
•
Tautomers are constitutional isomers that rapidly
interconvert. How is that different from resonance?
Practice with SkillBuilder 10.3
•
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10-36
Klein, Organic Chemistry 2e
10.7 Hydroboration-Oxidation
•
•
•
Hydroboration-oxidation for alkynes proceeds through
the same mechanism as for alkenes giving the antiMarkovnikov product
It also produces an enol that will quickly tautomerize
In this case, the tautomerization is catalyzed by the
base (OH-) rather than by an acid
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10-37
Klein, Organic Chemistry 2e
10.7 Hydroboration-Oxidation
•
In general, we can conclude
that a C=O double bond is
more stable than a C=C
double bond. WHY?
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10-38
Klein, Organic Chemistry 2e
10.7 Hydroboration-Oxidation
•
After the –BH2 and –H groups have been added across
the C=C double bond, in some cases, an undesired
second addition can take place
•
To block out the second unit of BH3 from reacting with
the intermediate, bulky borane reagents are often used
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10-39
Klein, Organic Chemistry 2e
10.7 Hydroboration-Oxidation
•
Some bulky borane reagents are shown below
•
Practice with conceptual checkpoint 10.20
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10-40
Klein, Organic Chemistry 2e
10.7 Hydroboration-Oxidation
•
Predict products for the following reaction
•
Draw the alkyne reactant and reagents that could be
used to synthesize the following molecule
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10-41
Klein, Organic Chemistry 2e
10.7 Hydration Regioselectivity
•
•
Markovnikov hydration leads to a ketone
Anti-Markovnikov hydration leads to an aldehyde
•
Practice with SkillBuilder 10.4
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10-42
Klein, Organic Chemistry 2e
10.8 Alkyne Halogenation
•
•
Alkynes can also undergo halogenation
Two equivalents of halogen can be added
•
You might expect the mechanism to be similar to the
halogenation of alkenes, yet stereochemical evidence
suggests otherwise – see next slide
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10-43
Klein, Organic Chemistry 2e
10.8 Alkyne Halogenation
•
When one equivalent of halogen is added to an alkyne,
both anti and syn addition is observed
•
The halogenation of an alkene undergoes anti addition
ONLY
The mechanism for alkyne halogenation is not fully
elucidated
•
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10-44
Klein, Organic Chemistry 2e
10.9 Alkyne Ozonolysis
•
When alkynes react under ozonolysis conditions, the pi
system is completely broken
•
The molecule is cleaved, and the alkyne carbons are
fully oxidized
Practice with conceptual checkpoint 10.25
•
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10-45
Klein, Organic Chemistry 2e
10.9 Alkyne Ozonolysis
•
Predict the product(s) for the following reaction
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10-46
Klein, Organic Chemistry 2e
10.10 Alkylation of Terminal Alkynes
•
•
As acids, terminal alkynes are quite weak
Yet, with a strong enough base, a terminal alkyne can
be deprotonated and converted into a good nucleophile
•
What has a higher pKa, NH3 or R-CC-H? WHY?
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10-47
Klein, Organic Chemistry 2e
10.10 Alkylation of Terminal Alkynes
•
The alkynide ion can attack a methyl or 1° alkyl
halide electrophile
•
Such reactions can be used to develop molecular
complexity
•
Alkynide ions usually act as bases with 2° or 3°
alkyl halides to cause elimination rather than
substitution
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10-48
Klein, Organic Chemistry 2e
10.10 Alkylation of Terminal Alkynes
•
Acetylene can be used to perform a double alkylation
•
Why will the reaction be unsuccessful if the NaNH2 and
Et-Br are added together?
•
Complex target molecules can be made by building a
carbon skeleton and converting functional groups
Practice with SkillBuilder 10.5
•
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10-49
Klein, Organic Chemistry 2e
10.11 Synthetic Stategies
•
Recall the methods for increasing the saturation of
alkenes and alkynes
•
But, what if you want to reverse the process or
decrease saturation? See next slide
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10-50
Klein, Organic Chemistry 2e
10.11 Synthetic Stategies
•
Halogenation of an alkene followed by two
dehydrohalogenation reactions can decrease saturation
•
We will have to wait until chapter 11 to see how to
convert an alkane into an alkene, but here is a preview
•
What conditions would you use in step B?
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10-51
Klein, Organic Chemistry 2e
10.11 Synthetic Stategies
•
In the alkene to alkyne conversion above, why is water
needed in part 3) of that reaction?
•
Practice with SkillBuilder 10.6
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10-52
Klein, Organic Chemistry 2e
10.11 Synthetic Stategies
•
Give necessary reaction conditions for the multi-step
conversions below
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10-53
Klein, Organic Chemistry 2e
Additional Practice Problems
•
Name the molecule
•
Draw the structure of 2,2-dimethyl-6-chloro-3-heptyne
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10-54
Klein, Organic Chemistry 2e
Additional Practice Problems
•
Give 2 sets of reagents that could be used to synthesize
1-pentyne through elimination reactions.
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10-55
Klein, Organic Chemistry 2e
Additional Practice Problems
•
Give a set of reagents that could be used to synthesize
cis-2-pentene from an addition reaction.
•
Give a set of reagents that could be used to synthesize
trans-2-pentene from an addition reaction.
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10-56
Klein, Organic Chemistry 2e
Additional Practice Problems
•
Give a set of reagents that could be used to synthesize a
ketone from an addition reaction.
•
Give a set of reagents that could be used to synthesize
an aldehyde from an addition reaction.
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10-57
Klein, Organic Chemistry 2e
Additional Practice Problems
•
Determine necessary reagents to complete the
synthesis below.
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10-58
Klein, Organic Chemistry 2e