1. 10.4 Preparation of Alkynes
Like alkenes, alkynes can also be prepared by elimination
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Klein, Organic Chemistry 1e

2. 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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Klein, Organic Chemistry 1e

3. 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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Klein, Organic Chemistry 1e

4. 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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Klein, Organic Chemistry 1e

5. 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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6. 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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Klein, Organic Chemistry 1e

7. 10.5 Reduction w/ a Poisoned Catalyst
Is this a syn or anti addition?
Practice with conceptual checkpoint 10.9
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Klein, Organic Chemistry 1e

8. 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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Klein, Organic Chemistry 1e

9. 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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Klein, Organic Chemistry 1e

10. 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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Klein, Organic Chemistry 1e

11. 10.5 Dissolving Metal Reductions
Mechanism: step 2 and 3
Draw the product for step 3 of the mechanism
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Klein, Organic Chemistry 1e

12. 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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Klein, Organic Chemistry 1e

13. 10.5 Dissolving Metal Reductions
Predict the product(s) for the following reaction
Practice with conceptual checkpoint 10.10
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Klein, Organic Chemistry 1e

14. 10.5 Summary of Reductions
Familiarize yourself with the reagents necessary to manipulate alkynes
Practice with conceptual checkpoint 10.11
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Klein, Organic Chemistry 1e

15. 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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Klein, Organic Chemistry 1e

16. 10.6 Hydrohalogenation of Alkynes
You might expect alkynes to undergo hydrohalogenation by a mechanism similar to alkenes
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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Klein, Organic Chemistry 1e

17. 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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Klein, Organic Chemistry 1e

18. 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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Klein, Organic Chemistry 1e

19. 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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Klein, Organic Chemistry 1e

20. 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
Copyright 2012 John Wiley & Sons, Inc.
Klein, Organic Chemistry 1e

21. 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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Klein, Organic Chemistry 1e

22. 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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Klein, Organic Chemistry 1e

23. 10.7 Hydroboration-Oxidation
Hydroboration-oxidation for alkynes proceeds through the same mechanism as for alkenes giving the anti-Markovnikov procudt
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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Klein, Organic Chemistry 1e

24. 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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Klein, Organic Chemistry 1e

25. 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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Klein, Organic Chemistry 1e

26. 10.7 Hydroboration-Oxidation
Some bulky borane reagents are shown below
Practice with conceptual checkpoint 10.20
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Klein, Organic Chemistry 1e

27. 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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Klein, Organic Chemistry 1e

28. 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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Klein, Organic Chemistry 1e

29. 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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Klein, Organic Chemistry 1e

30. 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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Klein, Organic Chemistry 1e

31. 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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Klein, Organic Chemistry 1e

32. 10.9 Alkyne Ozonolysis
Predict the product(s) for the following reaction
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Klein, Organic Chemistry 1e

33. 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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Klein, Organic Chemistry 1e

34. 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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Klein, Organic Chemistry 1e

35. 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
Copyright 2012 John Wiley & Sons, Inc.
Klein, Organic Chemistry 1e

36. 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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Klein, Organic Chemistry 1e

37. 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?
Copyright 2012 John Wiley & Sons, Inc.
Klein, Organic Chemistry 1e

38. 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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Klein, Organic Chemistry 1e

39. 10.11 Synthetic Stategies
Give necessary reaction conditions for the multi-step conversions below
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Klein, Organic Chemistry 1e

40. Study Guide for sections 10.4-10.11
DAY 27, Terms to know:
Sections geminal, vicinal, unimolecular, bimolecular, termolecular, enol, keto, tautomerization, carboxylates, alkynide ion
DAY 27, Specific outcomes and skills that may be tested on exam 4:
Sections
Be able to predict alkyne products from dihalides reacting with base, and be able to give a dihalide and reaction conditions that could be used to give a specific alkyne.
Given an alkyne and reaction conditions, be able to predict the product of an addition reaction including the proper stereochemistry and regiochemistry.
Given an alkyne and reaction conditions, be able to draw a reasonable mechanism including not limited to the dissolving metal reaction.
Be able to explain the factors including the rate and carbocation stability that suggests a termolecular hydrohalogenation reaction for alkynes rather than the standard mechanism seen with alkenes.
Be able to predict enol and keto products for both Markovnikov and anti-Markovnikov hydration reactions of alkynes, and be able to give reagents necessary to produce ketones and aldehydes using such reactions.
Be familiar with bulky borane reagents and why they are used in Hydroboration-Oxidation reactions.
Be able to predict the products for ozonolysis reactions starting with alkynes. Be able to give conditions for the conversion of an alkyne into two carbonyls or carboxylates using ozonolysis.
Give conditions for the formation of an alkynide ion, and be able to recognize when such ions form given specific conditions for forming them.
Be able to give reagents necessary to convert alkynide ions into alkynes forming new carbon-carbon bonds, and be able to predict alkyne products from reactions between alkynide ions and alkyl halides.
Be able to give reagents or predict products to complete short 2-5 step syntheses using all of the reactions learned up to this point. Being able to predict correct regio- and stereoselectivity is very important here.
Klein, Organic Chemistry 2e

41. Practice Problems for sections 10.4-10.11
Complete these problems outside of class until you are confident you have learned the SKILLS in this section outlined on the study guide and we will review some of them next class period
Klein, Organic Chemistry 2e

42. Day 28: EXAM 4 Day 29: Review for the comprehensive final
Klein, Organic Chemistry 2e