1. Organic Chemistry Second Edition Chapter 10 David Klein Alkynes
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Klein, Organic Chemistry 2e

2. 10.1 Alkynes Alkynes are molecules that incorporate a CC triple bond
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Klein, Organic Chemistry 2e

3. 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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Klein, Organic Chemistry 2e

4. 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 poison-tipped arrows by South American tribes
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Klein, Organic Chemistry 2e

5. 10.1 Alkyne Uses An example of a synthetic alkyne is ethynylestradiol
How do you think a CC triple bond affects the molecules geometry? Its rigidity? Its intermolecular attractions?
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
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Klein, Organic Chemistry 2e

6. 10.2 Alkyne Nomenclature
Alkynes are named using the same procedure we used in Chapter 4 to name alkanes with minor modifications
Identify the parent chain, which should include the CC triple bond
Identify and Name the substituents
Assign a locant (and prefix if necessary) to each substituent giving the CC triple bond the lowest number possible
List the numbered substituents before the parent name in alphabetical order. Ignore prefixes (except iso) when ordering alphabetically
The CC triple bond locant is placed either just before the parent name or just before the -yne suffix
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Klein, Organic Chemistry 2e

7. 10.2 Alkyne Nomenclature
Alkynes are named using the same procedure we used in Chapter 4 to name alkanes with minor modifications
Identify the parent chain, which should include the CC triple bond
Identify and name the substituents.
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Klein, Organic Chemistry 2e

8. 10.2 Alkyne Nomenclature
Alkynes are named using the same procedure we used in Chapter 4 to name alkanes with minor modifications
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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Klein, Organic Chemistry 2e

9. 10.2 Alkyne Nomenclature
Alkynes are named using the same procedure we used in Chapter 4 to name alkanes with minor modifications
List the numbered substituents before the parent name in alphabetical order. Ignore prefixes (except iso) when ordering alphabetically
The CC triple bond locant is placed either just before the parent name or just before the -yne suffix
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Klein, Organic Chemistry 2e

10. 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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Klein, Organic Chemistry 2e

11. 10.2 Alkyne Nomenclature Name the molecule below
Recall that when triple bonds are drawn their angles are 180°
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12. 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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Klein, Organic Chemistry 2e

13. 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

14. 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

15. 10.4 Preparation of Alkynes
Like alkenes, alkynes can also be prepared by elimination
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Klein, Organic Chemistry 2e

16. 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 2e

17. 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 2e

18. 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 2e

19. 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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Klein, Organic Chemistry 2e

20. 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 2e

21. 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 2e

22. 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 2e

23. 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 2e

24. 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 2e

25. 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 2e

26. 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 2e

27. 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 2e

28. 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 2e

29. 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 2e

30. 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
Vinylic
carbocation
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Klein, Organic Chemistry 2e

31. 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 2e

32. 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 2e

33. 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 2e

34. 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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Klein, Organic Chemistry 2e

35. 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 2e

36. 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 2e

37. 10.7 Hydroboration-Oxidation
Hydroboration-oxidation for alkynes proceeds through the same mechanism as for alkenes giving the anti-Markovnikov 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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Klein, Organic Chemistry 2e

38. 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 2e

39. 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 2e

40. 10.7 Hydroboration-Oxidation
Some bulky borane reagents are shown below
Practice with conceptual checkpoint 10.20
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Klein, Organic Chemistry 2e

41. 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 2e

42. 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 2e

43. 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 2e

44. 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 2e

45. 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 2e

46. 10.9 Alkyne Ozonolysis
Predict the product(s) for the following reaction
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Klein, Organic Chemistry 2e

47. 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 2e

48. 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 2e

49. 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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Klein, Organic Chemistry 2e

50. 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 2e

51. 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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Klein, Organic Chemistry 2e

52. 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 2e

53. 10.11 Synthetic Stategies
Give necessary reaction conditions for the multi-step conversions below
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Klein, Organic Chemistry 2e

54. Additional Practice Problems
Name the molecule
Draw the structure of 2,2-dimethyl-6-chloro-3-heptyne
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Klein, Organic Chemistry 2e

55. Additional Practice Problems
Give 2 sets of reagents that could be used to synthesize 1-pentyne through elimination reactions.
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Klein, Organic Chemistry 2e

56. 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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Klein, Organic Chemistry 2e

57. 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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Klein, Organic Chemistry 2e

58. Additional Practice Problems
Determine necessary reagents to complete the synthesis below.
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Klein, Organic Chemistry 2e