10.1 Alkynes. Alkynes are molecules that incorporate a C C triple bond Klein, Organic Chemistry 1e. Copyright 2012 John Wiley & Sons, Inc.

Transcription

1 10.1 Alkynes Alkynes are molecules that incorporate a C C triple bond. 10-1

2 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? 10-2

3 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. 10-3

4 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 molecule s geometry? Its rigidity? Its intermolecular attractions? 10-4

5 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. 10-5

6 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. 10-6

7 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. 10-7

8 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. 10-8

9 10.2 Alkyne Nomenclature In addition to the IUCAP 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 10.2 Alkyne Nomenclature Name the molecule below. Recall that when triple bonds are drawn, their angles are

11 10.3 Alkyne Acidity Recall that terminal alkynes have a lower pk a than other hydrocarbons. Acetylene is 19 pk a units more acidic than ethylene, which is times stronger. Does that mean that terminal alkynes are strong acids? 10-11

12 10.3 Alkyne Acidity Because acetylene (pk a =25) is still much weaker than water (pk a =15.7), a strong base is needed to make it react. Recall from Chapter 3 that 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

13 10.3 Alkyne Acidity Use ARIO to rationalize the equilibria below. A base s conjugate acid pk a must be greater than 25 for it to be able to deprotonate a terminal alkyne

14 10.4 Preparation of Alkynes Like alkenes, alkynes can also be prepared by elimination

15 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

16 10.4 Preparation of Alkynes Often, excess equivalents of NaNH 2 are used to shift the equilibrium toward the elimination products. NH 1-2 is quite strong, so if a terminal alkyne is produced, it will be deprotonated. That equilibrium will greatly favor products

17 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

18 10.5 Reduction of Alkynes Like alkenes, alkynes can readily undergo hydrogenation. Two equivalents of H 2 are consumed for each alkyne alkane conversion. The cis alkene is produced as an intermediate. WHY cis? 10-18

19 10.5 Reduction of Alkynes Poisoned Catalyst A deactivated or poisoned catalyst can be used to selectively react with the alkyne. Lindlar s catalyst and P-2 (Ni 2 B complex) are common examples of a poisoned catalysts

20 10.5 Reduction of Alkynes Poisoned Catalyst Is this a syn or anti addition? Practice with CONCEPTUAL CHECKPOINT

21 10.5 Reduction of Alkynes Dissolving Metal Reductions Reduction with H 2 gives syn addition. Dissolving metal conditions can give anti addition producing the trans alkene. Ammonia has a boiling point of 33 C, so the temperature for these reactions must remain very low. Why can t water be used as the solvent? 10-21

22 10.5 Reduction of Alkynes 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? 10-22

23 10.5 Reduction of Alkynes 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

24 10.5 Reduction of Alkynes Dissolving Metal Reductions Mechanism step 2 and 3: Draw the product for step 3 of the mechanism

25 10.5 Reduction of Alkynes Dissolving Metal Reductions Mechanism step 4: Do the pk a values for NH 3 and the alkene favor the proton transfer? 10-25

26 10.5 Reduction of Alkynes Dissolving Metal Reductions Predict the product(s) for the following reactions. Practice with CONCEPTUAL CHECKPOINT

27 10.5 Reduction of Alkynes Summary Familiarize yourself with the reagents necessary to manipulate alkynes. Practice with CONCEPTUAL CHECKPOINT

28 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? 10-28

29 10.6 Hydrohalogenation of Alkynes Modeled after the hydrohalogenation of alkenes, you might expect alkynes to react by the same mechanism. Yet, the mechanism above does not explain all observed phenomena: A slow reaction rate 3 rd order overall rate law Vinylic carbocations are especially unstable 10-29

30 10.6 Hydrohalogenation of Alkynes Kinetic studies on the hydrohalogenation of an alkyne suggest that the rate law is 1 st order with respect to the alkyne, and 2 nd order with respect to HX. What type of collision would result in such a rate law? Unimolecular, bimolecular, or termolecular? 10-30

31 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

32 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

33 10.7 Hydration of Alkynes Like alkenes, alkynes can also undergo acid catalyzed Markovnikov hydration. The process is generally catalyzed with HgSO 4 to compensate for the slow reaction rate that results from the formation of vinylic carbocation

34 10.7 Hydration of Alkynes HgSO 4 catalyzed hydration involves the mercury (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? 10-34

35 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

36 10.8 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

37 10.8 Hydroboration-Oxidation In general, we can conclude that a C=O double bond is more stable than a C=C double bond. WHY? 10-37

38 10.8 Hydroboration-Oxidation After the BH 2 and H groups have been added across the C=C double bond, in some cases, an undesired second addition can take place. R H B H H To block out the second unit of BH 3 from reacting with the intermediate, bulky borane reagents are often used. H R H H B BH 2 H Undesired product

39 10.8 Hydroboration-Oxidation Some bulky borane reagents are shown below. Practice with CONCEPTUAL CHECKPOINT

40 10.8 Hydroboration-Oxidation Predict products for the following reaction. Draw the alkyne reactant and reagents that could be used to synthesize the following molecule. O 10-40

41 10.8 Hydration Regioselectivity Markovnikov hydration leads to a ketone. Anti-Markovnikov hydration leads to an aldehyde. Practice with SKILLBUILDER

42 10.9 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

43 10.9 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

44 10.10 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

45 10.10 Alkyne Ozonolysis Predict the product(s) for the following reaction. O 3 H 2 O 10-45

46 10.11 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. Which has a higher pk a, NH 3 or R-C C-H? WHY? 10-46

47 10.11 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

48 10.11 Alkylation of Terminal Alkynes Acetylene can be used to perform a double alkylation. Why will the reaction be unsuccessful if the NaNH 2 and Et-Br are added together? 2 eq. N anh 2 2 eq. Et - B r Complex target molecules can be made by building a carbon skeleton and converting functional groups. Practice with SKILLBUILDER

49 10.12 Synthetic Strategies Recall the methods for increasing the saturation of alkenes and alkynes. But, what if you want to reverse the process or decrease saturation? 10-49

50 10.12 Synthetic Strategies 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? Step A Step B chapter 11 X 10-50

51 10.12 Synthetic Strategies In the alkene to alkyne conversion above, why is water needed in step 3) of that reaction? Practice with SKILLBUILDER

52 10.12 Synthetic Strategies Give necessary reaction conditions for the multi-step conversions below. Br + HO OH Br + En + En 10-52