1. ALKENE AND ALKYNE REACTIONS, CONTINUED Dr. Clower CHEM 2411 Spring 2014 McMurry (8 th ed.) sections 7.7-7.8, 7.10-7.11, 10.3-10.4, 8.2-8.8, 8.10, 8.12, 9.3-9.8, 7.1, 8.1, 9.2, 9.9

2. Reactions of Alkynes Similar to alkenes Can also react a second time Mechanism Which is faster, reaction with alkenes or alkynes? Alkenes have more stable carbocation intermediate I. Addition of HX and X 2 II. Hydration III. Reduction IV. Oxidation

3. I. Addition of HX Terminal alkynes: regiochemistry = Markovnikov Anti-Markovnikov if peroxides are present Internal alkynes (unsymmetrical) = mixture of products

4. Addition of HX

5. I. Addition of X 2 X 2 = Br 2 or Cl 2 Still anti addition

6. II. Hydration Markovnikov addition of water With alkenes: H 2 O in H 2 SO 4 or Hg(OAc) 2, H 2 O with reduction With alkynes: H 2 O in H 2 SO 4 with HgSO 4 Enol (a vinylic alcohol) rearranges to form a carbonyl Keto-enol tautomerism

7. Tautomers = constitutional isomers which rapidly interconvert Keto tautomer typically more stable than enol Catalyzed by acid or base

8. Draw the major product for each of the following reactions.

9. Hydration Anti-Markovnikov addition of water With alkenes: 1. BH 3 THF, 2. H 2 O 2, NaOH With alkynes: 1. BH 3 THF or Sia 2 BH, 2. H 2 O 2, NaOH Sia 2 BH = disiamylborane Sia = siamyl = sec-isoamyl More hindered than BH 3, so prevents addition of 2 borane molecules Product still undergoes keto-enol tautomerism Internal alkynes yield ketones Terminal alkynes yield aldehydes

10. Draw the major product for each of the following reactions.

11. Hydration Internal alkynes, both reagents give the same products Teminal alkynes, different products

12. III. Reduction Alkyne reduce to alkene or alkane Depends on the reagent/conditions used Types of reduction: Catalytic reduction Chemical reduction

13. Catalytic Reduction Alkyne → alkane Cannot stop reaction at alkene with these catalysts Can form alkene with Lindlar catalyst Pd + BaSO 4 /CaCO 3 + Pb salt + quinoline Syn addition gives cis alkene

14. Chemical Reduction Alkyne → trans alkene Reagents = 2 Li or 2 Na in NH 3 (l)

15. IV. Oxidation Oxidize with O 3 or KMnO 4 Both cleave C≡C Both oxidize to carboxylic acids Terminal alkynes give CO 2

16. Draw the major product for each of the following reactions.

17. Acidity of Alkynes Terminal alkynes are weak acids More acidic than alkenes or alkanes Form somewhat stable conjugate base = acetylide ion Electon pair close to nucleus because more s character

18. Acetylide Ion Strong base Stronger than HO - or RO - Not as strong as - NH 2 Acetylide ion can act as a base or a nucleophile

19. Acetylide Ion as a Nucleophile React with methyl or primary alkyl halides Undergo substitution reaction Form a new, larger alkyne Alkylation reaction C-C bond making reaction Example:

20. Alkylation Reactions

21. Acetylide Ion as a Base React with secondary or tertiary alkyl halides Undergo elimination reaction Dehydrohalogenation (eliminate H-X) Elimination reactions are used to synthesize alkenes and alkynes

22. Preparation of Alkenes Alcohol eliminates water to form an alkene Reagent = base Alkyl halide eliminates HX to form an alkene Reagent = acid

23. Preparation of Alkynes A two-step process from alkenes 1. Alkenes undergo addition of X 2 to make a vicinal dihalide 2. The vicinal dihalide undergoes 2 elimination reactions to yield the alkyne

24. How could you prepare 2-butyne from 2-butene? How could you prepare 2-pentyne from 3-pentanol?

25. Synthesis You will be given a product. Your goal is to determine how to make that product from simpler starting materials using reactions we have studied. Consider: How many carbons are in the starting material and product? Do you need to make any C-C bonds? If so, how will you do that? What functional groups are in the starting material? What can you do with those functional groups? What functional groups are in the product? How do you know how to make those functional groups? Try working backwards (retrosynthesis). Look McMurry section 9.9 for strategies and worked examples Example: Propose a synthesis of cis-3-hexene from acetylene. You may use any alkyl halide as a source of carbon.

26. Propose a synthesis of cis-3-hexene from acetylene.

27. Propose a synthesis of 2-butanone from ethylene.