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ALKEne and alkyne Reactions, continued

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 . Reactions of Alkynes. Similar to alkenes Can also react a second time Mechanism

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ALKEne and alkyne Reactions, continued

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  1. ALKEne and alkyne Reactions, continued Dr. Clower CHEM 2411 Spring 2014 McMurry (8th 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 • Addition of HX and X2 • Hydration • Reduction • 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 X2 • X2 = Br2 or Cl2 • Still anti addition

  6. II. Hydration • Markovnikov addition of water • With alkenes: H2O in H2SO4 or Hg(OAc)2, H2O with reduction • With alkynes: H2O in H2SO4with HgSO4 • Enol (a vinylic alcohol) rearranges to form a carbonyl • Keto-enol tautomerism

  7. Keto-enol tautomerism • 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. BH3•THF, 2. H2O2, NaOH • With alkynes: 1. BH3•THF or Sia2BH, 2. H2O2, NaOH • Sia2BH = disiamylborane • Sia = siamyl = sec-isoamyl • More hindered than BH3, 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 + BaSO4/CaCO3 + Pb salt + quinoline • Syn addition gives cis alkene

  14. Chemical Reduction • Alkyne → trans alkene • Reagents = 2 Li or 2 Na in NH3(l)

  15. IV. Oxidation • Oxidize with O3 or KMnO4 • Both cleave C≡C • Both oxidize to carboxylic acids • Terminal alkynes give CO2

  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 -NH2 • 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 • Alkenes undergo addition of X2 to make a vicinal dihalide • 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.

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