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Reactions of Alkenes

Reactions of Alkenes. Acid Catalyzed Hydration of Alkenes alkene reacts with water in the presence of a strong acid catalyst Markovniknov addition Rearrangements possible Conditions: acid catalyst (dilute H 2 SO 4 or H 3 PO 4 ) excess water. R. R. Reactions of Alkenes.

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Reactions of Alkenes

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  1. Reactions of Alkenes • Acid Catalyzed Hydration of Alkenes • alkene reacts with water in the presence of a strong acid catalyst Markovniknov addition Rearrangements possible • Conditions: • acid catalyst (dilute H2SO4 or H3PO4) • excess water R R

  2. Reactions of Alkenes • Acid catalyzed hydration to form alcohols suffers from several disadvantages: • equilibrium process • some reactions favor the alkene, not the alcohol • many alkenes are insoluble in aqueous acid • side reactions possible: • polymerization • rearrangement (carbonium ion intermediate)

  3. Reactions of Alkenes Example: Predict the major product formed during acid catalyzed hydrolysis (hydration) of 3,3-dimethyl-1-butene.

  4. Reactions of Alkenes • Hydration by Oxymercuration-Demercuration • milder, more universal method for formation of alcohols from alkenes • two step process: • (1) Hg(OAc)2 (aq) mercuric acetate • (2) NaBH4 sodium borohydride HgOAc H

  5. Reactions of Alkenes Mechanism of Oxymercuration: Step 1:Electrophilic attack to form mercurinium ion

  6. Reactions of Alkenes Step 2:Water attacks from the back side to open the ring. Step 3:Water removes a proton forming the organomercurial alcohol

  7. Reactions of Alkenes Step 4: Reduction of the organomercurial alcohol by sodium borohydride

  8. Reactions of Alkenes • Oxymercuration-demercuration gives the Markovnikov product.

  9. (1) H2O (2) - H+ NaBH4 Reactions of Alkenes • Ring opening of the mercurinium ion occurs from the back side: • anti addition of nucleophile H replaces the Hg(OAc) from the same side.

  10. Reactions of Alkenes • Alkoxymercuration-Demercuration • mercuration takes place in an alcohol solvent (instead of in water) • alcohol serves as nucleophile, attacking the mercurinium ion • ether forms as product

  11. Reactions of Alkenes Example: Predict the product formed in each of the following reactions:

  12. Reactions of Alkenes • Hydroboration-oxidation • produces anti-Markovnikov alcohols. • Two step process • (1) BH3.THF • (2) H2O2, OH- • syn addition

  13. Reactions of Alkenes • Hydroboration-oxidation occurs via syn addition of BH3 to the double bond. • Oxidation of the borane to the alcohol occurs with retention of configuration giving the overall syn addition H and OH to the double bond. H2O2 OH-

  14. Reactions of Alkenes Example: Predict the product formed in each of the following reactions.

  15. Reactions of Alkenes Example: How would you accomplish the following transformation? Notice: Product is anti-Markovnikov This reaction must involve more than one step…i.e. there will be an intermediate product. You can work forward or backwards to determine its identity.

  16. Reactions of Alkenes • Working forward: • The only reactions of alkyl bromides so far are substitution or elimination. • Substitution with OH- (E2 conditions) cannot occur at a 3o carbon and would give the alcohol on the wrong carbon anyway. • 1st order conditions would give mixture. • Try elimination to form the Sayzteff product

  17. Reactions of Alkenes • Use hydroboration-oxidation to form anti-Markovnikov alcohol. • Overall synthesis:

  18. Reactions of Alkenes • Halogenation • X2 adds to C=C to form vicinal dihalides • Conditions: • Cl2, Br2 or I2 • Solvent = CH2Cl2, CHCl3, or CCl4 • Note:Iodine is not used often because diiodides decompose easily

  19. Reactions of Alkenes Step 1: Formation of a halonium ion Step 2: Opening of halonium ion Anti addition

  20. Reactions of Alkenes Example: Predict the product formed in the following reaction. Note: Decolorization of a Br2/CCl4 solution is a common test for the presence of an alkene.

  21. Reactions of Alkenes • Formation of Halohydrins • formation of an alcohol with a halogen on the adjacent carbon atom • Conditions: • Cl2, Br2, or I2 • H2O as solvent

  22. Reactions of Alkenes • Formation of halohydrins: • Anti stereochemistry • Markovnikov product • OH on the more substituted carbon

  23. Reactions of Alkenes Example: Predict the product of the following reaction.

  24. Reactions of Alkenes • Catalytic Hydrogenation of Alkenes • formation of an alkane by addition of H2 across the double bond in the presence of a catalyst • Common catalysts: • Pt, Pd, Ni(heterogeneous) • (Ph3P)3RhCl (Wilkinson’s catalyst-homogeneous)

  25. Reactions of Alkenes • Hydrogenation occurs at the surface of the metal catalyst via a syn addition of H2. • H2 adsorbs onto the catalyst surface • Catalyst weakens the H-H bond • p bond of alkene adsorbs onto catalyst • H2 inserts into p bond

  26. Reactions of Alkenes Example: How would you accomplish the following transformation?

  27. Reactions of Alkenes • Formation of Cyclopropanes • addition of a carbene to a C=C bond • Possible Reagents: • CH2N2/hu or CH2N2/heat • CH2I2/Zn/CuCl (Simmons-Smith Reaction) • CHX3/NaOH (or KOH)/H2O • gives dihalocyclopropane

  28. Reactions of Alkenes • Examples: Simmons-Smith Reaction

  29. Reactions of Alkenes • Epoxidation of Alkenes • formation of a three membered cyclic ether by the reaction of an alkene with a peroxyacid • Concerted reaction • Retains stereochemistry present in the alkene

  30. Reactions of Alkenes • Common peroxyacids: • m-chloroperoxybenzoic acid (MCPBA) • peroxybenzoic acid • peroxyacetic acid

  31. Reactions of Alkenes • Examples: Draw the epoxide ring IN THE PLANE unless it’s fused to a larger ring.

  32. Reactions of Alkenes • Hydrolysis of Epoxides • formation of “anti” 1,2-diols (glycols) by acid catalyzed addition of water to the epoxide

  33. Reactions of Alkenes • Depending on the conditions chosen for epoxidation, the reaction may produce either the epoxide or the glycol. • Epoxide formation • MCPBA in CCl4 (or CH2Cl2 or CHCl3) • CH3CO3H • Glycol formation • peroxyacetic acid/H2O • MCPBA/CH2Cl2 then dilute H+ (aq)

  34. Reactions of Alkenes • Examples: H2O trans anti addition

  35. Reactions of Alkenes • Syn Hydroxylation of Alkenes • formation of 1,2-diol (glycol) • syn stereochemistry • Common Reagents: • OsO4/H2O2 • cold, dilute KMnO4/OH- (aq) or

  36. Reactions of Alkenes • Examples: Note: Decolorization of dilute KMnO4 is a common test for the presence of alkenes.

  37. Reactions of Alkenes • Oxidative Cleavage of Alkenes • formation of a mixture of ketones and/or carboxylic acids by cleavage of the C=C of an alkene with warm, concentrated KMnO4 Ketone cannot be oxidized further. Intermediate aldehyde is oxidized further. Mixture of ketone and carboxylic acid is formed in this example.

  38. Reactions of Alkenes • Examples:

  39. Reactions of Alkenes • Ozonolysis of Alkenes • formation of a mixture of aldehydes and ketones by oxidative cleavage of the C=C of an alkene using ozone (O3) and (CH3)2S. • Milder conditions prevent further oxidation of the aldehyde.

  40. Reactions of Alkenes Example: Give the product of the following reaction.

  41. Reactions of Alkenes Example: Show how you would accomplish the following syntheses.

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