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Acid-Catalyzed Hydration of Alkenes

Acid-Catalyzed Hydration of Alkenes. C. C. C. H. C. O H. Acid-Catalyzed Hydration of Alkenes. reaction is acid catalyzed; typical hydration medium is 50% H 2 SO 4 -50% H 2 O. +. H— O H. Follows Markovnikov's Rule. Hydration – Thermodynamics.

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Acid-Catalyzed Hydration of Alkenes

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  1. Acid-Catalyzed Hydration of Alkenes

  2. C C C H C OH Acid-Catalyzed Hydration of Alkenes • reaction is acid catalyzed; typical hydration medium is 50% H2SO4-50% H2O + H—OH Follows Markovnikov's Rule

  3. Hydration – Thermodynamics • Hydrohalogenation & hydration reactions are at equilibrium.

  4. Hydration – Thermodynamics • How could Le Châtelier’s principle be used to shift the equilibrium to the right or left? • SEE: CONCEPTUAL CHECKPOINT 9.11.

  5. Hydration – Thermodynamics • Similar to hydrohalogenation, the stereochemistry of hydration reactions is controlled by the geometry of the carbocation. • Draw the complete mechanism for the reaction above to show WHY a racemic mixture is formed. • SEE: SKILLBUILDER 9.3.

  6. CH3 H3C H 50% H2SO4 CH2CH3 CH3 C C C 50% H2O H3C CH3 OH Markovnikov'sRule (90%)

  7. CH3 50% H2SO4 CH2 50% H2O OH Follows Markovnikov's Rule (80%)

  8. CH3 H3C CH3 CH3 C C CH2 H3C OH Mechanism • involves a carbocation intermediate • is the reverse of acid-catalyzed dehydrationof alcohols to alkenes H+ + H2O

  9. Addition of Water to Alkene (alcohols)

  10. Addition of Water to Alkenes (alcohols)

  11. Question • Which alkene will undergo acid-catalyzed hydrolysis at the fastest rate? • A) B) • C) D)

  12. CH3 H3C CH3 CH3 C C CH2 H3C OH Principle of Microscopic Reversibility • In an equilibrium process, the same intermediates and transition states are encountered in the forward direction and the reverse, but in the opposite order. H+ + H2O

  13. H2O addition: Markovnikov's Rule

  14. Question • The product isolated from the acid-catalyzed hydration of (Z)-3-methyl-2-pentene is: • A) 2-ethyl-2-butanol • B) 2-ethyl-1-butanol • C) 3-methyl-2-pentanol • D) 3-methyl-3-pentanol

  15. Question • The product isolated from the acid-catalyzed hydration of (E)-3-methyl-2-pentene is: • A) chiral • B) achiral

  16. Oxymercuration-Demercuration • Because rearrangements often produce a mixture of products, the synthetic utility of Markovnikov hydration reactions is somewhat limited. • OXYMERCURATION-DEMERCURATIONis an alternative process that can yield Markovnikov products avoiding rearrangements

  17. Oxymercuration-Demercuration • OXYMERCURATION begins with mercuric acetate. • How would you classify the mercuric cation? • As a nucleophile or an electrophile? • As a Lewis acid or Lewis base?

  18. Oxymercuration-Demercuration • Similar to how we saw the alkene attack a proton previously, it can also attack the mercuric cation. • Resonance stabilizes the mercurinium ion. Can you draw a reasonable resonance hybrid?

  19. Oxymercuration-Demercuration • The mercurinium ion is also a good electrophile, and it can easily be attacked by a nucleophile, even a weak nucleophile such as water. • NaBH4 is generally used to replace the –HgOAc group with a –H group via a free radical mechanism.

  20. Addition of Alcohol to Alkenes (ethers)

  21. + E—Y E Y C C C C Stereochemistry of Addition to Alkenes • Regiochemistry withstanding, in order to understand the • stereochemistry of the product, you must consider two things: • (1) Stereochemistry of the starting alkene (cis or trans; Z or E) • (2) Stereochemistry of the addition (syn or anti)

  22. + E—Y E Y C C C C Stereochemistry of Addition to Alkenes • Optically inactive reactants produce optically inactive products. • (Racemic mixtures are optically inactive) • The correlary is that an optically active starting material • MAY produce an • optically active product depending on the mechanism.

  23. Question • The product isolated from the acid-catalyzed hydration of (E)- or (Z)-3-methyl-2-pentene is: • A) optically active • B) an optically inactive racemic mixture • C) an optically inactive enantiomer

  24. Hydroboration-Oxidation of Alkenes

  25. Hydroboration-Oxidation • To achieve anti-Markovnikov hydration, hydroboration-oxidation is often used. • Note that the process occurs in two steps.

  26. OH Synthesis Suppose you wanted to prepare 1-decanol from 1-decene? • Needed: a method for hydration of alkenes with a regioselectivity opposite to Markovnikov's rule.

  27. 1. hydroboration 2. oxidation OH Synthesis Two-step reaction sequence called hydroboration-oxidation converts alkenes to alcohols with a regiochemistry opposite to Markovnikov's rule.

  28. H BH2 C C C C Hydroboration Step + H—BH2 Hydroboration can be viewed as the addition ofborane (BH3) to the double bond. But BH3 is not the reagent actually used.

  29. H BH2 C C C C Hydroboration Step + H—BH2 Hydroboration reagents: H Diborane (B2H6)normally used in an ether-like solventcalled "diglyme" BH2 H2B H

  30. H BH2 C C C C •• + O – BH3 Hydroboration Step + H—BH2 Hydroboration reagents: Borane-tetrahydrofurancomplex (H3B-THF)

  31. H BH2 H OH C C C C Oxidation Step H2O2, HO– Organoborane formed in the hydroborationstep is oxidized with hydrogen peroxide.

  32. 1. B2H6, diglyme 2. H2O2, HO– OH Example (93%)

  33. H OH 1. H3B-THF CH3 CH3 C C C C 2. H2O2, HO– CH3 H Example H3C CH3 H3C H (98%)

  34. Features of Hydroboration-Oxidation • hydration of alkenes • regioselectivity opposite to Markovnikov's rule • no rearrangement • stereospecific syn addition

  35. OH Example 1. B2H6, diglyme 2. H2O2, HO– (82%)

  36. Stereochemistry of Hydroboration-Oxidation

  37. Features of Hydroboration-Oxidation • hydration of alkenes • regioselectivity opposite to Markovnikov's rule • no rearrangement • stereospecific syn addition

  38. H CH3 CH3 HO H H syn Addition • H and OH become attached to same face of double bond 1. B2H6 2. H2O2, NaOH only product is trans-2-methylcyclopentanol(86%) yield

  39. Question • Hydroboration-oxidation of which one of the following yields a primary alcohol as the major product? • A) B) • C) D)

  40. H OH 1. H3B-THF CH3 CH3 C C C C 2. H2O2, HO– CH3 H Question H3C CH3 H3C H The product is achiral The product is optically active The product is a racemic mixture The product is a single enantiomer

  41. Conversion of Alkenes to Vicinal Halohydrins

  42. Addition of Halogens in the Presence of Water (halohydrins)

  43. C C C C + X2 X X alkenes react with X2 to form vicinal dihalides

  44. C C C C C C C C + X2 X X alkenes react with X2 to form vicinal dihalides alkenes react with X2 in water to give vicinal halohydrins + H2O + X2 X OH + H—X

  45. H H OH Cl H H Examples H2O + Br2 BrCH2CH2OH H2C CH2 (70%) Cl2 H2O anti addition: only product

  46. H H OH Cl H H Examples H2O + Br2 BrCH2CH2OH H2C CH2 (70%) Cl2 H2O anti addition: only product

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