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6.10 Acid-Catalyzed Hydration of Alkenes

6.10 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. CH 3. H 3 C. H. 50% H 2 SO 4. CH 2 CH 3. CH 3. C. C. C. 50% H 2 O. H 3 C. CH 3. O H.

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6.10 Acid-Catalyzed Hydration of Alkenes

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  1. 6.10Acid-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

  3. CH3 H3C H 50% H2SO4 CH2CH3 CH3 C C C 50% H2O H3C CH3 OH Follows Markovnikov's Rule (90%)

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

  5. 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

  6. H H3C + : H O C CH2 H3C H slow H H3C + : : + O C CH3 H3C H Mechanism Step (1) Protonation of double bond +

  7. H H3C + : : + O C CH3 H3C H fast CH3 H + : C CH3 O H CH3 Mechanism Step (2) Capture of carbocation by water

  8. CH3 H H : C CH3 O : : O H H CH3 fast CH3 H .. + + : : H C O CH3 O H H CH3 Mechanism Step (3) Deprotonation of oxonium ion + +

  9. Relative Rates Acid-catalyzed hydration • ethylene CH2=CH2 1.0 • propene CH3CH=CH2 1.6 x 106 • 2-methylpropene (CH3)2C=CH2 2.5 x 1011 • The more stable the carbocation, the fasterit is formed, and the faster the reaction rate.

  10. 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

  11. 6.11Thermodynamics of Addition-Elimination Equlibria

  12. Hydration-Dehydration Equilibrium How do we control the position of the equilibrium and maximize the product?

  13. Le Chatelier’s Principle A system at equilibrium adjusts so to minimize any stress applied to it. For the hydration-dehydration equilibria, the key stress is water. Adding water pushes the equilibrium toward more product (alcohol). Removing water pushes the equilibrium toward more reactant (alkene).

  14. Le Chatelier’s Principle At constant temperature and pressure a reaction proceeds in a direction which is spontaneous or decreases free energy (G). The sign of G is always positive, but G can be positive or negative. G = Gproducts – Greactants Spontaneous when G < 0

  15. Le Chatelier’s Principle For a reversible reaction: aA + bB  cC + dD The relationship between G and Go is: R = 8.314 J/(mol.K) and T is the temperature in K

  16. Le Chatelier’s Principle At equilibrium G = 0 and the following becomes true: Substituting Keq into the previous equation gives: Go = - RT lnKeq Reactions for Go positive are endergonic and for Go negative are exergonic.

  17. 6.12Hydroboration-Oxidation of Alkenes

  18. 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.

  19. 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.

  20. 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.

  21. 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

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

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

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

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

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

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

  28. 6.13Stereochemistry of Hydroboration-Oxidation

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

  30. 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

  31. 6.14Mechanism of Hydroboration-Oxidation

  32. 1-Methylcyclopentene + BH3 • syn addition of Hand B to double bond • B adds to less substituted carbon

  33. Organoborane Intermediate

  34. Add Hydrogen Peroxide • OH replaces B on same side

  35. trans-2-Methylcyclopentanol

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