Reactions of Alkenes

1. Reactions of Alkenes • Addition is the most common reaction of alkenes. • The π bond breaks and two σ bonds form. • There is a loss of an element of unsaturation.

2. Reactions of Alkenes • Electrophilic additions • HX • compare to free radical addition of HBr • acid-catalyzed hydration • oxymercuration-demercuration • alkoxymercuration-demercuration • hydroboration-oxidation • cationic polymerization • Reduction: Catalytic hydrogenation

3. Reactions of Alkenes • Addition of carbenes: cyclopropanation • Oxidative additions • X2 • halohydrin formation • epoxidation and acid-catalyzed ring opening • anti hydroxylation • syn hydroxylation • Oxidative cleavage • ozonolysis • potassium permanganate

4. Classification of Reactions • Oxidations • addition of O or O2 • addition of X2 • loss of H2 • Reductions • loss of O or O2 • loss of X2 • addition of H2 or H- • Neither

5. Electrophilic Addition • The π bond acts as a nucleophile. It can attack an electrophile. • General mechanism for many electrophilic additions:

6. Orientation of Addition • Also called regiochemistry • Markovnikov’s rule • The addition of a proton acid to the double bond of an alkene results in a product with the acid proton bonded to the C atom that already holds the greater number of H atoms. (Result of empirical observations) • (Extended) In an electrophilic addition to an alkene, the electrophile adds in such a way as to generate the most stable intermediate.

7. Electrophilic Addition of HX • carbocation intermediate • Markovnikov product • produces alkyl halides • solvent, if needed: CH2Cl2, CH3CN, etc.

8. Free-Radical Addition of HBr • Requires the presence of a peroxide and heat. • Due to unfavorable energetics, this mechanism does not apply to HCl or HI.

9. Free-Radical Addition of HBr • free-radical intermediate • The more substituted radical is more stable, just like for the carbocation.

10. Free-Radical Addition of HBr • anti-Markovnikov product,“the peroxide effect” • This reaction is much faster than the uncatalyzed ionic addition. anti-Markovnikov product

11. Addition to C=C Be able to recognize a peroxide!

12. Acid-Catalyzed Hydration • Adds H and OH to the double bond. • Step 1: protonation by the acid • carbocation intermediate

13. Acid-Catalyzed Hydration • A dilute acid like H2SO4or H3PO4 is needed as a catalyst. • Excess water is needed to drive equilibrium toward products and to prevent the reverse reaction from happening.

14. Acid-Catalyzed Hydration • Step 2: nucleophilic attack by water

15. Acid-Catalyzed Hydration • Step 3: Deprotonation by water. • Acid is regenerated • Markovnikov product • produces alcohols

16. Acid-Catalyzed Hydration • Disadvantages • It is an equilibrium process and sometimes the alkene is favored. • Many alkenes are insoluble in aqueous acid. • Side reactions are possible. • polymerization • rearrangement (why?) • The acid used may attack other functional groups present.

17. Hydration by Oxymercuration-Demercuration - A Better Way • A more universal and milder method for making alcohols from alkenes. • Can be used on more alkenes than acid-catalyzed hydration. Again, adds H and OH to the C=C. • A 2-step process • requires mercuric acetate Hg(OAc)2 in water • followed by NaBH4 in base. • Mercurinium ion intermediate…no carbocation, so no rearrangement.

18. Hydration by Oxymercuration-Demercuration • Step 1: oxymercuration • Mercuric acetate Hg(OAc)2in water dissociates into the acetate ion and +HgOAc, an electrophile. • A mercurinium ion intermediate forms.

19. Hydration by Oxymercuration-Demercuration • Water attacks from the back side. • Water attacks the more substituted C atom because it has more of a positive charge.

20. Hydration by Oxymercuration-Demercuration • “Product” is the organomercurial alcohol. Anti addition of HgOAc.

21. Hydration by Oxymercuration-Demercuration • Step 2 - demercuration • Sodium borohydride in base (OH- - to keep it from reacting with H2O) reduces the mercury compound to the alcohol. • This is the most commonly used method to hydrate alkenes in the laboratory…but be careful! Mercury compounds are toxic.

22. Hydration by Oxymercuration-Demercuration • Markovnikov product: an alcohol • anti addition of H and HgOAc

23. Alkoxymercuration-Demercuration • Used to add an alkoxy group (OR, an ether) to a double bond. • Requires mercuric acetate Hg(OAc)2in the appropriate alcohol followed by NaBH4 in base. • Again, there is a mercurinium ion intermediate not a carbocation, so there is no rearrangement.

24. Alkoxymercuration-Demercuration • Step 1: alkoxymercuration • Mercuric acetate Hg(OAc)2in the appropriate alcohol dissociates into the acetate ion and +HgOAc, an electrophile. • A mercurinium ion intermediate forms.

25. Alkoxymercuration-Demercuration • ROH attacks the more substituted C atom because it can share the positive charge.

26. Alkoxymercuration-Demercuration • “Product” is the organomercurial ether. Anti addition of HgOAc.

27. Alkoxymercuration-Demercuration • Step 2: demercuration • NaBH4 in base (OH- - to keep it from reacting with H2O) reduces the mercury compound to the ether. • Markovnikov product: an ether • anti addition of H and HgOAc

28. Alkoxymercuration-Demercuration

29. Hydration by Hydroboration • Results in anti-Markovnikov addition of H and OH. • 2-step process • requires borane and THF (BH3•THF) • followed by H2O2 in OH-

30. Hydration by Hydroboration • Step 1: • Addition of BH3, an electrophile, to C=C. transition state

31. Hydration by Hydroboration • Step 1 continued: • Borane (BH3) adds in one step, with B adding to the less hindered C atom. transition state

32. Hydration by Hydroboration • Step 2: • BH2 is removed by oxidation with H2O2 in base (OH-). • This results in -BH2 being replaced with -OH.

33. Hydration by Hydroboration • Because the borane adds in one step, the hydration of the alkene is syn, with the H and the OH both adding to the same side of the double bond. When borane adds to the other side of C=C, the (R) enantiomer is the product. The result is a racemic mixture.

34. Hydration by Hydroboration How would you accomplish this change? Hint: work backward

35. Cationic Polymerization • Can occur with an alkene and a trace of acid. isobutylene, cold

36. Cationic Polymerization • Once the carbocation is formed, it can attack the pi bond of another alkene molecule. • The polymer will grow until loss of a proton terminates chain growth.

37. Catalytic Hydrogenation • Adds H2 to the double bond. • Heterogeneous catalysis • Hydrogenation takes place on the surface of the solid metal catalyst (Pt, Pd, or Ni). • PH2 = 1 atm, solvent = alcohol or alkane • syn addition Both molecules adsorb to the surface of the catalyst.

38. Catalytic Hydrogenation

39. Catalytic Hydrogenation • Homogeneous catalysis is also possible. • Some catalysts are soluble, such as Wilkinson’s catalyst (Ph3P)3RhCl. • Other soluble catalysts are chiral and may be used to convert an optically inactive alkene to an optically active product.

40. Reactions of Alkenes • Addition of carbenes: cyclopropanation • Oxidative additions • X2 • halohydrin formation • epoxidation and acid-catalyzed ring opening • anti hydroxylation • syn hydroxylation • Oxidative cleavage • ozonolysis • potassium permanganate

41. Cyclopropanation • A carbene is an uncharged, reactive intermediate that adds to a double bond to form a cyclopropane. • Methylene is the simplest :CH2 • It is a strong electrophile (like BH3) due to an unfilled octet. • A carbene will add a C to the double bond.

42. Cyclopropanation • From carbenes or carbenoids • Carbene from diazomethane • N2CH2 N2 + :CH2 • Toxic, explosive • Inserts into C-H bonds (too aggressive) • Not desirable for cyclopropanation

43. Cyclopropanation • From carbenes or carbenoids • Carbene from alpha elimination • Requires a halogenated compound that has at least one somewhat acidic H • Requires a strong base to remove the H • CHBr3 + KOH  :CBr2 + H2O + Br-

44. Cyclopropanation • From carbenes or carbenoids • Carbenoid • ICH2ZnI, Simmons-Smith reaction • CH2I2 + Zn(Cu)  ICH2ZnI

45. Cyclopropanation - Examples alpha elimination Simmons-Smith reaction cis/trans stereochemistry is preserved.

46. Oxidative Addition of X2 • adds X2 to the C=C (an oxidation) • halonium ion intermediate (-ium = cation) • mostly for Cl2 and Br2 addition • diiodide products decompose too easily Step 1 Br2 is electrophilic by induction.

47. Electrophilic Addition • The π bond acts as a nucleophile. It can attack an electrophile. • General mechanism for many electrophilic additions.

48. Oxidative Addition of X2 • Ring strain and (+) charge on halogen make halonium ion electrophilic. • Halide ion acts as nucleophile. • Backside attack • Best solvents: CH2Cl2, CHCl3, CCl4 Step 2

49. Oxidative Addition of X2 • anti addition: halide attacks opposite the halonium ion

50. Oxidative Addition of Br2 • Since bromine is colored, this reaction can be used as a test for the presence of a double bond. • Br2(l) is dark brown. • If it adds to a C=C, the dibromide product is usually colorless.