1 / 70

Reactions of Alkenes

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. Reactions of Alkenes. Electrophilic additions HX compare to free radical addition of HBr acid-catalyzed hydration

cael
Download Presentation

Reactions of Alkenes

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  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 strong acid is needed as a catalyst • dilute H2SO4 or H3PO4 • 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 strong acid used may affect 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.

  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 OH • This will be important as we consider the next reaction of alkenes, alkoxymercuration-demercuration.

  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.

  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 OR

  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. • 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.

More Related