2. © 2011 Pearson Education, Inc. 1 Organic Chemistry 6 th Edition Paula Yurkanis Bruice Chapter 4 The Reactions of Alkenes

3. © 2011 Pearson Education, Inc. 2 Electrophilic Addition of Alkenes: Reaction and Synthesis Reaction of an alkene is an addition reaction. synthesis of an alkene an elimination reaction

4. © 2011 Pearson Education, Inc. 3 Electrophilic Addition of Alkenes

5. © 2011 Pearson Education, Inc. 4 Addition of Hydrogen Halides

6. © 2011 Pearson Education, Inc. 5 What is the product?

7. © 2011 Pearson Education, Inc. 6 a more stable carbocation Carbocation Formation Is the Rate- Limiting Step

8. © 2011 Pearson Education, Inc. 7 Carbocation Stabilities Alkyl groups decrease the concentration of positive charge in the carbocation

9. © 2011 Pearson Education, Inc. 8 Stabilization of a Carbocation

10. © 2011 Pearson Education, Inc. 9 Species Stability Predicted by Hyperconjugation Radical Stability Carbocation Stability Carbanion Stability

11. © 2011 Pearson Education, Inc. 10 Molecular Orbital Diagram in a Hyperconjugation System

12. © 2011 Pearson Education, Inc. 11 Hammond postulate: the transition state will be more similar to the species that it is closer to energetically Exergonic reaction: early transition state Endergonic reaction: late transition state

13. © 2011 Pearson Education, Inc. 12 The transition state is more similar in structure to the species to which it is more similar in energy

14. © 2011 Pearson Education, Inc. 13 In an electrophilic addition reaction, the more stable carbocation forms faster. Reaction Coordinate Diagram for the Addition of H + to 2-Methylpropene

15. © 2011 Pearson Education, Inc. 14 Electrophilic Addition Reactions Are Regioselective

16. © 2011 Pearson Education, Inc. 15 In a regioselective reaction, one constitutional isomer is the major or the only product. Markovnikov’s Rule The electrophile adds to the sp 2 carbon that is bonded to the greater number of hydrogens:

17. © 2011 Pearson Education, Inc. 16 Addition of Water to Alkene What is the electrophile? What nucleophile is present in the greatest concentration?

18. © 2011 Pearson Education, Inc. 17 Acid Catalyzed Addition of Alcohol Mechanism: Varying the alkene and alcohol starting materials will afford structurally diverse ether products:

19. © 2011 Pearson Education, Inc. 18 Carbocations can trap nucleophiles: Carbocations can rearrange: Carbocations can trap alkene nucleophiles: Carbocations can lose a proton to afford an alkene: The Fate of Carbocations:

20. © 2011 Pearson Education, Inc. 19 Rearrangement of Carbocations 1,2-hydride shift a more stable carbocation

21. © 2011 Pearson Education, Inc. 20 a more stable carbocation Rearrangement of Carbocations 1,2-methyl shift

22. © 2011 Pearson Education, Inc. 21 A carbocation does not always rearrange: Why? Because rearrangement has not resulted in a more a stable carbocation.

23. © 2011 Pearson Education, Inc. 22 Carbocation Rearrangement with Ring Expansion Rearrangement favored by relief of ring strain and tertiary carbocation formation.

24. © 2011 Pearson Education, Inc. 23 Carbocation Reactions in Nature

25. © 2011 Pearson Education, Inc. 24 Mechanism for the addition of bromine to an alkene Addition of a halogen to an alkene

26. © 2011 Pearson Education, Inc. 25 Bromonium Ion Formation The same reaction with chlorine affords a chloronium ion:

27. © 2011 Pearson Education, Inc. 26 Addition of halogens to an alkene in the presence of water affords a halohydrin, a compound with an alcohol and a halogen on adjacent carbons. Addition of halogens to an alkene in the presence of a primary alcohol affords a halo ether by the same mechanism. Addition of Halogens in the Presence of Water or Alcohol

28. © 2011 Pearson Education, Inc. 27 Consider the two possible transition states: Trapping of the Bromonium or Chloronium Ion by Nucleophilic Solvents Is Regioselective Therefore:

29. © 2011 Pearson Education, Inc. 28 Oxymercuration of an Alkene Trapping of the cyclic mercurium ion by water is regioselective:

30. © 2011 Pearson Education, Inc. 29 Demercuration by Reduction Reduction increases the number of C-H bonds or decreases the number of C-O, C-N, or C-X bonds Reduction Mechanism:

31. © 2011 Pearson Education, Inc. 30 The Oxymercuration-Demercuration Reaction A means of avoiding rearrangements

32. © 2011 Pearson Education, Inc. 31 Addition of a Peroxyacid to an Alkene An Oxidation Reaction Mechanism of epoxidation: Oxidation decreases the number of C-H bonds or increases the number of C-O, C-N, or C-X bonds

33. © 2011 Pearson Education, Inc. 32 Naming of Epoxide There are three ways of naming an epoxide: As an epoxy-substituted alkane. As an alkene, just add the word oxide for an epoxy compound. As an oxirane ring derivative. Alkene Oxide Naming Alkane and Oxirane Naming

34. © 2011 Pearson Education, Inc. 33 The Hydroboration Reaction Formally the Anti-Markovnikov addition of water Markovnikov addition of water Anti-Markovnikov addition of water Hydroboration is a two-step process: 1. Trialkylborane formation 2. Hydroperoxide and base treatment

35. © 2011 Pearson Education, Inc. 34 Anti-Markovnikov Addition of BH 3 favored. Markovnikov Addition of HBr favored A pericyclic reaction: Alkylborane Formation Regioselectivity due to carbocation formation is the transition state and steric interaction between borane and the alkene substituents :

36. © 2011 Pearson Education, Inc. 35 Trialkylborane Formation Insertion of B-H bonds into alkene bonds continues until the trialkylborane is formed: Driving force for trialkylborane formation: Stabilization of the vacant boron p-orbital by hyperconjugation.

37. © 2011 Pearson Education, Inc. 36 Oxidation of theTrialkylborane Followed by Base-Catalyzed Hydrolysis Treatment with hydrogen peroxide in aqueous hydroxide: Borate Ester, the Oxidation Product Hydrolysis:Bond Cleavage Mediated by a Water Component, i.e. Hydroxide

38. © 2011 Pearson Education, Inc. 37 Hydroboration Examples Do not forget to number the reagents. This is a two-step reaction!

39. © 2011 Pearson Education, Inc. 38 Addition of Hydrogen to Alkenes

40. © 2011 Pearson Education, Inc. 39 Catalytic Hydrogenation of an Alkene

41. © 2011 Pearson Education, Inc. 40 Potential Energies of Pentene Isomers The most stable alkene has the smallest heat of hydrogenation.

42. © 2011 Pearson Education, Inc. 41 Relative Stabilities of Alkyl-Substituted Alkenes Alkyl substituents stabilize both alkenes and carbocations. The most stable alkene has the greatest number of alkyl groups bonded to its sp 2 carbon.

43. © 2011 Pearson Education, Inc. 42 Steric Strain in Alkenes

44. © 2011 Pearson Education, Inc. 43 Partial Hydrogenation Cis - Alkene Unsaturated fat with cis double bonds. An oil at room temperature. Trans - AlkeneSaturated Chain Partially Saturated Chain Partially saturated fat with cis and the relatively stable trans double bond. A butter-like solid at room temperature.