Chapter 4-1. Alkenes: Reactions and Synthesis

1. Chapter 4-1. Alkenes: Reactions and Synthesis Based on McMurry’s Organic Chemistry, 6th edition

2. Alkene Reactions • Addition • Substitution • Diels-Alder • Cleavage

3. Addition Reaction • The characteristic reaction of alkenes is addition—the  bond is broken and two new  bonds are formed.

4. Diverse Reactions of Alkenes • Alkenes react with many electrophiles to give useful products by addition reactions (often through special reagents) • alcohols (add H-OH) • alkanes (add H-H) • halohydrins (add HO-X) • dihalides (add X-X) • halides (add H-X) • diols (add HO-OH)

6. Types of Additions

7. Addition Reactions • Syn addition • Anti addition • Carbocation formation • Radical formation

8. Syn & Anti Addition to Alkenes • Because the carbon atoms of a double bond are both trigonal planar, the elements of X and Y can be added to them from the same side or from opposite sides.

9. syn-Addition versus anti-Adition syn addition anti addition

10. Syn Addition Reactions • Hydrogenation • Hydroxylation • Hydroboration

11. Anti Addition Reactions • Hydroxylation • Halogenation • Halohydrin formation

12. Hydroxylation • Syn addition • Anti addition

13. Anti addition • Halogenation • Halohydrin formation Alkene Addition Reactions via Halonium Ions

14. Carbocation Formation • Hydration • Alcohol formation • HX Addition • Alkyl halide formation

15. Electrophilic Addition The most common reaction of alkenes is electrophilic addition. E+ Nu-- loosely held  electrons E + Carbocation intermediate E+ electrophile nucleophile E+ Nu Nu--

16. Electrophilic Addition of HX to Alkenes • General reaction mechanism: electrophilic addition • Attack of electrophile (such as HBr) on  bond of alkene produces carbocation and bromide ion • Carbocation is itself an electrophile, reacting with nucleophilic bromide ion

17. Electrophilic Addition for Syntheses • The reaction is successful with HCl and with HI as well as HBr. Note that HI is generated from KI and phosphoric acid

18. Regioselectivity ofHydrogen Halide Addition:Markovnikov's Rule

19. Orientation of Electrophilic Addition: Markovnikov’s Rule • In an unsymmetrical alkene, HX reagents can add in two different ways, but one way may be preferred over the other • If one orientation predominates, the reaction is regiospecific • Markovnikov observed in the 19th century that in the addition of HX to alkene, the H attaches to the carbon with the most H’s and X attaches to the other end (to the one with the most alkyl substituents) • This is Markovnikov’s rule

20. Markovnikov's Rule When an unsymmetrically substituted alkene reacts with a hydrogen halide, the hydrogen adds to the carbon that has the greater number of hydrogen substituents, and the halogen adds to the carbon that has the fewer hydrogen substituents.

21. Example of Markovnikov’s Rule • Addition of HCl to 2-methylpropene is regiospecific – one product forms where two are possible • If both ends have similar substitution, then the reaction is not regiospecific

22. HBr CH3CH2CH CH3CH2CHCH3 CH2 acetic acid Br Mechanistic Basis for Markovnikov's Rule:

23. + CH3CH2CH—CH3 + Br – CH3CH2CH CH3CH2CHCH3 CH2 Br Mechanistic Basis for Markovnikov's Rule:Example 1 HBr

24. + CH3CH2CH—CH3 + Br – CH3CH2CH CH3CH2CHCH3 CH2 Br Mechanistic Basis for Markovnikov's Rule:Example 1 + CH3CH2CH2—CH2 primary carbocation is less stable: not formed HBr

25. Mechanistic BasisforMarkovnikov's Rule Protonation of double bond occurs in direction that gives more stable of two possible carbocations.

26. Markovnikov’s Rule

29. H CH3 HCl CH3 Cl 0°C Mechanistic Basis for Markovnikov's Rule:Example 3

30. H CH3 HCl CH3 Cl 0°C Mechanistic Basis for Markovnikov's Rule:Example 3 H H Cl – CH3 + HCl

31. H CH3 HCl CH3 Cl 0°C Mechanistic Basis for Markovnikov's Rule:Example 3 secondary carbocation is less stable: not formed H H H + CH3 Cl – CH3 + H HCl

32. Practice Problem:

33. Solution:

34. Problem: Major products?

35. Problem: Which alkene?

36. Stability of Carbocations and Markovnikov’s Rule • More stable carbocation forms faster • Tertiary cations and associated transition states are more stable than primary cations

38. Carbocation Structure and Stability • Carbocations are planar and the tricoordinate carbon is surrounded by only 6 electrons in sp2 orbitals • The fourth orbital on carbon is a vacant p-orbital • The stability of the carbocation (measured by energy needed to form it from R-X) is increased by the presence of alkyl substituents • Therefore stability of carbocations: 3º > 2º > 1º > +CH3

39. Carbocation Stability • stability due to hyperconjugation

41. Carbocation Rearrangements in Hydrogen Halide Addition to Alkenes

42. Carbocation Rearrangements • supports carbocation formation. • 2° carbocation rearranges to more stable 3° carbocation by a hydride shift. Alkyl groups can also rearrange by an alkyl shift.

43. Example Account for the formation of • 2-chloro-3-methylbutane and • 2-chloro-2-methylbutane.

44. Carbocation Rearrangements

45. Carbocation Rearrangements

46. Free Radical Addition of HBr (not HCl or HI) Peroxides 0% 100% Anti-Markovnikov Product Mechanism: Free radical chain reaction • Initiation • R-O-O-R 2 R-O. • R-O. + HBr R-O-H + Br. warm

47. Propagation a. b. . 30 radical –stable Br. . What is the alternative reaction in step (a)? Answer: It is possible to form a 20 radical. However, since it is less stable, it does not form!

48. Addition of Water to Alkenes: according to Markovnikov’s Rule • Hydration of an alkene is the addition of H-OH to to give an alcohol • Acid catalysts are used in high temperature industrial processes: ethylene is converted to ethanol

49. The addition of H2O In the presence of a strong acid (e.g. H2SO4) and water, the addition reaction also takes place.

50. Another example: isobutene