1. Chapter 5 – Alkene Addition Reactions
5.1 – An Overview of Electrophilic Addition Reactions 5.2 – Reactions of Alkenes with Halogens 5.3 – Writing Organic Reactions 5.4 – Conversion of Alkenes into Alcohols 5.5 – Ozonolysis of Alkenes 5.6 – Free-Radical Addition of Hydrogen Bromide to Alkenes 5.7 – Polymers: Free-Radical Polymerization of Alkenes 5.8 – Alkenes in the Chemical Industry

2. Overview of Addition Reactions
The carbon of the alkene with fewer alkyl groups becomes bonded to the less electronegative atom (the positive end of the reagent dipole)
5.1 An Overview of Electrophilic Addition Reactions

3. Addition of Bromine and Chlorine
p bond: electron rich; serves as a nucleophile
Halogens: electrophilic via an induced dipole
5.2 Reactions of Alkenes with Halogens

4. Addition of Chlorine and Bromine
The product and mechanism are solvent dependent.
One of the most common mechanisms involved a halonium ion.
Bromonium, chloronium, or iodonium ion.
5.2 Reactions of Alkenes with Halogens

5. Addition of Chlorine and Bromine
Why is a bromonium ion formed rather than a bromo-substitued carbocation?
5.2 Reactions of Alkenes with Halogens

6. Addition of Chlorine and Bromine
Mechanistic Details
The alkene acts a nucleophile and Br2 acts as an electrophile.
5.2 Reactions of Alkenes with Halogens

7. Addition of Chlorine and Bromine
Mechanism cont’d.
Bromine addition is completed when a bromide ion donates an electron pair to either of the ring carbons of the bromonium ion.
5.2 Reactions of Alkenes with Halogens

8. Halohydrins and Haloetherification
Formed when a solvent, present in great excess, with a nucleophilic oxygen is used (an hydroxyl group).
5.2 Reactions of Alkenes with Halogens

9. Halohydrins Other halogens can also be used.
Result: net addition of a hypohalous acid (HO-X).
While I2 addition to alkenes doesn’t occur, Iodohydrins can be produced and are useful in organic synthesis. /not what it says in the text /
5.2 Reactions of Alkenes with Halogens

10. Halohydrin Regioselectivity
Regioselectivity is observed in unsymmetrical alkenes.
High regioselectivity observed when one carbon of the double bond is disubstituted.
5.2 Reactions of Alkenes with Halogens

11. Regioselectivity: the rationale
About 90% of the positive charge resides on the tertiary carbon.
The C-Br bond is so long and weak that it is essentially a carbocation.
5.2 Reactions of Alkenes with Halogens

12. How do we know these things about how reactions happen
How do we know these things about how reactions happen ? – intermediates and mechanisms.
product distributions
relative rates

13. Conventions for Writing Reactions
Solvents are generally written under the arrow
Reactants and catalysts are written over the arrow
5.3 Writing Organic Reactions

14. Conventions for Writing Reactions
major organic product
reactants & conditions
organic starting material
percent yield
5.3 Writing Organic Reactions

15. Conversion of Alkenes into Alcohols
OH on most substituted C
OH on least substituted C
5.4 Conversion of Alkenes into Alcohols

16. Oxymercuration-Reduction of Alkenes
A two step reaction carried out in sequence.
Oxymercuration
Reduction
Net reaction = hydration of an alkene.
Highly regioselective.
5.4 Conversion of Alkenes into Alcohols

17. Oxymercuration - Intermediates
Oxymercuration – the alkene reacts with mercuric acetate in aqueous solution, adding –HgOAc and –OH across the double bond.
5.4 Conversion of Alkenes into Alcohols

18. Oxymercuration – Mechanism, part 2
Oxymercuration proceeds via a mercurinium ion
Very comparable to a bromonium ion.
5.4 Conversion of Alkenes into Alcohols

19. Oxymercuration – Mechanism cont’d.
The mercurinium ion reacts with the solvent water:
5.4 Conversion of Alkenes into Alcohols

20. Oxymercuration – A late, fast step
The addition is completed by the transfer of a proton to the acetate ion.
The strongest base present is acetate ion, not water.
The equilibrium lies far to the right.
5.4 Conversion of Alkenes into Alcohols

21. Oxymercuration – The reduction (NaBH4)
Reduction Step – conversion of the oxymercuration adducts into alcohols.
C-Hg bond is replaced by C-H bond, with the H coming from the borohydride.
For now, think of BH4- as  to BH3 + H-
5.4 Conversion of Alkenes into Alcohols

22. Oxymercuration – Summary
Highly regioselective.
No rearrangements occur (since there are no carbenium ion intermediates)
More convenient to run on a laboratory scale than hydration (acid-catalyzed reaction with water) as it is free of rearrangements and other side reactions.
5.4 Conversion of Alkenes into Alcohols

23. Hydroboration-Oxidation
Borane (BH3) adds –H and –BH2 regioselectively to alkene. Subsequent oxidation replaces the boron with an OH group.
Net result: the hydroxyl ends up bonded to the carbon with fewer alkyl substituents.
5.4 Conversion of Alkenes into Alcohols

24. Hydroboration-Oxidation
Hydroboration Step:
Boron, as the more electronegative end of B-H, becomes bonded to the carbon with fewer alkyl substituents.
5.4 Conversion of Alkenes into Alcohols

25. Hydroboration-Oxidation
Hydroboration Step:
Borane has three B-H bonds and each one of these can react in turn with an alkene. If excess borane is employed the second addition doesn’t occur; it’s not an essential element of the reaction.
Second Addition
5.4 Conversion of Alkenes into Alcohols

26. Hydroboration-Oxidation
Hydroboration Steps: Third Addition (not essential)
5.4 Conversion of Alkenes into Alcohols

27. Hydroboration-Oxidation
The accepted mechanism is concerted and avoids a carbocation.
Some degree of electron deficiency is present. This explains regioselectivity.
5.4 Conversion of Alkenes into Alcohols

28. Hydroboration-Oxidation
Oxidation Step:
Conversion of organoboranes into alcohols.
or more generally this includes R-BH2 → R-OH
The C-B bond is replaced with C-OH.
The oxygen comes from H2O2. I’ll show the mechanism but you don’t need to be able to reproduce it.
5.4 Conversion of Alkenes into Alcohols

29. Show mechanism for C–B goes to C–OH

30. Comparing Hydration Methods
Oxymercuration-reduction and hydroboration- oxidation are complementary reactions. Both are more selective than acid-catalyzed hydration.
Oxymercuration-reduction results in the –OH on the more highly substituted C.
Hydroboration-oxidation results in the –OH being predominantly on the less substituted C.
5.4 Conversion of Alkenes into Alcohols

31. Formation of Ozonides
The addition of ozone to alkenes breaks the C-C p-bond to give an unstable cycloaddition product.
This unstable product is spontaneously converted into an ozonide, breaking the second C-C bond.
5.5 Ozonolysis of Alkenes

32. Ozonolysis Addition of ozone is a concerted cycloaddition.
Usual second step, reaction with DMS, (CH3)2S. NOTE rearranged ozonide.
5.5 Ozonolysis of Alkenes

33. Ozonolysis: one mechanistic rationale
Spontaneous rearrangement of the initial product leads to the ozonide. (you don’t need this mechanism; besides, it’s not really correct)
5.5 Ozonolysis of Alkenes

34. Another Reaction of Ozonides
Treatment of an Ozonide with Water
If an ozonide is treated with water, hydrogen peroxide (H2O2) is formed, causing the following:
Aldehydes are converted to carboxylic acids.
This does not affect the ketone products.
5.5 Ozonolysis of Alkenes

35. Ozonolysis Summary
5.5 Ozonolysis of Alkenes

36. End of Quiz 2 coverage
Radical addition will not tested until later

37. Free Radical Addition of HBr
The peroxide effect - The addition of peroxides reverses the regioselectivity of HBr addition to alkenes. This is considered, but isn’t really, anti- Markovnikov regioselectivity
5.6 Free-Radical Addition of Hydrogen Bromide to Alkenes

38. An Explanation of the Peroxide Effect
Radical stabilities explain the observed reversed regiochemistry with peroxides present.
Formation of a tertiary radical is faster than formation of a primary radical.
5.6 Free-Radical Addition of Hydrogen Bromide to Alkenes

39. Free-Radicals
Free radical – any species with at least one unpaired electron.
5.6 Free-Radical Addition of Hydrogen Bromide to Alkenes

40. tertiary > secondary > primary
Carbon Free-Radicals
Radical Stability: free-radicals are stabilized by alkyl-group substitution at sp2 hybridized carbons.
tertiary > secondary > primary
5.6 Free-Radical Addition of Hydrogen Bromide to Alkenes

41. “Fishhook” Notation of radical formation
Heterolytic Bond Breaking:
Homolytic Bond Breaking:
A different curved-arrow notation (fishhook notation) is used for homolytic processes.
5.6 Free-Radical Addition of Hydrogen Bromide to Alkenes

42. Free-Radical Chain Reactions
Most free radical reactions are classified as free- radical chain reactions.
Three fundamental steps involved in free-radical chain reactions.
Initiation: net number of radicals increases
Propagation: net number of radicals remains constant
Termination: net number of radicals decreases
5.6 Free-Radical Addition of Hydrogen Bromide to Alkenes

43. Free-Radical Reactions: Initiators
Initiation: The free radicals that take part in subsequent steps of the reaction are formed from a free-radical initiator.
5.6 Free-Radical Addition of Hydrogen Bromide to Alkenes

44. Free-Radical H-Br addition reactions:
Initiation:
In the free-radical addition of HBr to alkenes, two initiation steps take place. 1. 2.
5.6 Free-Radical Addition of Hydrogen Bromide to Alkenes

45. Free-Radical H-Br addition reactions: steps
Propagation: radicals react with non radical starting materials to give other radicals. a. b.
Regenerated for another use in step a.
5.6 Free-Radical Addition of Hydrogen Bromide to Alkenes

46. Free-Radical H-Br addition reactions: steps
Termination: two radicals react to give non radical products.
5.6 Free-Radical Addition of Hydrogen Bromide to Alkenes

47. Bond-Dissociation Energies
Defined as the standard enthalpy of the reaction:
Requires homolytic (not heterolytic) bond breakage.
Measures the intrinsic strength of a chemical bond.
5.6 Free-Radical Addition of Hydrogen Bromide to Alkenes

48. Additional material
Tables of bond dissociation energies (such as p in the text) and radical heats of formation (p ) will likely be added here.

49. Bond-Dissociation Energies
BDE analysis can be used to explain why a peroxide effect is not observed in the addition of HCl and HI to alkenes.
Compare propagation steps:
5.6 Free-Radical Addition of Hydrogen Bromide to Alkenes

50. Polymers
In the presence of free-radical initiators, many alkenes react to form polymers.
This process is a free-radical polymerization.
5.7 Polymers: Free-Radical Polymerization of Alkenes

51. Polymers Initiation Step: First Propagation Step:
5.7 Polymers: Free-Radical Polymerization of Alkenes