1. Reactions Mechanisms of Electrophiles Addition to Alkenes
Based on McMurry’s Organic Chemistry, 7th edition

2. Reactions of Alkenes
Alkenes react with many electrophiles to give useful products

3. Addition of Halogens to Alkenes
Bromine and chlorine add to alkenes to give 1,2-dihaldes,
F2 is too reactive and I2 does not add

4. Addition of Br2 to Cyclopentene
Addition is exclusively trans

5. Mechanism of Bromine Addition
Br+ adds to an alkene producing a cyclic ion
Bromonium ion, bromine shares charge with carbon
Gives trans addition

6. Bromonium Ion Mechanism
Electrophilic addition of bromine to give a cation is followed by cyclization to give a bromonium ion
This bromonium ion is a reactive electrophile and bromide ion is a good nucleophile
Stereospecific anti addition

7. Formation of Bromonium Ion
Mutual polarization of electron distributions of Br2 and alkene Br 11

8. Formation of Bromonium Iond–
Electrons flow from alkene toward Br2 Br Br d+ d+ 11

9. Formation of Bromonium Ion– Br
p electrons of alkene displace Br– from Br Br + 11

10. Stereochemistry – : Br .. + Br .. .. : : Br ..
attack of Br– from side opposite C—Br bond of bromonium ion gives anti addition .. Br : .. 12

11. Cyclopentene +Br2 15

12. Bromonium ion – 17

13. Bromide ion attacks the bromonium ion from side opposite carbon-bromine bond– – 17

14. trans-Stereochemistry in vicinal dibromide19

15. Stereochemistry
Anti Addition (halogens enter on opposite sides); Stereoselective
Syn addition (on same side) does not occur for this reaction.

16. Mechanism, Step 1
Step 1, formation of cyclic bromonium ion.

17. Step 2

18. Stereochemistry, addition of Br2
S,S
enantiomers
R,R
R,R
enantiomers
Two compounds (R,R and S,S) formed in equal amounts. Racemic mixture.
S,S

19. Attack of the Bromide Ion
Inversion.
Starts as R
Becomes S
The carbon was originally R with the Br on the top-side. It became S when the Br was removed and a Br attached to the bottom this is SN2 Mechanism.

20. Regioselectivity
If Br2 is added to propene there is no regioselectivity issue.
If Br2 is added in the presence of excess alternative nucleophile, such as CH3OH, regioselectivity may become important.

21. Regioselectivity - 2
Consider, again, the cyclic bromonium ion and the resonance structures.
Stronger bond
Weaker bond
More positive charge
Expect the nucleophile to attack here. Remember inversion occurs.

22. Regioselectivity, Bromonium Ion
Bridged bromonium ion from propene.

23. Bromination of a substituted cyclohexene
Consider the following bromination.
Expect to form two bromonium ions, one on top and the other on bottom.
Expect the rings can be opened by attack on either carbon atom as before.
But NO, only one stereoisomer is formed. WHY?

24. Addition to substituted cyclohexene
The tert butyl group locks the conformation as shown.
The cyclic bromonium ion can form on either the top or bottom of the ring.
How can the bromide ion come in? Review earlier slide showing that the bromide ion attacks directly on the side opposite to the ring.

25. Addition to substituted cyclohexene
Attack as shown in red by incoming Br ion will put both Br into equatorial positions, not anti.
This stereoisomer is not observed. The bromines have not been kept anti to each other but have become gauche as displacement proceeds.
Observe
Ring is locked as shown. No ring flipping.
Be sure to allow for the inversion motion at the carbon attacked by the bromide ion.

26. Addition to substituted cyclohexene
Attack as shown in green by the incoming Br will result in both Br being axial and anti to each other
This is the observed diastereomer. We have kept the bromines anti to each other.

27. 7.3 Addition of Hypohalous Acids to Alkenes: Halohydrin Formation
This is formally the addition of HO-X to an alkene to give a 1,2-halo alcohol, called a halohydrin
The actual reagent is the dihalogen (Br2 or Cl2 in water in an organic solvent)

28. Mechanism of Formation of a Bromohydrin
Br2 forms bromonium ion, then water adds

29. Mechanism .. O : Br .. + .. O +
bromonium ion is intermediate
water is nucleophile that attacks bromonium ion .. Br : .. 12

30. For unsymmterical alkenes, halohydrin formation is
Markovnikov-like in that the orientation of the addition of
X-OH can be predicted by considering carbocation stability
more + charge on the
more substituted carbon
H2O adds in the second step and adds to the
carbon that has the most + charge and ends
up on the more substituted end of the double bond
Br adds to the double bond first (formation of
bromonium ion) and is on the least substituted
end of the double bond

31. Oxymercuration Intermediates
For hydration of an alkene
Use mercuric acetate in THF followed by sodium borohydride
Markovnikov orientation via mercurinium ion

32. Mechanism 1 2 3 4

33. Borane, a digression
Isoelectronic with a carbocation

34. 7.5 Addition of Water to Alkenes: Hydroboration
Borane (BH3) is electron deficient and is a Lewis acid
Borane adds to an alkene to give an organoborane

35. Hydroboration-Oxidation Forms an Alcohol from an Alkene
Addition of H-BH2 (from BH3-THF complex) to three alkenes gives a trialkylborane
Oxidation with alkaline hydrogen peroxide in water produces the alcohol derived from the alkene

36. Orientation in Hydration via Hydroboration
Regiochemistry is opposite to Markovnikov orientation
OH is added to carbon with most H’s
H and OH add with syn stereochemistry, to the same face of the alkene (opposite of anti addition)
STEREOSPECIFIC

37. Mechanism
Syn stereochemistry, anti-Markovnikov orientation now established.
Two reasons why anti-Markovnikov:
Less crowded transition state for B to approach the terminal carbon.
A small positive charge is placed on the more highly substituted carbon.
Next…

38. Cont’d

39. Mechanism of Hydroboration
Borane is a Lewis acid
Alkene is Lewis base
Transition state involves anionic development on B
The components of BH3 are added across C=C
More stable carbocation is also consistent with steric preferences

40. 7.6 Addition of Carbenes to Alkenes
The carbene functional group is “half of an alkene”
Carbenes are electrically neutral with six electrons in the outer shell
They add symmetrically across double bonds to form cyclopropanes

41. Formation of Dichlorocarbene
Base removes proton from chloroform
Stabilized carbanion remains
Unimolecular elimination of Cl- gives electron deficient species, dichlorocarbene
Orbital picture for the carbene is similar to that of a carbocation

42. Reaction of Dichlorocarbene
Addition of dichlorocarbene is stereospecific
cis alkenes give cis cyclopropanes
trans alkenes give trans cyclopropanes

43. Simmons-Smith Reaction
Equivalent of addition of CH2:
Reaction of diiodomethane with zinc-copper alloy produces a carbenoid species
Forms cyclopropanes by cycloaddition

44. 7.7 Reduction of Alkenes: Hydrogenation
Addition of H-H across C=C
Reduction in general is addition of H2 or its equivalent, or a loss of O from the molecule
Requires Pt or Pd as powders on carbon and H2
Hydrogen is first adsorbed on catalyst
Reaction is heterogeneous (process is not in solution)

45. Hydrogen Addition- Selectivity
Selective for C=C. No reaction with C=O, C=N
Polyunsaturated liquid oils become solids
If one side is blocked, hydrogen adds to other
STEREOSPECIFIC

46. Catalytic Hydrogenation Mechanism
Heterogeneous – reaction between phases
Addition of H-H is syn

47. Oxidation of Alkenes: Epoxidation and Hydroxylation
Oxidation is addition of O, or loss of H
Epoxidation results in a cyclic ether with an oxygen atom
Stereochemistry of addition is syn
MCPBA in CH2Cl2 are the usual conditions
Addition of acid results in a trans-1,2-diol
meta-Chloroperoxybenzoic acid (mCPBA)
Treatment of the epoxide with aqueous acid give a trans diol

48. Osmium Tetroxide Catalyzed Formation of Diols
Hydroxylation - converts to syn-diol
Osmium tetroxide, then sodium bisulfate
Via cyclic osmate di-ester
Osmium is toxic, so catalytic amount and NMO are used

49. Oxidaton of Alkenes:Cleavage to Carbonyl Compounds
Ozone, O3, adds to alkenes to form molozonide
Reduce molozonide to obtain ketones and/or aldehydes

50. Permanganate Oxidation of Alkenes
Oxidizing reagents other than ozone also cleave alkenes
Potassium permanganate (KMnO4) can produce carboxylic acids and carbon dioxide if H’s are present on C=C

51. Cleavage of 1,2-diols
Reaction of a 1,2-diol with periodic (per-iodic) acid, HIO4 , cleaves the diol into two carbonyl compounds
Sequence of diol formation with OsO4 followed by diol cleavage is a good alternative to ozonolysis

52. Free Radical Polymerization: Initiation
Initiation - a few radicals are generated by the reaction of a molecule that readily forms radicals from a nonradical molecule
A bond is broken homolytically

53. Polymerization: Propagation
Radical from initiation adds to alkene to generate alkene radical
This radical adds to another alkene, and so on many times
Chain propagation ends when two radical chains combine
Not controlled specifically but affected by reactivity and concentration
Termination

54. 6.4: Electrophilic Addition of Hydrogen Halides to Alkenes
C-C -bond: H°= 368 KJ/mol
C-C -bond: H°= 243 KJ/mol
-bond of an alkene can
act as a nucleophile!!
Electrophilic addition reaction
Bonds broken Bonds formed
C=C -bond KJ/mol H3C-H2C–H KJ/mol
H–Br KJ/mol H3C-H2C–Br KJ/mol
calc. H° = -84 KJ/mol
expt. H°= -84 KJ/mol 54

55. Reactivity of HX correlates with acidity:
slowest HF << HCl < HBr < HI fastest
6.5: Regioselectivity of Hydrogen Halide Addition:
Markovnikov's Rule
For the electrophilic addition of HX across a C=C bond, the H (of
HX) will add to the carbon of the double bond with the most H’s
(the least substitutent carbon) and the X will add to the carbon of
the double bond that has the most alkyl groups. 55

56. Addition Reactions: HX to Alkenes
General Order of HX Reactivity:
HI > HBr > HCl > HF
Usually Dissolved in Solvent (CH3CO2H, CH2Cl2)
Can be Bubbled Through Solution as a Gas
Addition of HCl not Generally Useful (Works w/ Silica Gel)

57. Markovnikov’s Rule: Why?
Product Distribution Explained When Looking at Intermediates
Recall Discussion of Carbocation Stability (2° > 1°)
Major Product Formed From More Stable C+ Intermediate

58. Markovnikov’s Rule: C+ Stability
We Know 2° Carbocations More Stable Than 1°
Major Product Formed From More Stable C+ Intermediate
Means TS in 2° Carbocation Pathway Lower in Energy
Lower Energy of Activation
Activation Energies in 1° Carbocation Pathways Much Larger

59. Stereochemistry in Ionic Additions
Just as We Saw in SN1: C+ Has TWO FACES
Top and Bottom Attack Give Two Stereochemical Products
R and S Enantiomers Formed as a Racemic Mixture (50:50)

60. Regioselectivity (Orientation)
The incoming hydrogen attaches to the carbon with the greater number of hydrogens. This is regioselectivity. It is called Markovnikov orientation.

61. Mechanism
Step 1
Step 2

62. Regioselectivity (Orientation)
Primary carbocation
Secondary carbocation
Secondary carbocation more more stable and more easily formed.

63. Carbocation Stabilities
Order of increasing stability:
Methyl < Primary < Secondary < Tertiary
Order of increasing ease of formation:
Methyl < Primary < Secondary < Tertiary
Increasing Ease of Formation

64. For the electrophilic addition of HX to an unsymmetrically
substituted alkene:
• The more highly substituted carbocation intermediate is
formed.
• More highly substituted carbocations are more stable than
less substituted carbocations. (hyperconjugation)
• The more highly substituted carbocation is formed faster
than the less substituted carbocation. Once formed, the
more highly substituted carbocation goes on to the final
product more rapidly as well. 64

65. Note that the shifting atom or group moves with its electron pair.
6.7: Carbocation Rearrangements in Hydrogen Halide
Addition to Alkenes - In reactions involving carbocation
intermediates, the carbocation may sometimes rearrange if a
more stable carbocation can be formed by the rearrangement.
These involve hydride and methyl shifts.
Note that the shifting atom or group moves with its electron pair.
A MORE STABLE CARBOCATION IS FORMED. 65