1. Iran University of Science & Technology
Organic Chemistry
Sh.javanshir
Faculty of Chemistry
Iran University of Science & Technology

2. Chapter 4-2. Alkenes: continue

3. Oxidation of Alkenes: Hydroxylation
Hydroxylation adds OH to each end of C=C
Stereochemistry of addition is syn
Product is a 1,2-dialcohol or diol (also called a glycol)

4. Syn Hydroxylation of Alkenes
Two reagents:
Osmium tetroxide (expensive!), followed by hydrogen peroxide or sodium bisulfate
Cold, dilute aqueous potassium permanganate, followed by hydrolysis with base 4

5. syn -Glycol Formation syn glycols are made with OsO4 or KMnO4 OsO4
osmium tetroxide
KMnO4
potassium permanganate

6. SYN ADDITION GIVES CIS GLYCOLSH 3 C H H 3 H O 2 O H O s 4 N a H S O 3 H O H
cis C H 3 C H 3 H C H C H 2 O C H
syn
conformation C H 3 3 C H O H N a H S O 3 C O s 4 H O H
meso
cis -2-butene
remember: addition is syn
result is cis

7. Formation of Diols via Osmium Tetroxide
Hydroxylation - converts to syn-diol
Osmium tetroxide, then sodium bisulfate
Via cyclic osmate di-ester

8. Mechanism Notice the transfer of 2e- onto Os = REDUCTION OXIDIZED
cyclic osmate di-ester
Notice the
transfer of
2e- onto Os
= REDUCTION
OXIDIZED
Both of the hydroxyl
oxygens in the glycol
come from OsO4
REDUCED

9. syn-Glycol Formation (II)
potassium
permanganate
syn hydroxylation

11. Problem: Which Alkene?

13. Cleavage Reactions of Alkenes
Ozonolysis
Hot Potassium Permanganate

14. Oxidative Cleavage Both the pi and sigma bonds break. C=C becomes C=O.
Two methods:
Warm or concentrated or acidic KMnO4.
Ozonolysis
Used to determine the position of a double bond in an unknown.

15. Ozonolysis Reaction with ozone forms an ozonide.
Ozonides are not isolated, but are treated with a mild reducing agent like Zn or dimethyl sulfide.
Milder oxidation than permanganate.
Products formed are ketones or aldehydes.

16. Ozone .. .. .. .. .. .. .. .. .. : - : : - electric discharge or
cosmic rays .. .. .. .. : .. .. - .. + : + : .. .. -
EQUIVALENT RESONANCE STRUCTURES

17. Ozonolysis of alkenes
Ozonide
Ozonides usually not isolated, but further reacted with reducing agents
Formation of two molecules each containing C=O (Carbonyl) groups

18. Ozonolysis Example
Ozonide
DMSO

19. Examples
Aldehydes
1-Butene
Ketone
2,3-Dimethyl-2-butene

20. WORKUP PROCEDURES FOR OZONOLYSIS
Two types of workup (decomposition of the ozonide)
are possible :
1. OXIDATIVE
Hydrogen peroxide is present
Aldehydes are oxidized to carboxylic acids.
Formaldehyde is oxidized to carbon dioxide,
which is lost as a gas.
2. REDUCTIVE
Add Zn and H2O or H3O+
METHOD A
METHOD B
Reduce the ozonide with Pd / H2 ,
and then add acid ( H3O+ ).
Aldehydes survive intact and are not oxidized with reductive
conditions.

21. AT ONE TIME OZONOLYSIS WAS WIDELY USED
FOR STRUCTURE PROOF BY DEGRADATION
Broken apart ( or degraded ) to
simpler pieces that are easier to
identify.
Unknown
compound
“At one time” =
before spectroscopy.
The original structure can be
deduced by reassembling the
pieces.

22. PROBLEM TO SOLVE
1) O3 / CH2Cl2
C7H12
2) H3O+
Pd / H2
C7H14
answer

23. WHAT WAS THE ORIGINAL STRUCTURE ?
H2O2
oxidative
workup

24. Oxidation of Alkenes: Potassium permanganate
With KMnO4 in basic solution, product is a 1,2-diol
In neutral or acidic KMnO4 solution the alkene will be broken into 2 carbonyl groups: 24

25. Example 25

26. Oxidation of Alkenes: Potassium permanganate
# of H atoms
Type of Product 2
CO2 1
-COOH
Ketone

28. Problem

30. Preparation of Alkenes: A Preview of Elimination Reactions

31. Alkene Synthesis Overview
E2 dehydrohalogenation (-HX)
E1 dehydrohalogenation (-HX)
Dehalogenation of vicinal dibromides (-X2)
Dehydration of alcohols (-H2O)

32. Removing HX via E2
Strong base abstracts H+ as X- leaves from the adjacent carbon.
Tertiary and hindered secondary alkyl halides give good yields.
Use a bulky base if the alkyl halide usually forms substitution products.

33. Some Bulky Bases
(CH3CH2)3N :
triethylamine

34. Hofmann Product Bulky bases abstract the least hindered H+
Least substituted alkene is major product.

35. E2: Cyclohexanes
Leaving groups must be trans diaxial.

36. Removing HX via E1 Secondary or tertiary halides
Formation of carbocation intermediate
Weak nucleophile
Usually have substitution products too

37. A Preview of Elimination Reactions
Alkenes are commonly made by elimination of HX from alkyl halide: (dehydrohalogenation); using heat and KOH

39. E2: Vicinal Dibromides Remove Br2 from adjacent carbons.
Bromines must be anti-coplanar (E2).
Use NaI in acetone, or Zn in acetic acid. Br
CH3 H

40. A Preview of Elimination Reactions
elimination of H-OH from an alcohol (dehydration); requires strong acids (sulfuric acid, 50 ºC)

41. Prob.: Mechanism?

42. Solution:

43. Polymerization
A polymer is a very large molecule consisting of repeating units of simpler molecules (monomers), formed by polymerization

44. Polymerization
An alkene (monomer) can add to another molecule like itself to form a chain (polymer).
Three methods:
Cationic, a carbocation intermediate
Free radical
Anionic, a carbanion intermediate (rare)

45. Polymerization CH2=CH2 + heat, pressure  -(CH2CH2)-n n = 10,000+
polyethylene
CH3CH=CH2 polymerization  -(CH2CH)-n
CH3
polypropylene
CH2=CHCl poly…  (CH2CH)-n Cl
polyvinyl chloride (PVC)

46. Cationic Polymerization
Alkene is treated with an acid.
Intermediate must be a stable carbocation.
=>

48. Termination

49. Cationic Polymerization
Dimerization of 2-methylpropene
(CH3)2C
CH2
monomer (C4H8)
H2SO4 +
two dimers (C8H16)
CH3CCH
CH3
C(CH3)2
CH3CCH2C
CH2 3

50. Mechanism of Dimerization+
CH3
H2C C
CH3C
CH3CCH2C
CH3 + 3

51. Mechanism of Dimerization
CH3CCH
CH3
C(CH3)2
CH3CCH2C
CH3
CH2
CH3CCH2C
CH3 + 3

52. Free Radical Polymerization of Alkenes
Alkenes combine many times to give polymer
Reactivity induced by formation of free radicals

53. 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

54. Polymerization: Propagation
Radical from intiation adds to alkene to generate alkene derived radical
This radical adds to another alkene, and so on many times

55. Polymerization: Termination
Chain propagation ends when two radical chains combine
Not controlled specifically but affected by reactivity and concentration

56. Overall mechanism for free-radical chain polymerization
•Initiation
I  2R•
R• + M  RM1•
•Propagation
RM1• + M  RM2• …
RMn• + M  RMn+1•
•Termination
RMn• + RMm•  dead polymer

58. Free-Radical Polymerization of Ethylene
H2C
CH2
200 °C
2000 atm O2
peroxides
polyethylene
CH2 6

59. .. RO
Mechanism •
H2C
CH2 7

60. ..
RO:
Mechanism
H2C
CH2 • 9

61. ..
RO:
Mechanism
H2C
CH2 •
CH2
H2C 10

62. ..
RO:
Mechanism
H2C
CH2
H2C
CH2 • 10

63. ..
RO:
Mechanism
H2C
CH2
H2C
CH2
CH2
H2C • 10

64. ..
RO:
Mechanism
H2C
CH2
H2C
CH2
H2C
CH2 • 10

65. ..
RO:
Mechanism
H2C
CH2
H2C
CH2
H2C
CH2
CH2
H2C • 10

66. Unsymmetrical Monomers
If alkene is unsymmetrical, reaction is via more highly substituted radical

67. Free-Radical Polymerization of Propene
H2C
CHCH3
polypropylene CH
CH3
Uses: kitchenware food containers,
fibres for making hard-wearing carpets. 6

68. .. RO
Mechanism •
H2C
CHCH3 7

69. ..
RO:
Mechanism
H2C
CHCH3 • 9

70. ..
RO:
Mechanism
H2C
CHCH3 •
CHCH3
H2C 10

71. ..
RO:
Mechanism
H2C
CHCH3
H2C
CHCH3 • 10

72. ..
RO:
Mechanism
H2C
CHCH3
H2C
CHCH3
CHCH3
H2C • 10

73. ..
RO:
Mechanism
H2C
CHCH3
H2C
CHCH3
H2C
CHCH3 • 10

74. ..
RO:
Mechanism
H2C
CHCH3
H2C
CHCH3
H2C
CHCH3
CHCH3
H2C • 10

75. Poly(phenylethene) or Polystyrene peroxides nC6H5-CH=CH2 (-CH-CH2-)n reflux in kerosene C6H5
Stiffer than poly(ethene), greater Van der Waals’
force due to the benzene rings.
Uses: Toys, cups, refrigerator parts.
Expanded polystyrene for packaging, heat
and sound insulation.

77. Chain Branching During Polymerization
During radical propagation chain can develop forks leading to branching
One mechanism of branching is short chain branching in which an internal hydrogen is abstracted

78. Branching of the Polymer Chain

79. Anionic Polymerization
Alkene must have an electron-withdrawing group like C=O, CN, or NO2.
Initiator: Grignard or organolithium reagent.
=>

82. Natural Rubber
Natural rubber comes from the rubber tree (Hevea brasiliensis) and is a white, milky liquid called latex. Most rubber comes from Malaysia and other nations in East Asia.
Latex can also be seen as the white fluid in dandelion stalks. The latex from the tree is actually a suspension of rubber particles in water. Rubber is a polymer of isoprene. Natural rubber is relatively reactive, and is especially vulnerable to oxidation.
Long chains can be stretched, but then return to original structure.
Chains slide past each other and can be pulled apart easily.
Structure is cis-1,4-polyisoprene. n

83. Vulcanization
Discovered accidentally by Goodyear(1839) , dropped rubber and sulfur on a hot stove.
Sulfur produces cross-linking, strengthening the rubber. Cross-linking also increases the elasticity.
Hardness can be controlled by varying the amount of sulfur.
The optimum amount of sulfur to be added to the rubber is about 10% by weight. Adding an excess of sulfur produces a very brittle and inelastic substance called ebonite.

84. Synthetic Rubber
With a Ziegler-Natta catalyst, a polymer of 1,3-butadiene can be produced, in which all the additions are 1,4 and the remaining double bonds are all cis.
It may also be vulcanized.

85. Polyamides: Nylon
Usually made from reaction of diacids with diamines, but may also be made from a single monomer with an amino group at one end and acid group at other.

86. Polyesters
Dacron® and Mylar®: polymer of terephthalic acid and ethylene glycol.
Made by the transesterification of the methyl ester.

87. Polycarbonates Esters of carbonic acid.
Carbonic acid is in equilibrium with CO2 and water, but esters are stable.
React phosgene with bisphenol A to obtain Lexan® for bulletproof windows.

88. Cycloalkenes Cyclopropene and cyclobutene have angle strain.
Larger cycloalkenes, such as cyclopentene and cyclohexene, can incorporate a double bond into the ring with little or no angle strain. 13

89. Cycloalkenes- 3 to 7 “cis” olefins
rings not large enough to accommodate trans double bonds
C8 limited stability as trans

90. Stereoisomeric cycloalkenes
cis-cyclooctene and trans-cyclooctene are stereoisomers
cis-cyclooctene is 39 kJ/ mol more stable than trans-cyclooctene H H H
cis-Cyclooctene
trans-Cyclooctene 15

91. more stable configuration of double bond predominatesBr
KOCH2CH3
ethanol +
(15%)
(85%)
more stable configuration of double bond predominates 11

92. Stereoisomeric cycloalkenes
trans-cyclooctene is smallest trans-cycloalkene that is stable at room temperature
cis stereoisomer is more stable than trans through C11 cycloalkenes
cis and trans-cyclododecene are approximately equal in stability H
trans-Cyclooctene 15

93. Stereoisomeric cycloalkenes
trans-cyclooctene is smallest trans-cycloalkene that is stable at room temperature
cis stereoisomer is more stable than trans through C11 cycloalkenes
cis and trans-cyclododecene are approximately equal in stability
cis-Cyclododecene
trans-Cyclododecene 15