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. Carbocation formation Radical formation
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
anti addition 18

10. Syn Addition Reactions
Hydrogenation
Hydroxylation
Hydroboration

11. Anti Addition Reactions
Hydroxylation
Halogenation
Halohydrin formation

12. Hydroxylation
Syn addition
Anti addition

13. Halogenation Halohydrin formation
Anti addition
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 of Hydrogen Halide Addition: Markovnikov's Rule7

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. 8

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. Mechanistic Basis for Markovnikov's Rule:
acetic acid
HBr
CH2
CH3CH2CH Br
CH3CH2CHCH3 9

23. Mechanistic Basis for Markovnikov's Rule: Example 1
CH3CH2CH—CH Br – +
HBr
CH2
CH3CH2CH Br
CH3CH2CHCH3 9

24. Mechanistic Basis for Markovnikov's Rule: Example 1+
CH3CH2CH2—CH2
primary carbocation is less stable: not formed
CH3CH2CH—CH Br – +
HBr
CH2
CH3CH2CH Br
CH3CH2CHCH3 9

25. Mechanistic Basis for Markovnikov's Rule
Protonation of double bond occurs in direction that gives more stable of two possible carbocations. 12

26. Markovnikov’s Rule

29. Mechanistic Basis for Markovnikov's Rule: Example 3Cl
CH3
0°C
HCl 14

30. Mechanistic Basis for Markovnikov's Rule: Example 3+
CH3
Cl –
HCl
CH3 H Cl
CH3
0°C
HCl 14

31. Mechanistic Basis for Markovnikov's Rule: Example 3
secondary carbocation is less stable: not formed H H H +
CH3 +
CH3
Cl – H
HCl
CH3 H Cl
CH3
0°C
HCl 14

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 Alkenes15

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

44. Carbocation Rearrangements

45. Carbocation Rearrangements

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

47. . . Propagation a. b. What is the alternative reaction in step (a)?
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

51. Polymerization
In the presence of only a catalytic amount of H+ and a high concentration of the olefin, polymerization takes place.

52. Problem: Which starting Alkenes?

53. Anti-Markovnikov addition of H2O
Anti-Markovnikov addition of water is achieved via a hydroboration reaction, i.e. addition of BH3 to the double bond.

54. hydroboration-oxidation
Anti-Markovnikov orientation. 
100% yields. 
no rearrangements 

55. Addition of Water to Alkenes: Hydroboration
Herbert Brown (HB) invented hydroboration (HB)
Borane (BH3) is a Lewis acid
Borane adds to an alkene to give an organoborane

56. The mechanism (I)
The nucleophilic alkene attacks the electron poor boron atom

57. 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

58. 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)

59. Hydroboration, Electronic Effects give Non-Markovnikov
More stable carbocation is consistent with steric preferences

60. BH3 can add to more olefins

62. Mechanism of oxidation (III)
Syn Stereoselective:
OH has same positional orientation as the B in the trialkylborane

63. The overall result

65. Predict the Product
=>
syn addition

66. Addition of Halogens to Alkenes
Bromine and chlorine add to alkenes to give 1,2-dihalides, an industrially important process
F2 is too reactive and I2 too unreactive
Cl2 reacts as “Cl+ Cl-”; Br2 is similar

67. Addition of Halogens to Alkenes
Bromine and chlorine add to alkenes to give 1,2-dihalides, an industrially important process
F2 is too reactive and I2 too unreactive
Cl2 reacts as “Cl+ Cl-”; Br2 is similar

68. Addition of Br2 to Cyclopentene
Addition is exclusively trans (stereospecific)

69. Mechanism of Bromine Addition
Br+ adds to an alkene producing a cyclic cation: a bromonium ion, in which bromine shares charge with carbon

70. Mechanism of Bromine Addition
Since the Br blocks one face, one must get anti (trans) addition

72. For a cyclohexene, anti addition corresponds to trans diaxial addition

73. TESTS FOR C=C DOUBLE BONDS

74. PERMANGANATE TEST FOR A C=C DOUBLE BOND
Baeyer Test
POSITIVE TEST
KMnO4 reacts
implies the presence
of a double bond
KMnO4
brown
sludge
MnO2
(+)
purple
NEGATIVE TEST
(-)
KMnO4 is not
decolorized
compound with a
suspected double bond
implies that a double
bond is not present

75. BROMINE TEST FOR A C=C DOUBLE BOND
POSITIVE TEST
Br2 reacts
implies the presence
of a double bond
Br2 / CCl4
(+)
purple
colorless
NEGATIVE TEST
(-)
Br2 is not decolorized
(does not react)
compound with a
suspected double bond
implies that a double
bond is not present

76. Halohydrin Formation
If a halogen is added in the presence of water, a halohydrin is formed
Water is the nucleophile, instead of halide.
Product is Markovnikov and anti.

77. Regioselectivity in Halohydrin Fromation
Nucleophilic attack occurs at the more substituted carbon end of the bridged halonium ion because that carbon is better able to accommodate the partial positive charge in the transition state.

78. Regiospecificity
The most highly substituted carbon has the most positive charge, so nucleophile attacks there.

80. Predict the Product
Predict the product when the given alkene reacts with chlorine in water.

81. Reduction of Alkenes: Catalytic Hydrogenation
Addition of H-H across C=C
Reduction in general is addition of H2 or its equivalent

82. Catalytic Hydrogenation
Alkene + H2  Alkane
Catalyst required, usually Pt, Pd, or Ni.
Finely divided metal, heterogeneous
Syn addition

83. Syn Addition

84. Mechanism of catalytic hydrogenation.B X Y H 8

85. Mechanism of catalytic hydrogenation.B Y C C A X H H H H 9

86. Mechanism of catalytic hydrogenation.B X Y H H H H 9

87. Mechanism of catalytic hydrogenation.B Y A X H H H C C H 9

88. Mechanism of catalytic hydrogenation.B X Y H H 9

89. Mechanism of catalytic hydrogenation.B X Y H H 9

90. Selectivity in Hydrogen Addition
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

91. Solid Fats from Liquid Oils

92. fats & oils: triglycerides
CH2—O—CCH2CH2CH2CH2CH2CH2CH2CH2CH3
| O
CH—O—CCH2CH2CH2CH2CH2CH2CH2CH2CH3
CH2—O—CCH2CH2CH2CH2CH2CH2CH3
“saturated” fat

93. Stereoselectivity
A reaction in which a single starting material can give two or more stereoisomeric products but yields one of them in greater amounts than the other (or even to the exclusion of the other) is said to be stereoselective. 20

94. Example of stereoselective reaction
H3C
CH3 H
Example of stereoselective reaction
H2, cat
CH3
H3C H
CH3
H3C H
Both products correspond to syn addition of H2. 21

95. Example of stereoselective reaction
H3C
CH3 H
Example of stereoselective reaction
H2, cat
CH3
H3C H
But only this one is formed. 21

96. Example of stereoselective reaction
H3C
CH3 H
Example of stereoselective reaction
H2, cat
Top face of double bond blocked by this methyl group
CH3
H3C H 21

97. Example of stereoselective reaction
H3C
CH3 H
Example of stereoselective reaction
H2, cat
CH3
H3C H
H2 adds to bottom face of double bond. 21

98. Six-membered ring formation (4+2)
Diels-Alder Reaction
Six-membered ring formation (4+2)
Diene + Dienophile

99. Chapter 4-1, Questions
25, 26, 33, 36, 37, 40