Chapter 6; Addition Reaction of Alkenes part 2

1
Chapter 6 Addition Reaction of Alkenespart 2

• HX Addition
• Electrophilic Addition (Markovnikov Product)
• Free Radical Mechanism (Anti-Mark Product)
• Hydration ( H2O)
• Halogenation/ Hydrohalogenation
• Reduction or Hydrogenation ( H2 )
• Oxidation
• Multi-step Synthesis

2
Addition of Halogens to Alkenes

• electrophilic addition to double bond
• forms a vicinal dihalide

X2 Cl2 or Br2
F2 explosive I2 endothermic
3
Example
Br2
CH3CHCHCH(CH3)2
CH3CH
CHCH(CH3)2
Br
Br
(100)
4
Stereochemistry of Halogen Addition

• anti addition

Anti Addition Two Bromines add to opposite
sides of the ring
5
Example
H
Cl2
H
trans-1,2-Dichlorocyclooctane73 yield only
product
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Mechanism is electrophilic addition

• Br2 is not polar, but it is polarizable
• two steps (1) formation of bromonium ion
• electrophilic attack
• (2) nucleophilic attack on bromonium ion by
  bromide

NET REACTION
CH2CH2 Br2 -gt Br-CH2-CH2-Br
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Step 1a Formation of Bromonium Ion
Mutual polarizationof electron distributionsof
Br2 and alkene
Electrons flow from alkenetoward Br2
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Step 1b Electrophilic Addition to form Bromonium
Ion
Part i
Br-
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Step 1b Lone Pair on Bromine Stabalizes
Carbocation and Forms Cyclic Bromonium Ion
Part ii
Br-
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Step 2 Bromide Ion Must Attack from Oppositte
Side of Cyclic Bromonium Ion (anti addition)
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Example
Br2
trans-1,2-Dibromocyclopentane80 yield only
product
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X2
X
X
alkenes react with X2 to form vicinal dihalides
alkenes react with X2 in water to give vicinal
halohydrins
H2O
X2
X
OH
HX
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Examples
H2O

H2C
BrCH2CH2OH
Br2
CH2
(70)
Cl2
H2O
anti addition only product
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Mechanism 1) Cl2 is polarized and adds across
double bond. 2) Ion formed is stabalized by lone
pair of Cl.
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3) Water attacks chloronium ion from side
opposite (anti addition) carbon-chlorine bond.
This gives trans isomer
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Regioselectivity
(77)

• Markovnikov's rule applied to halohydrin
  formation the halogen adds to the carbon having
  the greater number of hydrogens.

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Hydrogenation (Reduction, H2) of Ethylene
Metal Catalyst
?
?
?
?
HH

• exothermic ?H 136 kJ/mol
• catalyzed by finely divided Pt, Pd, Rh, Ni

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Two spatial (stereochemical) aspects ofalkene
hydrogenation

• (1) syn addition of both H atoms to double bond
• (2) hydrogenation is stereoselective,
  corresponding to addition to less crowded face of
  double bond

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syn-Additon versus anti-Addition
syn addition
anti addition
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syn-Addition Metal catalyst breaks H-H bonds.
B
Y
C
C
A
X
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syn-Addition Addition of H2 across double bonds
takes place in two steps.
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Example of Stereoselective Reaction
H2, cat
Both productscorrespond tosyn additionof H2.
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Example of Stereoselective Reaction
H2, cat
Top face of doublebond blocked bythis methyl
group
But only thisone is formed.
H2 adds to bottom face of double bond.
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Oxidation of Alkenes

• Oxidation without bond cleavage (epoxidation)
• Oxidation with bond cleavage

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Epoxides three-membered rings that contain oxygen
Epoxidation of AlkenesNo C-C Sigma Bond Breaking

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Example
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Mechanism of Epoxidation
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Ozonolysis Strong Oxidizing Agent Breaks CC Bond
1. O3
2. H2O, Zn
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Multi-step Synthesis

• Making Alkenes
• Reactions of Alkenes
• Alkenes are intermediates in multi-step synthesis

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?-Elimination Reactions to Make Alkenes
dehydration of alcohols X H Y OH
H

H2O
OH
H
?
?
dehydrohalogenation of alkyl halides X H Y
Cl, Br, I
Base

HBr
Br
H
?
?
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Specific Addition Reactions
R
R
R
H
HO
HOH
H
C
R
R
1) BH3

2) H2O2/OH-
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Specific Addition Reactions
anti addition
syn addition
H
H
C
HH
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Oxidation of Alkenes

• Oxidation without bond cleavage
• Oxidation with bond cleavage

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Prepare cyclohexane from cyclohexanol
?
Multi-step synthesis uses alkenes as intermediate
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Prepare 1-bromo-2-methyl-2-propanol from
tert-butyl alcohol
?
(CH3)3COH
1) H2SO4 heat
2) Br2
H2O