Addition of Halogens

The remaining halide ion is a good
nucleophile which attacks the positively
charged halonium ion
© Prentice Hall 2001
Chapter 3
1
Addition of Halogens

If the reaction is carried out in water a
different product is obtained
© Prentice Hall 2001
Chapter 3
2
OxymercurationDemercuration



In oxymercuration-demercuration, an alkene is
treated first with mercuric acetate in aqueous
tetrahydrofuran
The product is treated with sodium borohydride
in the presence of hydroxide
The result is Markovnikov addition of water to the
double bond, yielding an alcohol
© Prentice Hall 2001
Chapter 3
3
OxymercurationDemercuration


Advantages over addition of H2O using
H2SO4
Oxymercuration-demercuration


doesn’t require the presence of a strong acid
is not subject to carbocation rearrangement
© Prentice Hall 2001
Chapter 3
4
OxymercurationDemercuration
© Prentice Hall 2001
Chapter 3
5
Oxymercuration and
Alkoxymercuration
Why does the nucleophile attack at the middle carbon
and not at the end carbon?
The transition state at the left is more stable than the
one at the right.
© Prentice Hall 2001
Chapter 3
6
Markovnikov’s Rule & OxymercurationDemercuration


Recall the modern version, i.e. the
electrophile adds to the carbon bonded
to the greatest number of hydrogens
In this case the electrophile is the
mercuric acetate
© Prentice Hall 2001
Chapter 3
7
Hydroboration-Oxidation

Hydroboration-oxidation is a convenient
way to add water to a double bond,
forming an alcohol
© Prentice Hall 2001
Chapter 3
8
Hydroboration-Oxidation


Viewed in the classical sense, the addition
appears to be in the anti-Markovnikov direction
The hydroxyl group bonds to the carbon with the
most hydrogens
© Prentice Hall 2001
Chapter 3
9
Hydroboration-Oxidation

As we consider the mechanism, we see that
the electrophile bonds to the carbon with the
most hydrogens (obeying the modern version
of Markovnikov’s rule)
© Prentice Hall 2001
Chapter 3
10
Hydroboration
© Prentice Hall 2001
Chapter 3
11
Oxidation
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Chapter 3
12
Hydroboration-Oxidation

In addition to the electronic effect, steric effects
may be important

Recall that three alkene molecules eventually
surround each boron
The structure on the left is less hindered than
the structure on the right

© Prentice Hall 2001
Chapter 3
13
Addition of Radicals
Markovnikov Addition
Anti-Markovnikov Addition
Source of Confusion regarding addition of HBr for
years
© Prentice Hall 2001
Chapter 3
14
Heterolysis & Homolysis
Heterolytic bond
cleavage or heterolysis
H
Br
H+
+
Br
H
+
Br
Homolytic bond
cleavage or homolysis
H
Br
Homolysis produces radicals, which are very reactive
species
© Prentice Hall 2001
Chapter 3
15
Radicals



are unpaired electron spins
tend to perpetuate unpaired spin
participate in chain reactions
© Prentice Hall 2001
Chapter 3
16
Radicals

Sources include:



Hydrogen peroxide
Alkyl peroxides
Light causes homolysis of the weak O-O
bond
© Prentice Hall 2001
Chapter 3
17
Radicals

Stability of alkyl radicals is similar to
stability of carbocations
© Prentice Hall 2001
Chapter 3
18
Radical Chain Reactions
© Prentice Hall 2001
Chapter 3
19
Radical Chain Reactions
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Chapter 3
20
Radical Chain Reactions
© Prentice Hall 2001
Chapter 3
21
Radical Addition is Unique to
Hydrogen Bromide
Why?
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Chapter 3
22
Radical Addition is Unique to
Hydrogen Bromide
© Prentice Hall 2001
Chapter 3
23
Addition of Hydrogen and
Relative Stabilities of Alkenes
© Prentice Hall 2001
Chapter 3
24
Addition of Hydrogen and
Relative Stabilities of Alkenes
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Chapter 3
25
Relative Stabilities of Alkenes
© Prentice Hall 2001
Chapter 3
26
Relative Stabilities of Alkenes
© Prentice Hall 2001
Chapter 3
27
Relative Stabilities of Alkenes
© Prentice Hall 2001
Chapter 3
28
Relative Stabilities of Alkenes
© Prentice Hall 2001
Chapter 3
29