Addition of Hydrogen Halides to Alkenes

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© Prentice Hall 2001

Chapter 3

1

Addition of Hydrogen Halides
to Alkenes


What about the following reaction?



Which
sp
2

carbon gets the hydrogen
and which gets the chlorine?

© Prentice Hall 2001

Chapter 3

2

Addition of Hydrogen Halides
to Alkenes


The more substituted carbocation is
preferred

© Prentice Hall 2001

Chapter 3

3

Stability of Carbocations


Alkyl groups (“R”s) tend to stabilize the
positive charge on the
sp
2

carbon of a
carbocation

© Prentice Hall 2001

Chapter 3

4

Stability of Carbocations


Alkyl groups are more polarizable than
hydrogen (i.e. they tend to release electrons
more easily than does hydrogen)


Also, alkyl groups can release electrons via
hyperconjugation

© Prentice Hall 2001

Chapter 3

5

Stability of Carbocations


Alkyl groups bonded to the
sp
2

carbon
of a carbocation tend to spread out the
positive charge, thereby stabilizing the
carbocation

© Prentice Hall 2001

Chapter 3

6

The Hammond Postulate

Carbocation formation
is endergonic
-

hence
transition state is
closest to the
carbocation

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Chapter 3

7

The Hammond Postulate


Formation of a tertiary carbocation should
be faster than formation of a primary
carbocation

© Prentice Hall 2001

Chapter 3

8

Regioselectivity
-

Markovnikov’s Rule


“When a hydrogen halide adds to an
asymmetrical alkene, the addition
occurs such that the halogen
attaches itself to the carbon atom of
the alkene bearing the least number
of hydrogen atoms”

© Prentice Hall 2001

Chapter 3

9

Markovnikov’s Rule


Modern equivalent statement:


The electrophile adds to the

sp
2

carbon
that is bonded to the greater number of
hydrogens

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Chapter 3

10

Addition of Water

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Chapter 3

11

Rearrangements of
Carbocations

1,2
-
Hydride Shift

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Chapter 3

12

Rearrangements of
Carbocations

1,2
-
Methyl Shift

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Chapter 3

13

Addition of Halogens


The atoms that make up the Br
2

or Cl
2

molecules are very electronegative


The halogen
-
halogen bond is weak

© Prentice Hall 2001

Chapter 3

14

Addition of Halogens


When


electrons approach a halogen
molecule, one end of the halogen molecule
accepts them releasing the shared bonding
pair to the other halogen atom to form a
halonium ion and a halide ion

© Prentice Hall 2001

Chapter 3

15

Addition of Halogens


The remaining halide ion is a good
nucleophile which attacks the positively
charged halonium ion

© Prentice Hall 2001

Chapter 3

16

Addition of Halogens


Chlorine and bromine are the only useful
halogen used for this reaction


Fluorine reacts explosively, and iodine
doesn’t react


Addition reactions typically are carried out
in CCl
4