© Prentice Hall 2001 Chapter 3 1

Addition of Hydrogen Halides to Alkenes

What about the following reaction?

Which sp2 carbon gets the hydrogen and which gets the chlorine?

CH3 C

CH3

CH2HCl CH3 C

CH3

Cl

CH3 CH3 CH

CH3

CH2Cl+ or

© 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 sp2 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 sp2 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

© Prentice Hall 2001 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 sp2 carbon

that is bonded to the greater number of hydrogens

© Prentice Hall 2001 Chapter 3 10

Addition of Water

© Prentice Hall 2001 Chapter 3 11

Rearrangements of Carbocations

1,2-Hydride Shift

© Prentice Hall 2001 Chapter 3 12

Rearrangements of Carbocations

1,2-Methyl Shift

© Prentice Hall 2001 Chapter 3 13

Addition of Halogens

The atoms that make up the Br2 or Cl2 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 CCl4