6.4

6.4

Electrophilic Addition of 

Electrophilic Addition of 

Hydrogen Halides to 

Hydrogen Halides to 

Alkenes

Alkenes

+

+

E

E

—

—

Y

Y

δ+

δ+

δ

δ

–

–

C

C

C

C

E

E

Y

Y

C

C

C

C

General equation for electrophilic addition

General equation for electrophilic addition

General equation for electrophilic addition

+

+

H

H

—

—

X

X

δ+

δ+

δ

δ

–

–

C

C

C

C

H

H

X

X

C

C

C

C

When EY is a hydrogen halide

When EY is a hydrogen halide

When EY is a hydrogen halide

CH

CH

3

3

CH

CH

2

2

CH

CH

2

2

CH

CH

3

3

H

H

H

H

CH

CH

3

3

CH

CH

2

2

CH

CH

2

2

CHCH

CHCH

2

2

CH

CH

3

3

Br

Br

(76%)

(76%)

CHCl

CHCl

3

3

, 

, 

-

-

30°C

30°C

C

C

C

C

Example

Example

Example

HBr

HBr

Electrophilic addition of hydrogen halides 

Electrophilic addition of hydrogen halides 

to alkenes proceeds by rate

to alkenes proceeds by rate

-

-

determining 

determining 

formation of a carbocation intermediate.

formation of a carbocation intermediate.

Mechanism

Mechanism

Mechanism

Electrons flow from

Electrons flow from

the 

the 

π

π

system of the

system of the

alkene (electron rich) 

alkene (electron rich) 

toward the positively

toward the positively

polarized proton of 

polarized proton of 

the hydrogen halide.

the hydrogen halide.

Mechanism

Mechanism

Mechanism

X

X

H

H

H

H

C

C

C

C

Mechanism

Mechanism

Mechanism

..

..

..

..

:

:

C

C

C

C

+

+

..

..

..

..

:

:

X

X

:

:

–

–

X

X

H

H

H

H

C

C

C

C

Mechanism

Mechanism

Mechanism

..

..

..

..

:

:

C

C

C

C

+

+

..

..

..

..

:

:

X

X

:

:

–

–

H

H

C

C

C

C

..

..

..

..

X

X

:

:

6.5

6.5

Regioselectivity of

Regioselectivity of

Hydrogen Halide Addition:

Hydrogen Halide Addition:

Markovnikov's Rule

Markovnikov's Rule

When an unsymmetrically substituted 

When an unsymmetrically substituted 

alkene reacts with a hydrogen halide, 

alkene reacts with a hydrogen halide, 

the hydrogen adds to the carbon that 

the hydrogen adds to the carbon that 

has the greater number of hydrogen 

has the greater number of hydrogen 

substituents, and the halogen adds to 

substituents, and the halogen adds to 

the carbon that has the fewer hydrogen 

the carbon that has the fewer hydrogen 

substituents.

substituents.

Markovnikov's Rule

Markovnikov's Rule

acetic acid

acetic acid

Br

Br

CH

CH

3

3

CH

CH

2

2

CHCH

CHCH

3

3

Example 1

Example 1

Example 1

Markovnikov's Rule

Markovnikov's Rule

CH

CH

2

2

CH

CH

3

3

CH

CH

2

2

CH

CH

HBr

HBr

(80%)

(80%)

CH

CH

3

3

CH

CH

3

3

CH

CH

3

3

C

C

Br

Br

(90%)

(90%)

C

C

C

C

Markovnikov's Rule

Markovnikov's Rule

Example 2

Example 2

Example 2

acetic acid

acetic acid

HBr

HBr

CH

CH

3

3

CH

CH

3

3

H

H

H

H

0°C

0°C

CH

CH

3

3

Cl

Cl

CH

CH

3

3

(100%)

(100%)

Markovnikov's Rule

Markovnikov's Rule

Example 3

Example 3

Example 3

HCl

HCl

6.6

6.6

Mechanistic Basis

Mechanistic Basis

for

for

Markovnikov's Rule

Markovnikov's Rule

Protonation of double bond occurs in 

Protonation of double bond occurs in 

direction that gives more stable of two 

direction that gives more stable of two 

possible carbocations.

possible carbocations.

Br

Br

CH

CH

3

3

CH

CH

2

2

CHCH

CHCH

3

3

Mechanistic Basis for Markovnikov's Rule:

Mechanistic Basis for Markovnikov's Rule:

Example 1

Example 1

CH

CH

2

2

CH

CH

3

3

CH

CH

2

2

CH

CH

acetic acid

acetic acid

HBr

HBr

Br

Br

CH

CH

3

3

CH

CH

2

2

CHCH

CHCH

3

3

Mechanistic Basis for Markovnikov's Rule:

Mechanistic Basis for Markovnikov's Rule:

Example 1

Example 1

CH

CH

2

2

CH

CH

3

3

CH

CH

2

2

CH

CH

HBr

HBr

CH

CH

3

3

CH

CH

2

2

CH

CH

—

—

CH

CH

3

3

+   Br 

+   Br 

–

–

+

+

Br

Br

CH

CH

3

3

CH

CH

2

2

CHCH

CHCH

3

3

Mechanistic Basis for Markovnikov's Rule:

Mechanistic Basis for Markovnikov's Rule:

Example 1

Example 1

CH

CH

2

2

CH

CH

3

3

CH

CH

2

2

CH

CH

HBr

HBr

CH

CH

3

3

CH

CH

2

2

CH

CH

—

—

CH

CH

3

3

+   Br 

+   Br 

–

–

+

+

CH

CH

3

3

CH

CH

2

2

CH

CH

2

2

—

—

CH

CH

2

2

+

+

primary carbocation is less stable:  not formed

primary carbocation is less stable:  not formed

Cl

Cl

CH

CH

3

3

CH

CH

3

3

H

H

Mechanistic Basis for 

Mechanistic Basis for 

Markovnikov's Rule:

Markovnikov's Rule:

Example 3

Example 3

0°C

0°C

HCl

HCl

Cl

Cl

CH

CH

3

3

CH

CH

3

3

H

H

CH

CH

3

3

H

H

H

H

+

+

Cl 

Cl 

–

–

Mechanistic Basis for 

Mechanistic Basis for 

Markovnikov's Rule:

Markovnikov's Rule:

Example 3

Example 3

HCl

HCl

Cl

Cl

CH

CH

3

3

CH

CH

3

3

H

H

CH

CH

3

3

H

H

H

H

+

+

+

+

H

H

CH

CH

3

3

H

H

Cl 

Cl 

–

–

Mechanistic Basis for 

Mechanistic Basis for 

Markovnikov's Rule:

Markovnikov's Rule:

Example 3

Example 3

HCl

HCl

secondary 

secondary 

carbocation is 

carbocation is 

less stable:  

less stable:  

not formed

not formed

6.7

6.7

Carbocation Rearrangements in 

Carbocation Rearrangements in 

Hydrogen Halide Addition 

Hydrogen Halide Addition 

to Alkenes

to Alkenes

HCl, 0°C

HCl, 0°C

CH

CH

3

3

CHC

CHC

H

H

(CH

(CH

3

3

)

)

2

2

+

+

CH

CH

3

3

CHC

CHC

H

H

(CH

(CH

3

3

)

)

2

2

Cl

Cl

(40%)

(40%)

CH

CH

3

3

CHC(CH

CHC(CH

3

3

)

)

2

2

+

+

H

H

CH

CH

3

3

CH

CH

2

2

C(CH

C(CH

3

3

)

)

2

2

Cl

Cl

(60%)

(60%)

Rearrangements sometimes occur

Rearrangements sometimes occur

Rearrangements sometimes occur

H

H

2

2

C

C

CHC

CHC

H

H

(CH

(CH

3

3

)

)

2

2