Electrophilic Addition to Alkenes

E

X

+

E

X

+

E

X

Hydrohalogenation (addition of HX)

Markovnikov’s Rule of Electrophilic Addition:
Electrophile adds in such a way as to generate the
most stable cation intermediate. (Usually, such that
electrophile is bound to less substituted carbon.)

H

3

C

H C

H

H

H

Br

+

H

3

C

Br

CH

3

H

3

C

Br

+

H

3

C

H

H

3

C

selectively

not

observed

H

3

C

We say reaction is regioselective.

Anti-Markovnikov Addition w/

HBr + peroxide

HBr + peroxide

For most Markovnikov-rule electrophilic additions, there are
alternative conditions that generate anti-Markovnikov preference.

O

H

3

C

H

H

3

C

Br

O

O

O

O

ben o l per o ide

H

3

C

not

H

3

C

H

H

Br

+

H

3

C

benzoy l per oxide

H

3

C

Br

CH

3

selectively

not

observed

Markovnikov Addition of H

2

O

Hydration

H

2

SO

4

/H

2

O

Harsh and reversible

H C

H

H

3

C

H

3

C

OH

t

H

3

C

H

3

C

H

H

H

3

C

H

3

C

HO

CH

3

H

3

C

H

3

C

OH

selectively

not

observed

Hydroxymercuration-

D

ti

1. Hg(OAc)

2

, H

2

O

2. NaBH

4

Demercuration

Milder, higher yielding

Diastereoselectivity of

Hydroxymercuration-Demercuration

Ph

Ph

OH

1. Hg(OAc)

2

, H

2

O

2. NaBH

4

OH

Ph

+

(S)

(R)

H & OH add to

opposite faces

CH

3

H

CH

3

CH

3

H

(R)

(S)

of alkene

enantiomers

Ph

=

Reaction is diastereoselective: (R,R) or (S,S) diastereomers are

not produced

Markovnikov Addition of ROH

Alkoxymercuration-Demercuration

l

i

l

H

3

C

H

3

C

H

H

1. Hg(OAc)

2

, ROH

2. NaBH

4

H

3

C

H

3

C

RO

CH

3

selectively

H

3

C

H

H

3

C

Hg

OAc

δ+

M

i i

i

A

ith H O

H

3

C

H

H

3

C

H

δ+

O

H

Mercurinium ion

trapped by ROH
instead of H

2

O

As with H

2

O,

reaction is
stereospecific

O

R

H

Anti-Markovnikov Addition of H

2

O

Hydroboration

1 BH

3

·THF

H

3

C

H

3

C

OH

H

3

C

H

3

C

H

H

1. BH

3

THF

2. H

2

O

2

, OH

-

l

ti

l

H

3

C

H

3

C

HO

CH

3

selectively

not

observed

CH

3

CH

3

H

H

H

OH

CH

3

+

1. BH

3

·THF

2. H

2

O

2

, OH

-

H & OH add to

same face of

alkene

OH

H