Organic Chemistry

Section 20.4 Oxidation of Alkenes with Peroxyacids 855

20.4 Oxidation of Alkenes with Peroxyacids

An alkene can be oxidized to an epoxide by a peroxyacid. The overall reaction

amounts to the transfer of an oxygen atom from the peroxyacid to the alkene.

Recall than an bond is weak and easily broken (Section 20.3).

The oxygen atom of the OH group of the peroxyacid accepts a pair of electrons from

the bond of the alkene, causing the weak bond to break heterolytically. The

electrons from the bond are delocalized onto the carbonyl group. The electrons

left behind as the bond breaks add to the carbon of the alkene that becomes elec-

tron deficient when the bond breaks. Notice that epoxidationof an alkene is a concert-

ed reaction: All the bond-forming and bond-breaking processes take place in a single step.

The mechanism for the addition of oxygen to a double bond to form an epoxide is

analogous to the mechanism described in Section 4.7 for the addition of bromine to a

double bond to form a cyclic bromonium ion. In one case the electrophile is oxygen,

and in the other it is bromine. So the reaction of an alkene with a peroxyacid, like the

reaction of an alkene with is an electrophilic addition reaction.

The addition of oxygen to an alkene is a stereospecific reaction. Because the reaction

is concerted, the bond cannot rotate, so there is no opportunity for the relative

positions of the groups bonded to the carbons of the alkene to change. Therefore, a

cis alkene forms a cis epoxide. Similarly, a trans alkene forms a trans epoxide.

H 3 CCH 3 H 3 CCH 3

H

C

O

RCOOH

O

RCOOH

O

C

H

H 3 CH

CH 3

C C

H

H 3 CH

HCHC 3

HHC C

C

O

cis-2-butene cis-2,3-dimethyloxirane

trans-2-butene trans-2,3-dimethyloxirane

H 3 CHC

HCH 3

C

O

+

sp^2

C¬C

CCCCBr

Br

Br

Br

+

−

+

Br 2 ,

mechanism for epoxidation of an alkene

CCCC

O

O

O H

C

R

O

R O

O

C H

p

O¬H

O¬O

p O¬O

R C

O

OOH

a weak bond

O¬O

+ RCOOH +

O O

RCOH

O

RCH CH 2

an alkene a peroxyacid

RCH CH 2

an epoxide a carboxylic acid

3-D Molecules:

Peroxyacetic acid;

m-Chloroperoxybenzoic acid

3-D Molecules:

cis-2,3-Dimethyloxirane;

trans-2,3-Dimethyloxirane

BRUI20_841_882r3 01-04-2003 1:11 PM Page 855