Organic Chemistry

226 CHAPTER 5 Stereochemistry

4. Addition Reactions that Form a Bromonium Ion Intermediate
   If two asymmetric carbons are created as the result of an addition reaction that forms a
   bromonium ion intermediate, only one pair of enantiomers will be formed. In other
   words, the addition of is a stereoselective reaction. The addition of to the cis
   alkene forms only the threo enantiomers.

Similarly, the addition of to the trans alkene forms only the erythro enantiomers.

Because the cis and trans isomers form different products, the reaction is stereospecif-

ic as well as stereoselective.

Because the addition of to the cis alkene forms the threo enantiomers, we know

that addition of is an example of anti addition because if syn addition had occurred

to the cis alkene, the erythro enantiomers would have been formed. The addition of

is anti because the reaction intermediate is a cyclic bromonium ion (Section 4.7).

Once the bromonium ion is formed, the bridged bromine atom blocks that side of the

ion. As a result, the negatively charged bromide ion must approach from the opposite

side (following either the green arrows orthe red arrows). Thus, the two bromine

atoms add to opposite sides of the double bond. Because only anti addition of can

occur, only two of the four possible stereoisomers are obtained.

Br 2

Br 2

Br 2

Br 2

H

H

H

Br

H C C

3 C

H 3 C

CH 2 CH 3

H Br Br CH 2 CH 3

Br

CC ++Br 2

+

H 3 C

H CH 2 CH 3

trans-2-pentene

erythro enantiomers

Fischer projections

erythro enantiomers

perspective formulas

H

CH 2 Cl 2

CH 3

H

H

Br

Br

CH 2 CH 3

CH 3

Br H

Br H

CH 2 CH 3

CC

Br 2

H

H

H H

H

Br

H C C

3 C

CH 2 CH 3 H 3 C

CH 2 CH 3

H Br Br

Br

CC ++Br 2

+

H 3 C CH 2 CH 3

H

cis-2-pentene

threo enantiomers

Fischer projections

threo enantiomers

perspective formulas

H

CH 2 Cl 2

CH 3

Br

Br

CH 2 CH 3

CH 3

Br H

HBr

CH 2 CH 3

CC

Br 2 Br 2

Tutorial:

Review halogenation

Br Br

CC

Br

Br−

C C

Br

CC

Br

addition of Br 2 is an anti addition

the Br’s have added to opposite

sides of the double bond

Br– adds to the side opposite

to where Br+ added

Br

Br

+

CC

the bromonium ion formed

from the reaction of Br 2

with cis-2-butene