Organic Chemistry

Section 5.19 Stereochemistry of Electrophilic Addition Reactions of Alkenes 221

The product is a racemic mixture because the carbon in the radical intermediate that
bears the unpaired electron is hybridized. This means that the three atoms bonded
to it are all in the same plane (Section 1.10). Consequently, identical amounts of the R
enantiomer and the Senantiomer will be obtained because HBr has equal access to
both sides of the radical. (Although the unpaired electron is shown to be in the top lobe
of the porbital, it actually is spread out equally through both lobes.)

PROBLEM 42

What stereoisomers are obtained from each of the following reactions?

a. d.

b. e.

c. f.

If a reaction creates an asymmetric carbon in a compound that already has an asym-
metric carbon, a pair of diastereomers will be formed. For example, let’s look at the
following reaction. Because none of the bonds to the asymmetric carbon in the reac-
tant is broken during the addition of HBr, the configuration of this asymmetric carbon
does not change. The bromide ion can approach the planar carbocation intermediate
from either the top or the bottom in the process of creating the new asymmetric car-
bon, so two stereoisomers result. The stereoisomers are diastereomers because one of
the asymmetric carbons has the same configuration in both isomers and the other has
opposite configurations in the two isomers.

H 3 C CH 3

H 3 C H

HBr

CC peroxide

H 3 C CH 3

H 3 C CH 2 CH 2 CH 3

H 2

Pt/C

CC

H 3 C CH 3

H 3 C H

HBr

CC CC

H+

H 2 O

H H

CH 3 CH 2 CH 2 CH 3

H 3 C

CH 3 CH 2 CH 2 CH CH 2 HBr

HCl

sp^2

CH 2 Br

CH 3 CH 2 C H

(R)-1-bromo-2-methyl-

butane

(S)-1-bromo-2-methyl-

butane

Br

CH 3

+

CH 3 CH 2 CH 2 Br

CH 3

H

CH 2 Br C

CH 3 CH 2 CH^3

H

C

H 3 C

Cl

C CH

H

CH 2

Cl

H 3 C

C C

CH 3

Br

H

H Cl

H 3 C

C C

CH 3

Br

H

H

H 3 C

Cl

C CHCH 3

H

HBr

Br

+ +

diastereomers

stereoisomers

(R)-3-chloro-1-butene

configuration does

not change new asymmetric

carbon