Organic Chemistry

Section 5.9 Isomers with More than One Asymmetric Carbon 199

PROBLEM 21

Draw the stereoisomers of 2,4-dichlorohexane. Indicate pairs of enantiomers and pairs of

diastereomers.

1-Bromo-2-methylcyclopentane also has two asymmetric carbons and four
stereoisomers. Because the compound is cyclic, the substituents can be in either the cis
or the trans configuration. The cis isomer exists as a pair of enantiomers, and the trans
isomer exists as a pair of enantiomers.

1-Bromo-3-methylcyclobutane does not have any asymmetric carbons. C-1 has a
bromine and a hydrogen attached to it, but its other two groups
are identical; C-3 has a methyl group and a hydrogen attached to it, but its other two
groups are identical. Because the compound does not have a
carbon with four different groups attached to it, it has only two stereoisomers, the cis iso-
mer and the trans isomer. The cis and trans isomers do not have enantiomers.

1-Bromo-3-methylcyclohexane has two asymmetric carbons. The carbon that is
bonded to a hydrogen and a bromine is also bonded to two different carbon-containing
groups and
so it is an asymmetric carbon. The carbon that is bonded to a hydrogen and a methyl
group is also bonded to two different carbon-containing groups, so it is also an asym-
metric carbon.

Because the compound has two asymmetric carbons, it has four stereoisomers. Enan-
tiomers can be drawn for the cis isomer, and enantiomers can be drawn for the trans isomer.

1-Bromo-4-methylcyclohexane has no asymmetric carbons. Therefore, the com-
pound has only one cis isomer and one trans isomer.

H

Br

H

CH 3

cis-1-bromo-4-methylcyclohexane

H

Br

CH 3

H

trans-1-bromo-4-methylcyclohexane

cis-1-bromo-3-methylcyclohexane trans-1-bromo-3-methylcyclohexane

H

Br

H

CH 3

H

Br

H

CH 3

H

Br

H

CH 3

H

Br

H

CH 3

H

Br

H

CH 3

asymmetric carbon

asymmetric carbon

*

*

these two groups are different

(¬CH 2 CH(CH 3 )CH 2 CH 2 CH 2 ¬ ¬CH 2 CH 2 CH 2 CH(CH 3 )CH 2 ¬),

H

H

H

H

Br

CH 3

Br

CH 3

cis-1-bromo-3-methylcyclobutane trans-1-bromo-3-methylcyclobutane

(¬CH 2 CH(Br)CH 2 ¬)

(¬CH 2 CH(CH 3 )CH 2 ¬)

H

Br

H

CH 3

cis-1-bromo-2-methylcyclopentane

H

Br

CH 3

H

trans-1-bromo-2-methylcyclopentane

CH 3

H

H

Br

H

CH 3

H

Br

3-D Molecules:

(1R,2S)-1-Bromo-2-methylcy-

clopentane; (1S,2R)-1-Bromo-

2-methylcyclopentane;

(1R,2R)-1-Bromo-2-methylcy-

clopentane; (1S,2S)-1-Bromo-

2-methylcyclopentane