Organic Chemistry

Section 5.1 Cis–Trans Isomers 183

Movie:

Isomerism

PROBLEM 1

a. Draw three constitutional isomers with molecular formula

b. How many constitutional isomers can you draw for

5.1 Cis–Trans Isomers

Cis–trans isomers (also called geometric isomers) result from restricted rotation
(Section 3.4). Restricted rotation can be caused either by a double bond or by a cyclic
structure. As a result of the restricted rotation about a carbon–carbon double bond, an
alkene such as 2-pentene can exist as cis and trans isomers. The cis isomerhas the hy-
drogens on the same sideof the double bond, whereas the trans isomerhas the
hydrogens on opposite sidesof the double bond.

Cyclic compounds can also have cis and trans isomers (Section 2.14). The cis iso-
mer has the hydrogens on the same side of the ring, whereas the trans isomer has the
hydrogens on opposite sides of the ring.

PROBLEM 2

Draw the cis and trans isomers for the following compounds:

a. 1-ethyl-3-methylcyclobutane c. 1-bromo-4-chlorocyclohexane

b. 2-methyl-3-heptene d. 1,3-dibromocyclobutane

H H

H 3 C CH 3

cis-1,4-dimethylcyclohexane

H CH 3

H 3 C H

trans-1,4-dimethylcyclohexane

cis-1-bromo-3-chlorocyclobutane

H

Br

Cl

H

trans-1-bromo-3-chlorocyclobutane

H

H

Cl

Br

C 4 H 10 O?

C 3 H 8 O.

isomers

constitutional isomers stereoisomers

isomers that contain

chirality centers

cis–trans

isomers

3-D Molecules:

cis-2-Pentene; trans-2-Pentene

cis-2-pentene trans-2-pentene cis-2-pentene

CC

H 3 C

H H

CH 2 CH 3

trans-2-pentene

CC

H 3 C

H

H

CH 2 CH 3