Chemistry - A Molecular Science

Chapter 13 Organic Chemistry

302

Geometric isomers

Sigma bonds are cylindrical, and the groups bound by them rotate around the bond to adopt different relative positions. Figure 13.11a

shows just two of the relative positions of

the fluorine atoms during a rotation about the sigma bond. However, groups connected by double bonds cannot rotate relative to

one another without breaking the

bond because π

the rotation would move the two p orbitals used in the

bond away from one another and π

remove their overlap. Consequently, the transformation in Figure 13.11b cannot occur without breaking the C=C bond, so the two molecules are

geometric isomers

. A

geometric isomer in which two groups are

on the same side of a double bond is the

cis

isomer,

while a geometric isomer in which the groups are on opposite sides of a double

bond is called a

trans

isomer.

Example 13.5

Draw and name the two geometric isomers of 2-hexene. 2-hexene is H

C-CH=CH-CH 3

-CH 2

-CH 2

and, as shown in the margin, it can exist in both a 3

trans

and a

cis

form. In the

trans

form, the two C-C bonds (dark lines) are on opposite

sides of the double bond; but, in the

cis

isomer, they are on the same side. Thus, the

molecule shown in Example 13.2d is named

trans

-2-hexene to show that it is the

geometric isomer in which the C-C bonds

are on opposite sides of the double bond.

Name the molecule shown in the figure labeled Example 13.5b. There are two carbon atoms with a double bond, so the compound is an ethene. Two chlorine atoms make it a dichloroethene. The two chlorine atoms are on the same side of the double bond, so the molecule is

cis

-dichloroethene.

Alkenes can convert between the

cis

and

trans

isomers if enough energy is supplied to

break the

bond. For example, consider the molecule π

retinal

, which is derived

from

β−

carotene (Figure 13.8) in the body. Like

β−

carotene, retinal absorbs visible light,

and

the energy of the absorbed photon promotes an electron from a

to a π

• orbital. The π

result is that the excited state contains is one electron in the

orbital and one in the π

• π

orbital, which results in a bond order of zero for the

bond shown in red in Figure 13.12; π

i.e

., the bond is a single bond in the excited st

ate. Rotation can occur about the resulting

single bond and convert the

cis

isomer (Figure 13.12a) to the

trans

isomer (Figure 13.12b).

This simple process is the driving force responsible for vision.

H CC

H

HH F

F

F CC

H

HH F

H

same molecule

H CCF

H F

F CCH

H F

cis

isome

r

tran

sisome

r

(a)

x (b)

Figure 13.11 Rotation can occur about single bonds but not about double bonds. a) The two structures of C

H 2

F 42

are not isomers because

the groups can rotate around the single bond. b) The two structures of C

H 2

F 22

are isomers because the groups

cannot rotate around a double bond.

tran

s-2-hexene

cis

-2-hexene

Example 13.5a Cl

Cl^

Example 13.5b

OH

O

H

hν

cis bonds

trans bonds

(a)

(b)

Figure 13.12

cis

(a) and

trans

(b) isomers of retinal

© by

North

Carolina

State

University