Organic Chemistry

and trans-1,2-dichloroethene have dipole moments of zero because the bond dipole

moments cancel (Section 1.15).

Cis and trans isomers can be interconverted (in the absence of any added reagents)

only when the molecule absorbs sufficient heat or light energy to cause the bond to

break, because once the bond is broken, rotation can occur about the remaining bond

(Section 2.10). Cis–trans interconversion, therefore, is not a practical laboratory process.

PROBLEM 7

a. Which of the following compounds can exist as cis–trans isomers?

b. For those compounds, draw and label the cis and trans isomers.

1. 
   
   
   
   3. 
      
   
   
   
   


2. CH 3 C CHCH 3 4. CH 3 CH 2 CH CH 2

CH 3

CH 3 CH CHCH 2 CH 3 CH 3 CH CHCH 3

> 180 ºC

or

h

cis-2-pentene trans-2-pentene

H

CH 2 CH 3

C

CH 2 CH 3

H

C

H

C

H 3 C

H

C

H 3 C

p s

p

1 m 2

118 CHAPTER 3 Alkenes• Thermodynamics and Kinetics

3-D Molecules:

cis-Retinal; trans-Retinal

CIS–TRANS INTERCONVERSION IN VISION

When rhodopsin absorbs light, a double bond interconverts between the cis and trans forms. This process plays an

important role in vision (Section 26.7).

cis-2-butene

bp = 3.7 ºC

m = 0.33 D

CH 3

H

H 3 C

H H H

C C

cis-1,2-dichloroethene

bp = 60.3 ºC

m = 2.95 D

Cl Cl

C C

H 3 C

H

C

trans-2-butene

bp = 0.9 ºC

m = 0 D

CH 3

H

C

trans-1,2-dichloroethene

bp = 47.5 ºC

m = 0 D

Cl

Cl H

H

C C

N opsin

N opsin

OPSIN

OPSIN

rhodopsin metarhodopsin II

(trans-rhodopsin)

cis double bond

light

trans double bond

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