Organic Chemistry

Section 3.5 The E,ZSystem of Nomenclature 119

PROBLEM 8

Which of the following compounds have a dipole moment of zero?

3.5 The E,ZSystem of Nomenclature

As long as each of the carbons of an alkene is bonded to only one substituent, we

can use the terms cis and trans to designate the structure of the alkene. If the hydrogens

are on the same side of the double bond, it is the cis isomer; if they are on opposite

sides of the double bond, it is the trans isomer. But how would you designate the iso-

mers of a compound such as 1-bromo-2-chloropropene?

For a compound such as 1-bromo-2-chloropropene, the cis–trans system of nomen-

clature cannot be used because there are four different substituents on the two vinylic

carbons. The E,Zsystem of nomenclature was devised for these kinds of situations.*

To name an isomer by the E,Zsystem, we first determine the relative priorities of

the two groups bonded to one of the carbons and then the relative priorities of the

two groups bonded to the other carbon. (The rules for assigning relative priorities

are explained next.) If the high-priority groups are on the same side of the double

bond, the isomer has the Zconfiguration (Zis for zusammen, German for “together”).

If the high-priority groups are on opposite sides of the double bond, the isomer has the

Econfiguration (Eis for entgegen, German for “opposite”).

The relative priorities of the two groups bonded to an carbon are determined

using the following rules:

• Rule 1.The relative priorities of the two groups depend on the atomic numbers
  of the atoms that are bonded directly to the carbon. The greater the atomic
  number, the higher is the priority.
  For example, in the following compounds, one of the carbons is bonded to
  a Br and to an H:

Br Cl

Cl

Br

the Z isomer the E isomer

high priority high priority

CH 3

C

H

C

CH 3

C

H

C

sp^2

sp^2

sp^2

the Z isomer

low priority

high priority

low priority

high priority

the E isomer

C C

high priority

low priority

low priority

high priority

C C

sp^2

sp^2

Which isomer is cis and which is trans?

Cl

CH 3

C

H

C

Br CH 3

Cl

C

H

C

Br

sp^2

Cl

Cl

C

H

C

H H

H

C

Cl

C

H Cl

H

C

Cl

C

H H

Cl

C

Cl

C

H

ABCD

The Zisomer has the high-priority

groups on the same side.

*IUPAC prefers the Eand Zdesignations because they can be used for all alkene isomers. Many

chemists, however, continue to use the cis and trans designations for simple molecules.

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