Nomenclature

Limitation of cis-trans in nomenclature

In order to designate the arrangement of groups about the double bond as cis or trans, both carbons of the double bond must have one group the same. An example of a compound where this is not the case is shown. Thus the cis-trans nomenclature has been replaced by the E-Z nomenclature below that applies to all compounds containing carbon-carbon double bonds.

E-Z nomenclature

To assign the configuration about a double bond as either E- or Z- one must first assign relative priorities to the two substituents bonded to each of the double-bonded carbons.

Priorities depend on atomic number. Atoms of higher atomic number are assigned higher priorities.

If the atom directly bonded to the double-bonded carbons is the same for two or more of the substituents, compare the atomic number of the second atoms out, and then the third, and so on, until a difference becomes apparent.

Also a doubly-bonded carbon in a substituent group (one of a, b, c or d) is assigned a higher priority than a singly-bonded carbon

Once the priorities have been assigned, the configuration of the double bond can be assigned as shown below.

E- for entgegen
(opposite in German)
substituents
of higher priority
on opposite sides

Z- for zusammen
(together in German)
substituents
of higher priority
on the same side

The compound shown above is E because
CH₃ is the higher priority group on C-3
CH₂Cl is the higher priority group on C-2
AND these groups are on opposite sides of the double bond.