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Basic IUPAC Organic Nomenclature
Cis- and trans- terminology

If alkenes have two different substituents at each end of the C=C then they can exist as stereoisomers (as geometric isomers ).
This is because there is restricted rotation of the double bond due to the pi bond.

For example:
If we consider the general alkene unit shown below, then the alkene can exist as cis and trans isomers only if R1is not equal to R2 AND R3 is not equal to R4.

subs alkene


There are two ways to name these types of isomers, one is the cis / trans method described here, the other is E/Z and it's described on the next page.

Misconception Alert!

cis ¹ Z and trans ¹ E

In general terms there is NO specific relationship between cis and trans / E and Z as they are based on fundamentally different naming rules.

1,2-disubstituted alkenes are described as:

cis-alkene
trans-alkene
cis-
trans-


For example,  but-2-ene, where both R = methyl :

trans-but-2-ene trans-2-butene

cis-but-2-ene cis-2-butene



Tri- or tetrasubstituted alkenes are described as cis- and trans- based on the relative arrangement of the groups that form the parent hydrocarbon carbon chain that gives the root name. In the example shown the below, the longest carbon chain that gives the root name is highlighted in blue:

trans-alkene

cis-alkene
trans-3-methylhex-3-ene

cis-3-methylhex-3-ene

 © Dr. Ian Hunt, Department of Chemistry, University of Calgary