cis- and trans-alkenes

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 which means they don't readily interconvert.

Examples:

all terminal alkenes i.e. those with a C=CH₂ unit can not exist as cis- and trans- isomers.
similarly, all 1,1-symmetrically disubstituted alkenes i.e. those with a C=CR₂ unit can not exist as cis- and trans- isomers.
the terms cis- and trans- are assigned based on the relative arrangement of the alkyl groups that form the root name on the alkene unit. In general terms, that means including the principal functional group.
alkenes with the R₁-CH=CH-R₂ unit can exist as cis- and trans- isomers.

If we consider the general alkene unit shown below, then the alkene can exist as cis and trans isomers only if R₁ is not equal to R₂ AND R₃ is not equal to R₄.

There are two ways to name these types of isomers, one is the cis / trans method which is described here, the other is E / Z method that is 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- if the two alkyl groups, R-, that form the longest chain (i.e. the root name) are on the same side of the C=C
trans- if the two alkyl groups, R-, that form the longest chain (i.e. the root name) are on opposite sides of the C=C.
these terms are inserted into the name as prefixes.


cis-	trans-

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

trans-but-2-ene
cis-but-2-ene

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-3-methylhex-3-ene		cis-3-methylhex-3-ene