Answer
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.
- 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-, are on the same side of the C=C
- trans- if the two alkyl groups, R-, 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
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