Basic Organic Nomenclature

Alkenes - Cis / Trans Isomers

Family name: suffix: -ene

The double bond, in contrast to single bonds, is rigid and will not allow for rotation about the double bond. This introduces a new complexity to organic nomenclature, stereo isomerism. Stereo isomers are groups of molecules which have the same molecular formula and the atoms are connected in the same bonding pattern, but the atoms are located differently in space. This is most easily demonstrated by the example of 2-butene.
 
 
Compound Name
Line Drawing
3D Model
2-butene isomer I
2-butene isomer II

Both compounds are 2-butene, but clearly they differ in the physical location of the atoms with respect to each other. For simple alkenes, where there is only one alkyl group on each end of the double bond, the two isomers are distinguished using the cis / trans nomenclature introduced in cyclic alkanes. For the 2-butene isomers, isomer I is trans-2-butene (or t-2-butene), and isomer II is cis-2-butene (or c-2-butene). Trans refers to a geometry where the two alkyl groups are on opposite sides of the double bond, while cis refers to the geometry where the two alkyl groups are on the same side on the double bond. For more complex molecules where there are two alkyl groups on one or both ends of the double there is another way to name these compounds based on the Cahn-Ingold-Prelog rules. This subject will be dealt with in the pages on Stereochemistry.

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