Stereoisomers principally refer to cis- and trans- isomers, sometimes called geometric isomers. Examples are cis-2-butene and trans-2-butene. Like constitutional isomers, geometric isomers have same molecular formula (e.g., C4H8 for both cis-2-butene and trans-2-butene), but since the atoms are connected in the same way (there is a double bond between carbons 2 & 3 for each), the only difference is the geometry of the molecule. Alternatively, one can imagine that these two geometric isomers are simply viewed differently. It's all about how you view them.
Another example of stereoisomers would be the cis- and trans- isomers for the groups of atoms on a cyclic structure, such as cis-1,2-dimethylcyclohexane, or trans-1,2-dimethylcyclohexane. Even though double bounds are not present in cyclohexane, we can have cis- and trans- isomers, because, like those of many alkenes, the atoms, or groups of atoms, are locked in space.
The term cis- refers to groups being on the same side of the double bond (e.g., for geometric isomers) or on the same side of the molecule for a substituted cyclohexane molecule. Conversely, trans- refers to the groups in question being either on the opposite side of a double bond, or on opposite sides (above or below) the ring structure in a molecule like cyclohexane.
While cis- and trans- are most often used to describe stereoisomers, the Cahn Ingold designations of (E) or (Z) can also be used.
Copyright © Dr. Donald L. Robertson (Modified: 10/19/2010)