Basic Organic Nomenclature
4. Functional Groups with Suffix Only
III. Alkenes (ii)
Stereoisomerism in the Alkene FamilyThe geometry at, and the nature of, the double bond leads to the possibility of stereoisomers a feature not possible with the linear C,C triple bond.
The geometry at each of the carbons of a C,C double bond is approximately trigonal planar. The
nature of the C,C double bond is such that there is a high energy barrier to rotation about the
bond so that at normal conditions of temperature there is no rotation possible.
Stereoisomers are compounds that have
the same connectivity
Stereoisomerism in the alkene family occurs with as few as four carbons in the molecule. For butene four isomers exist:
In this view of both cis- and trans-but-2-ene
note that connectivity is the same for both molecules, and that the difference between the two is the spacial orientation of the two methyl groups attached to the C,C double bond. Interchanging the two compounds is not possible without breaking the pi bond. As in the nomenclature of ring compounds, trans indicates across and cis indicates same side.
The occurance of this type of stereoisomerism depends on the presence of two different groups at each end of the double bond, considered separately:
When using the cis/trans nomenclature, the cis or trans prefix refers to the way that the main chain passes through the double bond, as can be seen in the following illustrations:
Review Alkenes (i)
Next page : Alkenes (iii) E-Z Nomenclature
Main index page of MolecularModels.ca.
This document page last modified (mo/day/year) :