Organic Chemistry Molecular Models: Structure and Nomenclature

The two possible geometries of 2-butene are cis-2-butene and trans-2-butene; cis indicates that substituents are arranged on the same side of the double bond, while trans indicates opposite sides. These are stereoisomers; the atoms are connected in the same way, but arranged differently in space.

For alkenes, there is a more general way of indicating cis/trans orientation: E/Z nomenclature. Z stands for the German word zusammen (together) and corresponds to cis; E stands for entgegen (opposite) and corresponds to trans. The use of these designations is discussed in your texts and will be covered in lecture; for our present purpose it is sufficient to point out that another way of naming the two diastereomers of 2-butene is as Z- or E-2-butene.

cis-2-butene Z-2-butene			trans-2-butene E-2-butene

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