Introduction to Organic Nomenclature: 5. III. Alkenes(ii)

Basic Organic Nomenclature

An Introduction

Dave Woodcock,
Associate Professor Emeritus UBC (Okanagan)
©1996,2000, 2008

4. Functional Groups with Suffix Only
III. Alkenes (ii)
Stereochemistry (ii)

Index

Stereoisomerism in the Alkene Family

The 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.
Putting these two together gives a picture of a rigid, planar structure at a C,C double bonded site, as can be seen in the model of ethene, CH₂=CH₂, where all six atoms of the molecule are in the same plane.

Stereoisomers are compounds that have

(that is all the atoms are connected together in exactly the same way)

The cis/trans isomerism noted for cyclic compounds is an example of stereoisomerism already met.

Stereoisomerism in the alkene family occurs with as few as four carbons in the molecule. For butene four isomers exist:

Alkene Structure	IUPAC Name	Model
	but-1-ene
	trans-but-2-ene
	cis-but-2-ene
	methylpropene

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: