Ch. 3 - 2-Butene


In this chapter we will learn cis and trans isomerism caused by restricted rotation about the double bond (that is, higher energy barriers) by studying dichloroethylene, maleic acid and fumaric acid as examples. Z, E expression will also be introduced for more complicated compounds in which cis/trans nomenclature is not applicable.

Furthermore, we will learn that the differentiation between the geometrical isomerism and the conformational isomerism should be discussed in terms of the magnitude of the barrier of rotation of any kind of bond rather than in terms of difference between the rotation about a single bond and that about a double bond.
The s-trans and s-cis isomerism in dienes will also be introduced.

Goal of this chapter

At the end of the study of this chapter, you will be able to understand the followings:

  1. To point out the presence of trans and cis isomers of a compound with a double bond, and distinguish these two isomers when the isomerism is involved.
  2. To expand the concept of cis and trans isomerism to compounds with a double bond other than C=C bond.
  3. To name complicated compounds by Z, E nomenclature on the basis of the sequence rule.
  4. To distinguish conformers of dienes.

New terms and concepts

If you have learned about half of the following terms, go directly to the Question.

Else, please click the terms which you have not yet learned, and review them.

Now, you may read through Summary to ensure the point, or go to Questions to test yourself.

Go to Questions.

3.1 Geometrical isomer
3.2 Z and E nomenclature
3.3 Conformation and configuration
3.4 Nomenclature of stereochemistry



S3.1 cis, trans isomerism (geometrical isomerism)

cis isomers: isomers with similar ligands at the same side of the double bond
trans isomers: isomers with similar ligands at the opposite side of the double bond

go to 3.1 Cis-trans isomerism

S3.2 E, Z nomenclature

Z (or seqcis): ligands with higher priority are placed on the same side of a double bond
E (or seqtrans): ligands with higher priority are placed on the opposite side of a double bond


go to 3.2 E, Z nomenclature

S3.3 Conformers and configurational isomers

Conformers: stereoisomers with energy barrier lower than 100kJ mol-1
Configurational isomers: stereoisomers with energy barrier higher than 100 kJ mol-1

go to 3.3 Configuration and conformation