Geometrical Isomerism

Isomers are compounds that have the same molecular formula, but have different arrangements of atoms. Isomerism is very important in organic chemistry, as it gives rise to the millions of organic compounds that exist and, in some ways, helps to function as a coding mechanism.

There are two categories of isomerism: 

· Structural

· Stereisomerism

Structural isomerism:

As in alkanes, ethene (C₂H₄) and propene (C₃H₆) can have only one structure but alkenes higher than propene have different structures. Alkenes possessing C₄H₈ as molecular formula can be written in the following three ways:

Structures I and III, and II and III are the examples of chain isomerism whereas structures I and II are position isomers.

Geometrical isomerism:

Doubly bonded carbon atoms have to satisfy the remaining two valences by joining with two atoms or groups. If the two atoms or groups attached to each carbon atom are different, they can be represented by YX C = C XY like structure.

YX C = C XY can be represented in space in the following two ways :

There are two types of stereoisomerism,

· Optical

· Geometric

Optical Isomers

These are isomers which possess a non-superimposable mirror image. However, we will not be covering these in this course.

Geometric Isomers

This type of isomerism occurs in compounds that have some restricted rotation. Geometric isomerism is not only a feature of organic compounds, as inorganic compounds display geometric isomerism.  In geometric isomerism, there must be a least two different groups attached to the carbon on which there is restricted rotation.  

There are two ways to name geometric isomers,

· Cis-trans

· E-Z notation

Cis-Trans 

For cis-trans geometric isomerism to occur, at least one of the groups must appear on both carbons. If groups that are the same appear on the same side, the isomer is referred to as ‘cis’ and if they appear on different sides it is ‘trans’. Examples of cis-trans isomerisms are found below in figures. Notice the location of the chlorine in below figure, when the chlorine is on the same side, it is cis and when it is on different sides, as in the image on the right, it is trans.

The cis and trans isomers for 2-butene.

The cis and trans isomers for 1, 2-dichloropropene.

E-Z Notation 

This notation can be applied to all geometric isomers and is therefore preferred by IUPAC. However, it is a more complicated notation and so many prefer to use the cis-trans notation, when possible. If we consider the tri-substituted alkene in the example below, we would recognise that the cis-trans notation could not be used, although the compound displays geometric isomerism.

A tri-substituted alkene.

The Cahn-Ingold-Prelog rules are applied to determine the priority of the groups where is restricted rotation. If the groups that have the same priority are on opposite sides of the double bond, then the compound is stated as E. If these groups are the on the same side, then the compound is stated as Z.