(Geometric Isomerism & Optical Isomerism)


Isomerism is the phenomenon whereby certain compounds, with the same molecular formula, exist in different forms owing to their different organisations of atoms. The concept of isomerism illustrates the fundamental importance of molecular structure and shape in organic chemistry.


What are Isomers?

Isomers are molecules that have the same molecular formula, but have a different arrangement of the atoms in space. That excludes any different arrangements which are simply due to the molecule rotating as a whole, or rotating about particular bonds.

For example, both of the following are the same molecule. They are not isomers. Both are butane.



Structural Isomers are molecules which have the same molecular formula but have different connectivities (The Order They Are Put Together). Alkanes can be very simple examples of this.

With the structural formula C4H10 there are two different isomers possible.

As the number of Carbons in an alkane increases, the number of structural isomers also increases.

Types of Structural Isomerism

Chain isomerism

These isomers arise because of the possibility of branching in carbon chains.

For example, there are two isomers of butane, C4H10. In one of them, the carbon atoms lie in a "straight chain" whereas in the other the chain is "branched".

Pentane, C5H12, has three chain isomers. If you think you can find any others, they are simply twisted versions of the ones below. If in doubt make some models.

Position isomerism

In position isomerism, the basic carbon skeleton remains unchanged, but important groups are moved around on that skeleton.

For example, there are two structural isomers with the molecular formula C3H7Br. In one of them the bromine atom is on the end of the chain, whereas in the other it's attached in the middle.

Another similar example occurs in alcohols such as C4H9OH

These are the only two possibilities provided you keep to a four carbon chain, but there is no reason why you should do that. You can easily have a mixture of chain isomerism and position isomerism - you aren't restricted to one or the other.

So two other isomers of butanol are:

Functional group isomerism

In this variety of structural isomerism, the isomers contain different functional groups - that is, they belong to different families of compounds (different homologous series).

For example, a molecular formula C3H6O could be either propanal (an aldehyde) or propanone (a ketone).

Another example is propan-1-ol and propan-2-ol

Different Functional groups

Example- The molecular formula C2H60 represents both Ethanol and Methoxymethane.

Cyclic alkanes are isomeric with alkenes, e.g. cyclopropane and propene



Stereoisomers have the same structure and bond order but their atoms and groups of atoms are arranged differently in space. They have different spatial arrangements and their molecules are not superimposable. There are two types:

Geometric Isomerism

Involves a double bond, usually C=C, that does not allow free rotation about the double bond (unlike a C-C single bond). They are not superimposable.

These isomers occur where you have restricted rotation somewhere in a molecule.

Think about what happens in molecules where there is unrestricted rotation about carbon bonds - in other words where the carbon-carbon bonds are all single. The next diagram shows two possible configurations of 1,2-dichloroethane.

But what happens if you have a carbon-carbon double bond - as in 1,2-dichloroethene

These two molecules aren't the same.

In one, the two chlorine atoms are locked on opposite sides of the double bond. This is known as the Trans isomer. (trans : from latin meaning "across" - as in transatlantic).

In the other, the two chlorine atoms are locked on the same side of the double bond. This is know as the Cis isomer. (cis : from latin meaning "on this side")

The most likely example of geometric isomerism you will meet at an introductory level is but-2-ene. In one case, the CH3 groups are on opposite sides of the double bond, and in the other case they are on the same side.

Optical isomerism

Introduction to Chiral or Optical Isomers:
Some chemical compounds have optical activity in the sense that these compounds have the ability to rotate the plane of polarized light. Polarized light has light waves all traveling parallel to each other. Ordinary light has light waves traveling in all directions. When polarized light is passed through a solution of an optically active compound, the plane of polarization is rotated to the right or the left. The angle of rotation can be measured in a polarimeter.

An optically active organic compound can be identified by finding a chiral carbon.

A chiral carbon is one that has four different "groups" attached to it.

The groups can be anything from a single H to functional groups to one or more other carbons. See bromochloroiodomethane on the left - it has 3 halogens and one hydrogen.

With optical isomerism, there is no difference in connectivity and no double bonds. The isomerism is to do with the arrangement of the atoms in space. It arises through the presence of a Chiral Centre. Optical isomers are Non Superimposable Mirror Images of each other; a set of optical isomers are called enantiomers.

Optically active compounds exist in two isomeric forms. The isomer that rotates the plane of polarized light to the left (counterclockwise) is called levorotatory (l). The other isomer that rotates the light to the right (clockwise) is called dextrorotatory (d).

The optical isomers are mirror images of each other. The isomers result from the tetrahedral geometry around the chiral carbon center. To draw mirror images, write the structure of the first isomer, draw a plane (dotted line) to represent the mirror, and finally draw the mirror image behind the plane.


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