Isomerism: Definition, Types, Structural Isomerism

Isomerism and its Types

In organic chemistry, isomers are particles with the very same molecular formula (i.e., the same number of atoms of each element), but different structural or spatial arrangements of the atoms within the molecule. The reason there is such an enormous variety of organic compounds– more than 10 million– remains in part down to isomerism.

Definition of Isomerism

Isomerism is the phenomenon in which more than one compounds have the same chemical formula however various chemical structures. Chemical compounds that have similar chemical formulae but differ in properties and the plan of arrangement of the atoms in the molecule are called isomers. Therefore, the compounds that exhibit isomerism are called isomers.

The word “isomer” is originated from the Greek words “isos” and “meros”, which mean “equal parts”. This term was coined by the Swedish chemist Jacob Berzelius in the year 1830.

Types of Isomerism

Generally, there are 2 types. They are:

  • Structural Isomerism
  • Stereoisomerism

However, these have again many subtypes.

Structural Isomerism

Isomers are structural isomers when they have the very same molecular formula however different structures. Structural isomerism is even more of the following types. Let’s learn more about these types.

Further Reading:  Oxygen [Occurrence, Properties, Allotropes, Isotopes and Uses of Oxygen]
1) Chain Isomerism

Isomers are chain isomers when 2 or more compounds have the exact same molecular formula however vary in the branching of carbon atoms.


2) Position isomerism

Isomers are position isomers when the two or more compounds differ in the position of the functional group or substituent atoms.


3) Functional Isomerism

Isomers are functional isomers when the two or more compounds have an identical molecular formula but vary in the functional group present.


4) Metamerism

This is shown by compounds due to the existence of different alkyl chains on either side of the functional group.


Stereo Isomerism

Stereoisomerism is a phenomenon in which compounds have the same molecular formula however vary in the relative positioning or orientation of atoms in space. Stereoisomers are the compounds displaying stereoisomerism. We can, even more, categorize stereoisomerism into:

(a) Geometrical isomerism:

The isomers which have the same structural formula however differ in the spatial arrangement of the groups around the double bond are called geometrical isomers and the phenomenon is referred to as geometrical isomerism. This isomerism is shown by alkenes or their derivatives. In cis-isomer, similar groups reside on the same side, while the similar groups when lying on opposite sides, the isomer is referred to as trans.


(b) Optical isomerism:

This kind of isomerism occurs from various plans of the arrangement of atoms or groups in space, leading to two isomers which are the mirror images of each other. Optical isomers include an asymmetric (chiral) carbon atom (a carbon atom attached to four different atoms or groups) in their molecules. Optical isomers have similar chemical and physical properties and vary only in their behavior towards plane-polarized light.

Further Reading:  Vapour Pressure

The isomer which turns the plane-polarized light to left is called laevo (1) while that rotates the plane-polarized light to the right is called Dextro (d).



Other Isomerism displayed by sugars
Ketose-Aldose isomerism

Glucose and fructose are isomers of each other having the same chemical (molecular) formula C6H12O6, however, they differ in structural formula with respect to their D and L isomers are mirror images of each other. These two kinds are called enantiomers. Most of the monosaccharides in living beings belong to the D-series.



When two monosaccharides differ from each other in their configuration around a single uneven carbon (aside from anomeric carbon) atom, they are referred to as epimers of each other.

For instance, galactose and mannose are 2 epimers of glucose. They vary from glucose in the configuration of groups (H and OH) around C-4 and C-2 respectively. Galactose and mannose are not epimers of each other as they vary in configuration at 2 asymmetric carbon atoms around C-2 and C-4.



α and β Anomerism

The primary type of glucose and fructose in a solution is not an open chain. Rather, the open-chain form of these sugar in solution cyclize into rings. An extra uneven center is created when glucose cyclizes. Carbon-1 of glucose in the open-chain type ends up being an uneven carbon in the ring type and 2 ring structures can be formed. These are:

  • – α-D-glucose
  • – β-D-glucose
Further Reading:  Niobium: Occurrence, Properties, Uses, and Isotopes of Niobium

The classification α implies that the hydroxyl group attached to C-1 is below the plane of the ring, β implies that it is above the plane of the ring. The C-1 carbon is called the anomeric carbon atom and so, α and β types are anomers.


Frequently Asked Questions

Q1: What is the condition for geometric isomerism?

Ans: The necessary condition for the compound in having geometric isomerism is that the two groups attached to the same carbon must be different.

Q2: Why there is free rotation around a single bond?

Ans: The single bond formation is by linear overlap of sp3 orbitals of two carbon atoms. So, the sigma electron density is between two nuclei therefore groups attached to carbon atoms are capable of free rotation.

Q3: Why there is no rotation around double bond?

Ans: A double bond has a sigma and pi bond. So, the sigma electron density is between two nuclei and pi-electron density is above and below the line joining two nuclei which causes hindrance in movement. Hence there is no free rotation around the double bond.