Isomerism – Meaning, Examples and Types

Isomerism

A type of structural isomerism where molecules with the same molecular formula have different connectivity of their atoms. For example, two molecules of ethanol (CH3CH2OH) have the same molecular formula, but they are different molecules because the atoms are connected differently. One is the gas ethane (CH3CH3) and the other is liquid ethanol.

What do you mean by Isomerism? & What is an Isomer?

• Isomerism a phenomenon exhibited by molecules that have the same molecular formula but different structural formulae.
• Isomers can distinguished by their different physical and chemical properties.
• An isomer is a molecule that has the same molecular formula but different structural formulae.

Examples of Isomers-

There are two types of isomers- structural isomers and stereoisomers.

1. Structural isomers are molecules that have the same atoms, but the atoms connected in different ways. For example, the molecules ethane and ethanol have the same atoms, but the atoms connected differently in each molecule.
2. Stereoisomers are molecules that have the same atoms and the same bonds, but the molecules arranged differently in space. For example, the molecules of glucose and fructose have the same atoms and the same bonds, but the molecules arranged differently in space. This is because the molecules of glucose are all right-handed, while the molecules of fructose are all left-handed.

Types of Isomers or Isomerism –

There are two types of isomerism: structural isomerism and stereoisomerism.

1. Structural isomerism is a type of isomerism in which molecules with the same molecular formula but different structures called isomers. For example, butane and isobutane are structural isomers because they have the same molecular formula (C4H10) but different structures.
2. Stereoisomerism is a type of isomerism in which molecules with the same molecular formula and same structure called isomers. For example, 2-butanol and 2-methyl-2-propanol are stereoisomers because they have the same molecular formula (C4H10O) and same structure, but they are not identical.

Structural isomerism can further divided into following subtypes-

Conformational isomerism- In this type of isomerism, the atoms or groups of atoms within a molecule are still in the same order, but the orientation of the molecule is different. This can be due to rotation around a single bond, or a double bond.

• Enantiomerism- This type of isomerism occurs when there is a mirror image of the molecule. The atoms are still in the same order, but the orientation reversed. This can be due to a chiral carbon atom.
• Metamerism Isomerism – Those structural isomers which differ in their alkyl groups which are attached to their functional groups, known as metamerism isomers and the phenomenon known as metamerism isomerism.
• Optical Isomerism – Optical isomerism a phenomenon exhibited by certain molecules that have the same chemical formula but different shapes. These molecules called optical isomers, or enantiomers. The different shapes of the molecules are due to the presence of different groups of atoms bonded together in a particular way.
• Geometric Isomerism – Geometric isomerism a phenomenon exhibited by certain molecules that have the same chemical formula but different shapes. These molecules called geometric isomers, or constitutional isomers. The different shapes of the molecules are due to the presence of different groups of atoms bonded together in a particular way.

Example- diethyl ether & methyl propyl ether

The two ethers are very similar in their chemical properties. They are both colorless, volatile liquids that are soluble in water. They have a sweet, ether-like odor and are flammable.

Diethyl ether [on both sides alkyl group -ethyl(C2H5) is attached]

C2H5-O-C2H5

Tautomerism isomerism – Those structural isomers which differ in the position of hydrogen atoms or protons or electrons, are known as tautomers and this phenomenon known as tautomerism. It is also known as desmotropism (desmos- bond & tropism-turn). Tautomers occur in equilibrium state with each other as they are easily inter-convertible.

Enantiomers and diastereomers:

Stereoisomers are isomers that have the same molecular formula but a different arrangement of atoms in space. This can be due to a different sequence of bonded atoms, as in the case of optical isomers, or to the presence of asymmetric atoms, as in the case of chiral molecules.

There are two types of stereoisomers: enantiomers and diastereomers.

• Enantiomers are isomers that are mirror images of each other. They also called optical isomers because they rotate the plane of polarized light.
• Diastereomers are isomers that are not mirror images of each other. They may have different physical properties, such as melting point and boiling point, and they may have different reactivities.
• Ring-chain isomerism – Those structural isomers in which one isomer has open chain structure while another one has closed chain or ring structure, are known as ring-chain isomers and this phenomenon called ring-chain isomerism.
• Aldehyde – In chemistry, an aldehyde is an organic compound containing a functional group with the structure −CHO. This group composed of a carbon atom bonded to a hydrogen atom, in particular, aldehydes are formaldehyde, acetaldehyde, and propionaldehyde.

Examples- Butene & cyclobutane

Butene is a four-carbon alkene molecule with two double bonds. Cyclobutane is a four-carbon cycloalkane molecule with no double bonds.

Stereoisomerism

Those compounds which have same molecular formula but show different spatial arrangement of atoms in them are known as stereoisomers and the phenomenon known as stereoisomerism.

There are two types of stereoisomers: enantiomers and diastereomers.

• Enantiomers are two stereoisomers that are mirror images of each other.
• Diastereomers are stereoisomers that are not mirror images of each other.

Example – 1,2-dichloroethene (C2H2Cl2)

1,2-dichloroethene is a colorless, flammable gas with a sharp, unpleasant odor. It used in the manufacture of plastics, solvents, and pesticides.