Isomerism- Definition, Classification, Structural Isomerism, Stereoisomerism, Practice Problems & FAQs

Suppose there is a class of 50 students having 5 rows and in each row, there are 10 seats. The teacher exchanged the first row of students with the last row as the last row was creating a disturbance in the classroom.

Can we say that before and after exchanges the first and the last row of students, the class will look exactly identical? Will there be any difference between the two setups?

The classroom will look identical if we look at it from one angle. But if we compare the sitting of students in two different setups, the total number of students in both cases will be the same but there will be some difference in the behavioural approach of the students in the second case. More decorum is maintained in the classroom as compared to the first setup.

A similar observation is seen in the case of two different compounds that have the same chemical formula but differ in physical and chemical properties. In 1830, Swedish chemist Jacob Berzelius coined the term isomers for these compounds.

Let’s discuss more about isomerism and the types of isomerism.

Table of Contents

• What is an Isomerism?
• Classification of Isomerism
• Types of Isomerism
• Practice Problems
• Frequently Asked Questions-FAQs

What is an Isomerism?

Isomerism is the phenomenon where two or more compounds have the same chemical formula but differ in one or more physical or chemical properties. Chemical compounds have identical formulae but different physical or chemical properties are named isomers.

“Isomer” word is derived from two Greek words “isos” and “meros”, which mean “equal” and “parts” respectively.

For example

Pentane, 2-methylbutane and 2,2-dimethylpropane are isomers of each other. They have the same molecular formula (C₅H₁₂) but have different physical and chemical properties.

Classification of Isomerism

Types of Isomerism

Structural isomerism: Structural isomerism is the phenomenon where chemical compounds have the same molecular formula but different chemical structures. Structural isomers differ in the functional group or the atom linked to the central atom or both. Structural isomerism is also called constitutional isomerism. In the structural isomers, the molecular formula remains the same though the functional group may or may not be the same.

There are six different types of structural isomerism:

• Chain isomerism: The phenomenon where structural isomers have the same molecular formula but differ in the length of the parent chain of carbon atoms is called chain isomerism. Chain isomerism is also called skeletal isomerism. For example, pentane, 2-methyl butane and 2,2-dimethlpropane are chain isomers of C₅H₁₂.

• Position isomerism: The phenomenon where structural isomers differ in the position of functional groups or substituent atoms or multiple bonds is called positional isomerism. For example: 2-chlorobutane and 1-chlorobutane are position isomers of each other.

• Functional isomerism: It is the type of isomers formed by compounds that have the same molecular formula but different functional groups. For example, propanone and propanal are functional isomers as they both have the same molecular formula but different functional groups.

• Metamerism: Metamerism is a phenomenon where structural isomers differ in the arrangement of alkyl groups present around the functional group. Metamerism is a rare type of isomerism and is mostly seen in chemical molecules containing divalent atoms attached by alkyl groups. For example, ethoxy ethane and methoxy propane show metamerism as the functional group are the same but the length of the alkyl group is different.

• Tautomerism: The phenomenon in which a single chemical compound exists in two or more interconvertible structures that are different in terms of the position of protons or electrons or hydrogen in each isomer. Tautomerism occurs in only a few chemical compounds and is caused by intramolecular proton transfer. For example Keto-enol tautomerism.

• Ring and chain isomerism: In ring-chain isomerism, one of the structural isomers exist in aliphatic chain form whereas the other one has a ring in it. For example Cyclopropane and Prop-1-ene.

Stereoisomerism: Isomers having the same chemical formula but differing in the spatial arrangement of atoms is called stereoisomers and the phenomenon is known as stereoisomerism. This phenomenon is observed in compounds that exist in a 3D space with different orientations.

There are two types of stereoisomerism:

Conformational isomerism: Conformational isomerism is defined as the spatial arrangement of atoms that gets converted into one another by rotation around C-C single bonds. It is also known as conformers or rotamers. Based on the angle of the rotation of the bond, different types of conformers can be obtained, mainly eclipsed and staggered forms. These conformers have different stability, but can be interconverted from one form to another when the difference in the amount of energy is supplied. For example, the conformation of ethane is

Configurational isomerism: It is a type of isomerism in which isomers differ in their configuration or the spatial arrangement of atoms that distinguishes one stereoisomer from another. There are two different types of configuration isomers:

1. Geometrical isomerism: Geometric isomerism is popularly called cis-trans isomerism. Chemical compounds differ in the arrangement of atoms or groups of atoms around the carbon-carbon double bond in geometric isomerism. This isomerism is shown due to the restricted rotation of the system present around the given atom in which double-bonded and ring carbon atoms act as a rotation-restricted system. For example

2. Optical isomerism: Optical isomerism is characterised by the formation of non-superimposable mirror images. The formation of non-superimposable mirror images occurs when chemical structures with the same molecular formula differ in the spatial arrangement of atoms. These non-superimposable mirror images are called enantiomers. For example:

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Practice Problems

Q. Which of the following pairs of molecules may represent isomers?

1. Pentane and butane
2. Ethane and ethene
3. Butane and 2-methyl propane
4. Cyclopropane and butane

Answer: (C)

Solution: Isomerism is the phenomenon in which two or more compounds have the same chemical formula but differ in one or more physical or chemical properties. Chemical compounds have identical formulae but different physical or chemical properties are named isomers. Butane and 2-methyl propane have the same molecular formula of C₄H₁₀ and therefore it may represent an isomeric pair but in other options, the molecular formula of compounds is different and cannot be considered as an isomeric pair.

So, option C is the correct answer.

Q. Considering the two compound X and Y, thet both are geometric isomers of each other. Select the correct option with respect to properties of the compounds X and Y.

1. Both X and Y have the same physical and chemical properties.
2. X and Y have the same physical properties but different chemical properties.
3. Both X and Y have the different physical and chemical properties.
4. X and Y have may or may not have different chemical properties.

Answer: (D)

Solution: Chemical compounds differ in the arrangement of atoms or groups of atoms around the carbon-carbon double bond in geometric isomerism. Usually, for geometric isomers, physical properties are different but chemical properties may or may not be different. So, the most appropriate answer would be option D.

Q. From the given set of compounds: pentane, butane, 2-methyl butane, propane. The number of pairs of isomers possible will be_____.

1. 1
2. 2
3. 4
4. 3

Answer: (A)

Solution: Chemical compounds have an identical formula but different physical or chemical properties are named isomers. Therefore, only pentane and 2-methyl butane can show isomerism because they have the same chemical formula (C₅H₁₂) and the number of possible pairs is equal to 1.

Q. Which type of isomerism is exhibited by 1-butanol and butan-1,2-diol molecule?

1. Chain isomerism
2. Functional isomerism
3. Positional isomerism
4. Will not show isomerism

Answer: (D)

Solution: As we know for a compound to exhibit isomerism it should have the same molecular formula but in the case of 1-butanol (C₄OH₉) and butan-1,2-diol (C₄O₂H₁₀) molecules, the molecular formulae are different and therefore it will not exhibit the isomerism.

Frequently Asked Questions-FAQs

Q. What is the difference between constitutional and configurational isomerism?
Answer: Constitutional or Structural isomerism is the phenomenon where chemical compounds have the same molecular formula but different chemical structures. Structural isomers differ in the way functional groups and other atoms are linked to the central atom. Structural isomerism is also called constitutional isomerism.

Whereas, configurational isomerism is a type of isomerism in which isomers differ in configuration i.e. the spatial arrangement of atoms is different.

Q. What is a non-superimposable mirror image?
Answer: A pair of stereoisomers that are non-superimposable mirror images of one another is referred to as an enantiomer. This means the molecules are made up of identical atoms that are bound together in the same way, i.e., they are connected in the same way but on the other hand, have a distinct 3D arrangement of atoms since they are mirror reflections of each other. Without breaking and reconstructing bonds, you can not superimpose one on top of the other.

Q. Why is rotation along double-bonded atoms difficult as compared with the single-bonded atoms?
Answer: Rotation around a single bond is easy, but it is difficult to rotate around a double bond. Because the electrons overlap both above and below the plane of the atoms, the pi bond inhibits rotation.

Q. How can the different isomers formed be separated?
Answer: Isomerism is the phenomenon in which two or more compounds have the same chemical formula but differ in one or more physical properties like melting point, boiling point, density etc. or chemical properties. Therefore the isomers can be separated by processes like fractional distillation, fractional crystallisation, chromatography technique etc.