You have landed on the right page to learn about Isomerism. During the study of organic chemistry, we come across many such compounds with different properties but the same molecular formula. “Isomer” comes from the Greek words “isos” and “meros,” which both imply “equal pieces.” In the year 1830, the Swedish chemist Jacob Berzelius created the word.

For example, ethyl alcohol \(\left({{\text{C}}{{\text{H}}_2}{{\text{H}}_5}{\text{OH}}} \right)\) and dimethyl ether \(\left({{\text{C}}{{\text{H}}_3}{\text{OC}}{{\text{H}}_3}} \right)\) have different properties but the same molecular formula, \({{\text{C}}_2}{{\text{H}}_6}{\text{O}}.\) These compounds are known as isomers. To learn more about isomerism, read the below article.

Define Isomerism

The ability of organic compounds to exhibit isomerism is one of their most important properties. Isomers are two or more compounds that have the same molecular formula but differ in their physical and chemical properties. This phenomenon is known as isomerism.

Types of Isomerism

There are two types of isomerism: These are structural isomerism or constitutional isomerism and stereoisomerism.

Learn about Geometrical Isomerism here

Structural Isomerism or Constitutional Isomerism

The number, kind, and connectivity of atoms in a molecule are referred to as its constitution. The isomers which differ in the connectivity of their atoms are called structural isomers or constitutional isomers and this phenomenon is known as constitutional isomerism or structural isomerism.

For example, ethanol and dimethyl ether are isomers because they have the same molecular formula. \({{\text{C}}_2}{{\text{H}}_6}{\text{O}}.\) These are structural isomers because the atoms in each compound are connected differently. The following are some typical examples of various types of structural isomerism:

• I. Chain isomerism
• II. Position isomerism
• III. Functional isomerism
• IV. Metamerism
• V. Tautomerism
• VI. Ring chain isomerism

I. Chain Isomerism

Isomeric substances that differ only in the arrangement of carbon atoms forming the base chain are known as chain isomers and isomerism between them is known as chain isomerism. The isomers of this type differ in the carbon atom chain or skeleton. They can have a straight or branched chain of carbon atoms. It should be noted that compounds containing up to three carbon atoms do not exhibit chain isomerism because they are a continuous chain with no possibility of branching. It is shown in the case of compounds containing four or more carbon atoms. To illustrate chain isomerism, consider the molecules of alkanes, alkenes and alkynes.

a. Chain isomerism in alkanes: It is shown by the alkanes with four or more carbon atoms. For example,

i. The molecular formula \({{\rm{C}}4}{{\rm{H}}{10}}\) represents two chain isomers.

b. Chain isomerism in alkenes: The position of the double bond is not to be changed when writing the chain isomers for alkenes. For example,

i. The molecular formula \({{\text{C}}_4}{{\text{H}}_{8}}\) represents two chain isomers as shown below:

c. Chain isomerism in alkynes: In alkyne, the compounds with five or more carbon atoms exhibit chain isomerism in alkynes. For example, the molecular formula \({{\text{C}}_5}{{\text{H}}_{8}}\) represents the two chain isomers shown below.

II. Position Isomerism

Position isomers are two or more compounds that differ in the position of a constituent atom or functional group on the carbon skeleton, and the phenomenon is known as position isomerisation.
For example, \({{\text{C}}_3}{{\text{H}}_8}{\text{O}},\) represents two alcohols:

III. Functional Isomerism

Functional isomers are two or more compounds that have the same chemical formula but different functional groups. This phenomenon is referred to as functional isomerism. For example, the molecular formula \({{\text{C}}_3}{{\text{H}}_6}{\text{O}}\) represents an aldehyde and a ketone:

IV. Metamerism

Metamers are compounds with the same molecular formula but different atomic or group positions on either side of bridging functional groups. The nature of the alkyl groups surrounding a specific functional group distinguishes the metamers. As a result, they are related. The molecular formula \({{\text{C}}_4}{{\text{H}}_{10}}{\text{O,}}\) for example, accounts for three isomeric ethers that are metamers.

V. Tautomerism

Tautomers are functional isomers that exist simultaneously and in dynamic equilibrium. Tautomerism refers to isomerism. There are several types, but the most common is keto-enol tautomerism, which occurs as a result of the \(1,3\) migration of the hydrogen atom from carbon to oxygen atom and vice versa.
For example,

The presence of an \(\alpha \)- hydrogen, which is involved in \(1,3\) – migration, is required for an aldehyde or ketone to exhibit tautomerism. Tautomerism is not shown in the absence of an \(\alpha \) – hydrogen. For example, Acetone, acetaldehyde, and acetophenone can exhibit tautomerism due to the presence of \({\text{a}}\)—hydrogen, whereas benzaldehyde and benzophenone do not because no \(\alpha \) – hydrogen is present.

VI. Ring-chain Isomerism

Ring chain isomerism is a type of structural isomerism that is distinct from the other structural isomers mentioned above. It is found in the isomeric open-chain and cyclic compounds, as the name implies. Ring chain isomers are compounds that have the same molecular formula but have open-chain or cyclic structures. The phenomenon is known as ring-chain isomerism.

For example, the molecular formula \({{\text{C}}_3}{{\text{H}}_6}\) represents the ring chain isomers listed below.

Similarly, the molecular formula \({{\text{C}}_3}{{\text{H}}_4}\) also accounts for two such isomers.

Stereoisomerism

The compounds having the same molecular as well as same structural formulae but differing in the relative arrangement of the atoms or groups in space are called stereoisomers and this phenomenon is known as stereoisomerism.

Stereoisomerism is of the following two types:

I. Conformational isomers: Conformational isomers are stereoisomers that differ in the relative position of atoms within the molecule and can be interconverted simply by rotating about sigma bonds. In other words: the interconversion of these isomers does not require breaking and remaking of covalent bonds.
II. Configurational isomers: Configurational isomers are stereoisomers that can only be interconverted by breaking and remaking covalent bonds rather than simply rotating about sigma bonds. There are two kinds of configurational isomerism: geometrical isomerism and optical isomerism.

a. Geometrical isomerism or cis-trans isomerism: Geometrical or cis-trans isomerism refers to isomerism caused by restricted rotation around a bond in a molecule.

Geometrical isomerism is exhibited by a variety of compounds which may be classified as follows:
i. Compounds containing a double bond; \({\rm{C = C,}}\,{\rm{C = N,}}\,{\rm{N = N}}\)
ii. A compound containing cyclic structure; homocyclic, heterocyclic and fused ring system.
iii. Compounds with steric hindrance that restrict rotation around a single bond. For example, biphenyls.

Conditions for geometrical isomerism: A compound will show geometrical isomerism if it fulfils the following two conditions.
1. There should be restricted rotation about a bond in the molecule.
2. Both substituents on each carbon atom about which rotation is frozen should be different.

For example,
a. Geometrical isomerism in Alkenes:

b. Geometrical isomerism in cycloalkanes:

b. Optical isomerism: Certain compounds can exist in two stereoisomeric forms that cannot be superimposed. Despite this difference in atom arrangement, these isomers have identical physical properties such as melting point, boiling point, refractive index, density, solubility, and so on. In most of their chemical reactions, they also resemble one another. They differ only in their behaviour toward plane polarised light; one rotates the plane of polarised light to the right, while the other rotates it to the left, but to the same extent. This type of pair of compounds is known as an optical isomer.

Optical isomers are two compounds that have the same number and type of atoms, bonds, and different spatial arrangements of the atoms, but they do not have superimposable mirror images. Isomers that are non-superimposable mirror images of each other are enantiomers.

Summary

Isomers are two or more compounds that have the same molecular formula but differ in their physical and chemical properties. This phenomenon is known as isomerism. There are two types of isomerism, these are structural isomerism and stereoisomerism. Structural isomerism is of six types they are chain isomerism, functional isomerism, positional isomerism, metamerism, tautomerism, and ring-chain isomerism. Stereoisomerism is of two types they are conformational isomerism and configurational isomerism. Configurational isomerism is of two types. These are geometrical isomerism and optical isomerism.

FAQs

Q.1. What are the two types of isomers?
Ans: There are only two types of isomers. These are structural isomers and stereoisomers.

Q.2.What is isomerism and its types?
Ans: Isomers are two or more compounds that have the same molecular formula but different physical and chemical properties. This phenomenon is known as isomerism.
There are two types of isomerism. They are structural isomerism and stereoisomerism.
1. Structural isomerism
a. Chain isomerism
b. Positional isomerism
c. Functional isomerism
d. Metamerism
e. Tautomerism
f. Ring chain isomerism
2. Stereoisomerism
a. Constitutional isomerism
b. Conformational isomerism
i. Optical isomerism
ii. Geometrical isomerism

Q.3. What is isomerism? Explain different types of isomerism with examples?
Ans: Isomers are two or more compounds that have the same molecular formula but differ in their physical and chemical properties. This phenomenon is known as isomerism.
There are two types of isomerism. They are
1. Structural isomerism: The isomers which differ in the connectivity of their atoms are called structural isomers or constitutional isomers and this phenomenon is known as constitutional isomerism. Structural isomerism are of six types, They are
1. Chain isomerism: Isomeric substances that differ only in the arrangement of carbon atoms forming the base chain are known as chain isomers and isomerism between them is known as chain isomerism.

For example, The molecular formula \({{\text{C}}_4}{{\text{H}}_{10}}\) represents two chain isomers.
2. Positional isomerism: Position isomers are two or more compounds that differ in the position of a constituent atom or functional group on the carbon skeleton, and the phenomenon is known as position isomerisation.
For example, \({{\text{C}}_3}{{\text{H}}_8}{\text{O,}}\) represents two alcohols:
3. Functional isomerism: Functional isomers are two or more compounds that have the same chemical formula but different functional groups. This phenomenon is referred to as functional isomerism.
For example, the molecular formula \({{\text{C}}_3}{{\text{H}}_6}{\text{O}}\) represents an aldehyde and a ketone:
4. Metamerism: Metamers are compounds with the same molecular formula but different atomic or group positions on either side of bridging functional groups.
The molecular formula \({{\text{C}}_4}{{\text{H}}_{10}}{\text{O,}}\) for example, accounts for three isomeric ethers that are metamers.

5. Tautomerism: Tautomers are functional isomers that exist simultaneously and in dynamic equilibrium.

For example,

6. Ring chain isomerism: Ring chain isomers are compounds that have the same molecular formula but have open-chain or cyclic structures. The phenomenon is known as ring-chain isomerism.
For example, the molecular formula \({{\text{C}}_3}{{\text{H}}_6}\) represents the ring chain isomers listed below.

2. Stereoisomerism: The compounds having the same molecular as well as the same structural formulae but differing in the relative arrangement of the atoms or groups in space are called stereoisomers and this phenomenon is known as stereoisomerism.

Stereoisomers are of two types. They are conformational isomers and configurational isomers. Configurational isomers are of two types. They are

1. Geometrical isomers: Geometrical or cis-trans isomerism refers to isomerism caused by restricted rotation around a bond in a molecule.

For example,

2. Optical isomers: Optical isomers are two compounds that have the same number and type of atoms, bonds, and different spatial arrangements of the atoms, but they do not have superimposable mirror images.

For example,

Q.4. What is position isomerism with example?
Ans: When two or more compounds differ in the position of a constituent atom or functional group on the carbon skeleton, they are referred to as position isomers and the phenomenon is known as position. For example,\({{\text{C}}_3}{{\text{H}}_8}{\text{O}},\) represents two alcohols:

Q.5. What is constitutional isomerism?
Ans: The isomers which differ in the connectivity of their atoms are called structural isomers or constitutional isomers and this phenomenon is known as constitutional isomerism or structural isomerism.

Learn Isomerism in Coordination Compounds

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