Types of Isomerism: Structural and Stereo Isomerism

Isomerism is a phenomenon in which two or more chemical compounds have the same molecular formula. Isomerism is derived from Greek words isos, which means “equal,” and meros, which means “part.” 

There are two main types of isomerism, each of which are further divided into subgroups. These primary types are Structural Isomerism and Stereoisomerism. Let us go through the diagram to know more about the types of isomerism and its example.

What is Isomerism?

Isomerism refers to two or more compounds having the same molecular formula but distinct chemical and physical properties.

Isomerism is a widespread phenomenon in organic chemistry and is one reason for the existence of many organic compounds. Berzelius coined the term isomerism (Greek, isos meaning equal and meros meaning parts) to describe compounds with the same molecular formula but different physical or chemical properties.

Types of Isomerism

The two types of isomerism are-
a. Structural or constitutional isomerism
b. Stereoisomerism or spatial isomerism

Structural isomerism is caused by differences in chemical structures, whereas stereoisomerism is caused by differences in spatial arrangements of atoms or groups. Thus, the chemical structure of the two isomers can differ in various ways.

Depending upon the variety of structural differences, we have several types of structural isomerism, e.g., chain isomerism, position isomerism, functional isomerism, metamerism, ring chain isomerism, and tautomerism.
The type of isomerism exists between different compounds, as given below.

Structural Isomerism

Compounds with the same molecular formula but different structures, i.e., atom arrangements within the molecule, are structural isomers. Thus, the phenomenon is referred to as structural isomerism.

\(\left( {{\rm{C}}{{\rm{H}}_3} – {\rm{C}}{{\rm{H}}_2} – {\rm{O}} – {\rm{H}}} \right)\) and \(\left( {{\rm{C}}{{\rm{H}}_3} – {\rm{O}} – {\rm{C}}{{\rm{H}}_3}} \right)\)

Study Isomerism in Alcohols Here

Structural isomerism is of the following types: 

i. Chain Isomerism

Compounds with the same molecular formula but different arrangements of the carbon chain within the molecule are called chain or nuclear isomers. The phenomenon is called chain isomerism.

For example;

In the given structure, both \(1 – \) bromobutane and \(1 – \)Bromo \(-2 – \) methylpropane have the same molecular formula, but \(1 – \) bromobutane contains a straight chain of four carbon atoms, whereas \(1 – \) Bromo \(-2 – \) methylpropane has a chain of only three carbon atoms. Thus, the two differ in their carbon skeleton.

ii. Position Isomerism

Position isomerism occurs when two or more compounds with the same molecular formula have different positions of a double bond, triple bond, or functional group. The compounds are known as position isomers.
For example,

a. But \( – 1 – \) ene and But \( – 2 – \) ene have the same molecular formula, \({{\text{C}}_4}{{\text{H}}_8}\) and are position isomers because they have different positions of double bonds shown below

b. Propan \( – 1 – {\text{ol}}\) and Propan \( – 2 – {\text{ol}}\) have the same molecular formula but possess -\({\text{OH}}\) groups at different positions. Therefore, they are also position isomers.

iii. Functional Isomerism

Compounds having the same molecular formula but different functional groups are called functional isomers, and the phenomenon is called functional isomerism.

Many classes of organic compounds show functional isomerism among themselves are given below;

a. Alcohols and ethers show functional isomerism: Ethanol and methoxy methane have the same molecular formula, \({{\text{C}}_2}{{\text{H}}_6}{\text{O}}\) but contain different functional groups. Ethanol contains the \( – {\text{OH}}\) group and belongs to the alcohol family, whereas methoxymethane contains the \({\text{-O-}}\) group and belongs to the ether family.

b. Aldehydes, ketones, unsaturated alcohols, and unsaturated ether \(\left({{{\text{C}}_{\text{n}}}{{\text{H}}_{2{\text{n}}}}{\text{O}}} \right)\) show functional isomerism. For example, \({{\text{C}}_3}{{\text{H}}_6}{\text{O}}\) represents the functional isomers.

b. Carboxylic acid and ester \(\left( {{{\text{C}}_{\text{n}}}{{\text{H}}_{2{\text{n}}}}{{\text{O}}_2}} \right)\) show functional isomerism: For example, \({{\text{C}}_2}{{\text{H}}_4}{{\text{O}}_2}\) represents the following two functional isomers.

c. Aromatic alcohols, phenols, and ethers show functional isomerism: For example, \({{\text{C}}_7}{{\text{H}}_8}{\text{O}}\) represents the following functional isomers.

iv. Metamerism

Metamerism is a phenomenon that occurs when compounds have the same molecular formula but a different number of carbon atoms on either side of the functional group (i.e., \( – {\text{O}} – , – {\text{S}} – , – {\text{NH}}\)). Metamerism occurs when members of the same homologous family interact.

For example,

a. The molecular formula \({{\text{C}}_4}{{\text{H}}_{10}}{\text{O}}\) represents the following three ethers. All three compounds possess the same functional group, \( – {\text{O}} – \) and belong to the same family. The alkyl groups present on both sides of the group \( – {\text{O}} – \) are different in all three compounds. Hence, they are metamers.

b. Some metamers also show position isomers. For example, pentan \( – 2 – \) one and pentan \( – 3 – \) one may be regarded as position isomers as well as metamers.

or

v. Ring-Chain Isomerism

This type of isomerism occurs due to the difference in the linking mode of carbon atoms. When two or more compounds have the same molecular formula but differ in linking carbon atoms and have an open chain and closed chain structures, the phenomenon is called ring-chain isomerism. Thus, the compound is called ring-chain isomers.

For example,

a. Propane and cyclopropane have the same molecular formula \({{\text{C}}_3}{{\text{H}}_6}\) but have different modes of linking carbon atoms. Propane has an open chain structure, whereas cyclopropane has a cyclic structure.

vi. Tautomerism

It is a special kind of functional isomerism in which the isomers exist in dynamic equilibrium with each other. It results from the migration of a hydrogen atom from one polyvalent atom to the other within the same molecule, which requires the rearrangement of linkages. The resulting isomers are known as tautomers, and the phenomenon is tautomerism.

For examples;

a. Hydrocyanic acid \(\left({{\text{HCN}}} \right)\) exists in the following tautomeric forms;

b. Nitrous acid \(\left({{\text{HN}}{{\text{O}}_2}} \right)\) exists in the following tautomeric forms:

Keto-Enol Tautomerism

Tautomerism is of various types; among these, keto-enol tautomerism is the most important one.

In keto-enol tautomerism, a compound exists in two interconvertible forms, one containing a keto group known as keto-form and an alcoholic group known as enolic-form. The two forms continuously change into each other through the oscillation of a proton and \(\pi \)-electrons.

For example,

a. Acetyl acetone shows keto-enol tautomerism

b. Acetone shows keto-enol tautomerism

Characteristics of Tautomeric Changes

1. Tautomerism is caused by a hydrogen atom wandering between two polyvalent atoms in the molecule. The modification is accompanied by a reorganization of single and double bonds.
2. It is a reversible intermolecular change.
3. The two tautomeric forms are not equally stable. The less stable form is called the labile form.
4. Ordinary separation methods are difficult to use because the two tautomeric forms remain in dynamic equilibrium. However, special techniques can be used to separate the two tautomeric forms of a material.

Type of Isomerism in Alkanes

Alkanes do not have multiple bonds or functional groups. As a result, they can only exhibit chain isomerism. The first three members, namely methane, ethane, and propane, do not show chain isomerism because they cannot display different carbon atom arrangements in the main chain. As a result, they have only one type of structural formula.

Butane \(\left( {{{\text{C}}_4}{{\text{H}}_{10}}} \right)\)

Butane has two chain isomers, \({\text{n}}\)-butane and iso-butane. The isomers differ in their carbon skeleton; \({\text{n}}\)-butane possesses a straight chain, whereas iso-butane has a branched chain.

Pentane \(\left( {{{\text{C}}_5}{{\text{H}}_{12}}} \right)\)

Pentane has three chain isomers, \({\text{n}}\)-pentane, iso-pentane, and neo-pentane.

Hexane \(\left( {{{\text{C}}_6}{{\text{H}}_{14}}} \right)\)

Hexane has five chain isomers. Their structures and names are as follows;

From the above discussion, it is clear that the number of structural isomers increases with an increase in carbon atoms for alkanes.

StereoIsomerism

Stereoisomers have the same structural formula but have different relative arrangements of atoms of groups in space. Stereo isomerism is a phenomenon that occurs when two isomers have the same structural formula but different close arrangements of atoms of groups in space.

StereoIsomerism is mainly of two types.

1. Geometrical isomerism
2. Optical isomerism

Summary

From this article, we can conclude that isomers are compounds with different chemical and physical properties. The chemical structures of the two isomers differ with the spatial arrangement of atoms or groups in their molecules.

Differences in chemical structures cause structural isomerism. Stereoisomerism is caused by the difference in spatial arrangements of atoms and groups.

FAQs

Q.1. Give some isomerism examples.

• Ans: Examples of isomerism
• I. Chain isomerism: Example, \(1 – \) bromobutane and \(1 – \) bromo \(-2 – \) methylpropane
• II. Position isomerism: Example, But \(-1 – \) ene and But \(-2 – \) ene
• III. Functional isomerism: Ethanol and Methoxy ethane
• IV. Metamerism: \(1 – \)methoxypropane, \(1 \) methoxypropane and ethoxyethane
• V. Ring-chain isomeris: Propane and cyclopropane
• VI. Tautomerism: Hydrocynic acid
• VII. Geometrical isomerism: cis-but \(-2 – \) ene and trans-but-e-ene
• VIII.Optical isomerism: \({\text{d}}\)-lactic acid and \({\text{I}}\)-lactic acid.

Q.2. What are the \(3\) types of isomers?
Ans: Isomers are of two types, namely structural isomers and stereoisomers. Structural isomers are of six kinds, and stereoisomers are of two kinds.

Q.3. Which type of isomerism cannot be shown by benzaldoxime?
Ans: There is no optical, functional, geometrical isomerism in benzaldoxime.

Q.4. What are the \(4\) types of isomers?
Ans: There are only two types of isomers. They are;
i. Structural isomers
ii. Sterio isomers

Q.5. What are the different types of isomerism?
Ans: Depending upon the variety of structural differences, we have several types of structural isomerism, e.g., chain isomerism, position isomerism, functional isomerism, metamerism, ring chain isomerism, and tautomerism. The various types of isomerism could be summarised as given below.
1. Structural isomerism
I. Chain isomerism
II. Position isomerism
III. Functional isomerism
IV. Metamerism
V. Ring-Chain isomerism
VI. Tautomerism
2. Sterio isomerism
I. Geometrical isomerism
II. Optical isomerism

Q.6. What type of isomerism is shown by butane and isobutane?
Ans: Butane and isobutane exhibit chain isomerism.

Q.7. How to identify the type of isomerism?
Ans: If the Compound has the same molecular formula but different structures, i.e., different atom arrangements within the molecule, they are referred to as structural isomers. Compounds with the same structural formula but a different relative arrangement of atoms or groups in space are called stereoisomers.