Isomerism: Definition, Examples of Isomer compounds, Types, Sample Questions

Read Also: Carbon and it’s compounds

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What is Isomerism?

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In simple words, multiple compounds having the same molecular formulae but different structural formulae are referred to as isomers and this entire phenomenon is described as ‘Isomerism’. ‘Isos’ and ‘meros’ are Greek words that translate to ‘equal parts’. The term isomer is derived in 1830 by a Swedish Chemist, Jacob Berzelius. Isomer compounds will also reflect different chemical and physical properties.

 

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Examples of Isomer compounds

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1. Let’s take two Isomer compounds ‘Dimethyl ether’ and ‘Ethyl Alcohol’. Both of them have the same molecular formula, C₂H₆O, but their structural formulae are different.

2. Let’s now take three Isomer compounds ‘isopentane, ‘neopentane’, and ‘Pentane’. They have the same molecular formula, C₅H₁₂, but their structural formulae are different:

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Types of Isomerism

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Isomerism can be differentiated by primary types and their subtypes.

• Structural Isomerism

Ring Chain Isomerism, Chain Isomerism, Tautomerism, Functional Isomerism, Metamerism, and Positional Isomerism are the subtypes of Structural Isomerism.

• Stereoisomerism

Optical Isomerism & Geometric Isomerism are the two subtypes of Stereoisomerism.

Types of Isomerism

Structural Isomerism

The phenomenon when the compounds start showing isomerism because of their different structure is termed Structural Isomerism.

Read More: Isomerism in Coordination Compounds

Example - Isobutane and n-butane

Structural Isomerism can further be classified into various types such as chain isomerism, positional isomerism, functional isomerism, metamerism, ring-chain isomerism, and tautomerism.

Ring Chain isomerism

In the ring chain isomerism, there will be an open-chain structure in any one of the isomers and the remaining will have a closed chain or a ring structure. These kinds of compounds are known as ring chain isomers and this phenomenon is termed ring chain isomerism. The number of pi bonds in ring chain isomers will be different.

Example - Cyclopropane and Propene. Their molecular formula is C₃H₆

Chain isomerism

In chain isomerism, there will be different branches of carbon atoms in multiple isomers compounds but still, their molecular formula remains the same. Skeleton isomerism is the other term used to refer to chain isomerism.

Example- 2 methyl butane, 2-2 dimethyl propane, and n-pentane. Their molecular formula is C₅H₁₂

Tautomerism

In tautomerism, there will be a difference in isomer compounds only in the position of electrons, protons, and atoms. Tautomers take place in an equilibrium state as they can easily interchange.

Example - Keto-enol tautomerism

Functional Isomerism

In functional isomerism, the isomer compounds with different functional groups have the same molecular formula.

Example - dimethyl ether and ethanol. They both have the same molecular formula C₂H₆ O. The functional group of Ethanol is ‘OH’ while the Functional group of dimethyl ether is ‘R-O-R’.

Metamerism

In metamerism, each side of the functional group has the presence of different alkyl chains. Compared with other types of isomerism, metamerism is rarely found and is constrained to molecules with atoms.

Example - Methoxy-propane (CH₃OC₃H₇) & ethoxyethane (C₂H₅OC₂H₅)

Positional Isomerism

In Positional isomerism, the functional groups and substituents of a structural isomer will have different positions.

Example - 1-Chloropropane and 2-Chloropropane

Stereoisomerism

The phenomenon when compounds start showing different spatial arrangements of atoms even when their molecular formula is the same is termed Stereoisomerism.

Example - 1,2 - dichloroethene (C₂H₂CI₂)

The subtypes of Stereoisomerism are:

• Optical isomerism

In optical isomerism, the isomer Compounds differ in terms of optical activity but are mirror images of each other. In this subtype of Stereoisomerism, carbon atoms are connected to four different groups and this is known as the chiral center. Enantiomers is the term mostly used to refer to optical isomers.

In Dextro optical isomers plane of polarized light is rotated to the right. In Laevo optical Isomers, the plane of polarized light is rotated to the left.

• Geometric isomerism

In geometric isomerism, the spatial positions of molecules are locked because of the presence of a double bond or ring structure. It is often referred to as Cis-Trans isomerism. Groups connected to ring structure carbon atom must be different in geometric isomerism.

The video below explains this:

Isomerism in Coordination Compounds Detailed Video Explanation:

Read Also: Nomenclature of Coordination compounds

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Things to Remember

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• Isomerism is the phenomenon of the existence of two or more compounds having a similar chemical formula but varying structures.
• They exhibit different properties and this difference arises due to their varying spatial or structural arrangements.
• Dimethyl Ether and Ethyl Alcohol are an example of isomeric compounds.
• Isomerism can basically be differentiated into structural isomerism and stereoisomerism.
• The phenomenon when the compounds start showing isomerism because of their different structure is termed Structural Isomerism.
• The phenomenon when compounds start showing different spatial arrangements of atoms even when their molecular formula is the same is termed Stereoisomerism.

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Sample Questions