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Isomerism is the phenomenon of the existence of two or more compounds having a similar chemical formula but varying structures. The compounds exhibiting this phenomenon are known as isomers. They have different atom arrangements and properties despite having identical formulae. The difference arises due to their varying spatial or structural arrangements. Basically, we see two types of isomerism, which are structural isomerism and stereoisomerism.
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Read Also: Carbon and it’s compounds
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.
Examples of Isomer compounds
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- 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.
Molecular Formula: Dimethyl Ether Ethyl Alcohol | C2H6O |
Structural Formula of Dimethyl Ether | |
Structural Formula of Ethyl Alcohol |
- Let’s now take three Isomer compounds ‘isopentane, ‘neopentane’, and ‘Pentane’. They have the same molecular formula, C5H12, but their structural formulae are different:
Molecular Formula: Isopentane Neopentane Pentane | C5H12 |
Structural Formula of Isopentane | |
Structural Formula of Neopentane | |
Structural Formula of Pentane |
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.
Molecular Formula: Isobutane n-butane | C4H10 |
Structural Formula of isobutane | |
Structural Formula of n-butane |
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:
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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.
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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
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.
Sample Questions
Ques. Which compound among the following has no isomers? (1 mark)
(a) C7H16
(b) C5H10
(c) C3H8
(d) C4H8
Ans. c) C3H8
Ques. Which compound among the following is the functional group isomer of ethanol? (1 mark)
(a) ethanal, CH3CHO
(b) acetic acid, CH3COOH
(c) diethyl ether, (C2H5)2O
(d) dimethyl ether, (CH3)2O
Ans. d) Dimethyl ether, (CH3)2O
Ques. Which compound among the following does not represent geometric isomerism? (1 mark)
(a) 4-octene
(b) 2-pentene
(c) 3-hexene
(d) 1-hexene
Ans. d) 1-hexene
Ques. State an example of position isomerism. (2 marks)
Ans. 1-Chloropropane and 2-Chloropropane
Ques. How many subtypes are there for structural isomerism? (2 marks)
Ans. There are 6 subtypes of structural isomerism, which are:
Ring Chain isomerism, Chain isomerism, Tautomerism, Functional isomerism, Metamerism, and Positional isomerism
Ques. Give the difference between geometric and optical isomerism. (3 marks)
Ans. 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 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.
Ques. What is chain isomerism? Give an example. (2 marks)
Ans. 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
Read More: Tetravalency of Carbon
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