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

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Jasmine Grover

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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. 

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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  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.

Molecular Formula:

Dimethyl Ether

Ethyl Alcohol

C2H6O

Structural Formula of Dimethyl Ether
Structural Formula of Dimethyl Ether
Structural Formula of Ethyl Alcohol
Structural Formula of Ethyl Alcohol
  1. 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 Isopentane 
Structural Formula of Neopentane
Structural Formula of Neopentane
Structural Formula of Pentane
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

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 Isobutane
Structural Formula of n-butane
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

Ring Chain Isomerism
Ring Chain Isomerism

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

Chain Isomerism
Chain Isomerism

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

Tautomerism
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’.

Functional Isomerism of Ethanol and Dimethyl Ether
Functional Isomerism of Ethanol and Dimethyl Ether

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)

Metamerism
Metamerism

Positional Isomerism

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

Example - 1-Chloropropane and 2-Chloropropane

Positional Isomerism of 1-Chloropropane and 2-Chloropropane
Positional Isomerism of 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)

Stereoisomerism of 1,2 - dichloroethene
Stereoisomerism of 1,2 - dichloroethene

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.

Optical isomerism
Optical isomerism
  • 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.

Geometric isomerism
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

Position Isomerism

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

CBSE CLASS XII Related Questions

1.

Which of the following compounds would undergo aldol condensation, which the Cannizzaro reaction and which neither? Write the structures of the expected products of aldol condensation and Cannizzaro reaction. 
\((i) Methanal \)
\((ii) 2-Methylpentanal \)
\((iii) Benzaldehyde \)
\((iv) Benzophenone \)
\((v) Cyclohexanone \)
\((vi) 1-Phenylpropanone \)
\((vii) Phenylacetaldehyde \)
\((viii) Butan-1-ol \)
\((ix) 2, 2-Dimethylbutanal\)

      2.

      The rate constant for the decomposition of hydrocarbons is 2.418 x 10-5 s-1 at 546 K. If the energy of activation is 179.9 kJ/mol, what will be the value of pre-exponential factor.

          3.

          Give the IUPAC names of the following compounds:

          (i)CH3CH(Cl)CH(Br)CH3

          (ii)CHF2CBrClF

          (iii)ClCH2C≡CCH2Br

          (iv)(CCl3)3CCl

          (v)CH3C(p-ClC6H4)2CH(Br)CH3

          (vi)(CH3)3CCH=CClC6H4I-p

              4.

              Write down the electronic configuration of:
              (i) Cr3+ (iii) Cu+ (v) Co2+ (vii) Mn2+ 
              (ii) Pm3+ (iv) Ce4+ (vi) Lu2+ (viii) Th4+

                  5.
                  Write the Nernst equation and emf of the following cells at 298 K : 
                  (i) Mg(s) | Mg2+ (0.001M) || Cu2+(0.0001 M) | Cu(s) 
                  (ii) Fe(s) | Fe2+ (0.001M) || H+ (1M)|H2(g)(1bar) | Pt(s) 
                  (iii) Sn(s) | Sn2+(0.050 M) || H+ (0.020 M) | H2(g) (1 bar) | Pt(s) 
                  (iv) Pt(s) | Br2(l) | Br-  (0.010 M) || H+ (0.030 M) | H2(g) (1 bar) | Pt(s).

                      6.

                      How would you account for the following: 

                      1. Of the d4 species, Cr2+ is strongly reducing while manganese(III) is strongly oxidising. 
                      2. Cobalt(II) is stable in aqueous solution but in the presence of complexing reagents it is easily oxidised. 
                      3. The d1 configuration is very unstable in ions.

                          CBSE CLASS XII Previous Year Papers

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