Isomerism

                    

              Ethyl alcohol                                                    Dimethyl ether 

As we can see above, ethyl alcohol and dimethyl ether have different structural formula. But their molecular formula is same which is C2H6O. These types of compounds have not only different structural formulas but shows distinct properties as well. Thus, we can say a molecular formula can represent more than one compound. This phenomenon is known as isomerism. The word isomerism is made up of two words iso & meros, iso means same and meros means parts. So, word isomerism means same parts. Ethyl alcohol and dimethyl ether are called isomers of each other. 

What do you mean by Isomerism? & What is an Isomer? 

“Compounds that are represented by the same molecular formulae, but different structural formulae are called isomers and this phenomenon is known as isomerism.”

Isomers not only show different structural formula but different physical and chemical properties as well. 

Examples of Isomers- 

1. Ethyl alcohol and dimethyl ether are isomers of each other as both the compounds have the same molecular formula – C2H6O while different structural formulae. 

2. Compounds such as pentane, iso-pentane and neopentane are isomers of each other. These all three compounds have the same molecular formula- C5H12. But different structural formulae. 

                 

                                                                  Pentane


                                                                Isopentane 


                                                                        Neopentane

Types of Isomers or Isomerism –                                         

Isomerism can be divided into mainly two types- Structural isomerism & Stereoisomerism. These can be further subdivided into different subtypes, which is illustrated in the diagram above. 

Structural Isomerism - Compounds which show isomerism due to difference in their structures are known as structural isomers. This phenomenon is known as structural isomerism. 

Examples of structural isomers – n-Butane and isobutane. 

n-butane and isobutane have the same molecular formula- C4H10 but different structural formulae. 

                     Butane                                            Isobutane 


 Structural isomerism can be further divided into following subtypes- 

1. Chain isomerism 

2. Functional isomerism 

3. positional isomerism 

4. Metamerism isomerism 

5. Tautomerism isomerism 

6. Ring-chain isomerism 

 

1.Chain isomerism – Those structural isomers which differ in chain of carbon atoms are known as chain isomers and the phenomenon is termed as chain isomerism. Thus, chain isomers differ in the arrangement of C-atoms in straight or branched chain of compounds. 

Examples of chain isomers – Hexane – C6H14

Hexane 

                                                    2-methyl pentane 

                                                   3-methyl pentane 

                                                 2,2-dimethyl butane 

                                                 2,3- dimethyl butane 

Important note – To show chain isomerism, organic compound or carbon compound should have minimum ‘4’ carbon atoms. 


Molecule 

No. of chain isomers 

C4H10

2

C5H12

3

C6H14

5

C7H16

9

C8H18

18

C9H20

35

C10H22

75


2.Functional Isomerism – Those compounds which have same molecular formula, but different functional groups are called functional isomers and this phenomenon is known as functional isomerism or functional group isomerism. 

Example Alcohol and ethers 

Ethanol (functional group-OH) and dimethyl ether(functional group- R-O-R)- Molecular formula – C2H6O

Structural formula of ethanol –                   Structural formula of dimethyl ether- 

                                            


3.Positional isomerism – Those structural isomers which differ in position of substituents or functional group or multiple bonds, are known as positional isomers and this phenomenon is known as positional isomerism. 

Positional isomerism example – 

1. But-2-ene & But-1-ene (differ in position of double bond)

But-2-ene                                                              But-1-ene 

2. 1-butanol & 2-butanol (differ in position of functional group -OH)

 

                       1-butanol                                 2-butanol 

3. 1-chlorobutane & 2-chlorobutane (differ in position of substituent group -Cl)


 

1-chlorobutane                                      2-chlorobutane 

4.Metamerism Isomerism – Those structural isomers which differ in their alkyl groups which are attached to their functional groups, are known as metamerism isomers and the phenomenon is known as metamerism isomerism. 

Example- diethyl ether & methyl propyl ether 

 

Diethyl ether [on both sides alkyl group -ethyl(C2H5) is attached]

 

Methyl propyl ether [one side alkyl group-methyl(CH3) is attached while on another side alkyl group- propyl(C3H7) is attached]

5.Tautomerism isomerism – Those structural isomers which differ in the position of hydrogen atoms or protons or electrons, are known as tautomers and this phenomenon is known as tautomerism. It is also known as desmotropism (desmos- bond & tropism-turn). Tautomers occur in equilibrium state with each other as they are easily inter-convertible. 

To show tautomerism, compounds must contain highly electronegative elements with multiple bonds. 

Example – Keto-enol tautomerism in acetone 

   

Acetone (in keto form)                             Acetone(in enol form)


         

Ketone                                             Enol 

6.Ring-chain isomerism – Those structural isomers in which one isomer has open chain structure while another one has closed chain or ring structure, are known as ring-chain isomers and this phenomenon is called ring-chain isomerism

Examples- Butene & cyclobutane 

 

                Butene 

 

          Cyclobutane 

       

Stereoisomerism – Those compounds which have same molecular formula but show different spatial arrangement of atoms in them are known as stereoisomers and the phenomenon is known as stereoisomerism. 

Example – 1,2-dichloroethene (C2H2Cl2

 

Trans-1,2-dichloroethene                      Cis-1,2-dichloroetene 

  

Stereoisomerism can be subdivided into following subtypes-

1. Geometrical isomerism – It is also known as Cis-Trans isomerism. Those stereoisomers in which isomerism arises due to restricted rotation of carbon-carbon double bond are known as geometrical isomers and this phenomenon is known as geometrical isomerism. In cis-trans isomerism it should be noted that the groups attached to the double bonded carbon atom should be different

                                                   

       Cis                                                                   Trans 


Geometric Isomers Example – cis-2-butene & trans-2-butene 

               

           Cis-2-butene                                    Trans-2-butene 



2.Optical isomerism – Those stereoisomers which are mirror images of each other or differ in optical activity are known as optical isomers and this phenomenon is known as optical isomerism. 

Conditions for optical isomerism – 1. In optical isomers carbon atoms are attached to four different atoms or groups. 

2. Chiral center – Optical isomers have chiral centers, it means a carbon atom which is attached to the 4 different groups or atoms. 

3. Optical isomer which rotates plane polarized light towards right side is called dextro (d) while the optical isomer which rotates towards left side is called laevo(l). A mixture containing equal amounts of dextro & laevo isomers is called racemic mixture. Racemic mixtures show no effect on plane polarized light as rotations of d & l cancel out each other. 

Optical Isomers example or Example of d & l isomers 

                                 

      D-mannose                                              L-mannose 

Those optical isomers which are mirror images of each other are called enantiomers. While which are non-mirror images of each other are called diastereomers. 

Examples – Enantiomers –

 

    

Diastereomers – 

     

                Cis                                           Trans