What is isomerism?

 Isomerism is a Greek word, ‘Iso’ means same and ‘mer’ means molecular unit.

“The phenomenon of the existence of different compounds with same molecular formula but different structural formulae or different arrangements of atoms or groups in space is called isomerism and the compounds are called isomers.”

Isomers differ in physical or chemical or in both properties.

                                                        CH₃             

                                                          │  

CH₃─CH₂─CH₂─CH₃   and   CH₃─CH─CH₃

n-butane                Isobutane

n-butane and Isobutane differ in physical properties  

CH₃─CH₂─OH  and CH₃─O─CH₃

Ethyl alcohol         Di methyl ether

( differ in both properties)

Types of Isomerism:

Isomerism Types

There are two major types of isomerism.

A)  Structural Isomerism:

“The phenomenon of the existence of different compounds with same molecular formula but differ in structural formulae is called structural isomerism and the compounds are called structural isomers.”

Types of Structural Isomerism :

Structural isomerism is further divided into four types

(i)    Chain Isomerism or Skeletal Isomerism:

“The phenomenon of the existence of different compounds with same molecular formula but differ in the carbon chains or skeletons is called chain isomerism and the compounds are called chain isomers.”                                                                                                               

  For example

n-pentane,iso-pentane and neo-pentane.                        

ð Alkanes usually show chain isomerism. Lower alkanes having 3 or less carbon do not show isomerism.

(ii) Position Isomerism:

“The phenomenon of the existence of different compounds with same molecular formula but differ in the positions of the functional groups is called position isomerism and the compounds are called position isomers.”  

e.g (i)                                                             OH

                                                                           │

  CH₃─CH₂─CH₂─CH₂─OH    and         CH₃─CH─CH₂─CH₃

     1-butanol                                       2-butanol

e.g.(ii)  

   CH≡C─CH₂─CH₂─CH₃   and    CH₃─C≡C─CH₂─CH₃

   1-pentyne    and    2-pentyne

(iii)  Functional Group Isomerism:

 “The phenomenon of the existence of different compounds with same molecular formula but differ in functional groups is called functional group isomerism and the compounds are called functional group isomers.”

e.g (i)  CH₃─CH₂─OH and  CH₃─O─CH₃

  Ethyl alcohol               Dimethyl ether

e.g (ii)  Glucose     and      Fructose                    

(iv) Metamerism

“The phenomenon of the existence of different compounds with same molecular formula but having an unequal number of carbons (different alkyl groups) on two sides of functional group is called metamerism and the compounds are called metamers.”

e.g (i)  CH₃─CH₂─O─CH₂─CH₃  and  CH₃─O─CH₂─CH₂─CH₃

  Diethyl ether    and   Methyl n-propyl ether

e.g (ii)     

                  O                                            O  

                    ║                                               ║                  

  CH₃─CH₂─C─CH₂─CH₂─CH₃  andCH₃─C─CH₂─CH₂─CH₂─CH₃

   Ethyl n-propyl ketone       methyl n-butyl ketone

Ethers, ketones and amines often show metamerism

(v) Tautomerism:

“A special type of functional group isomerism in which proton transfer occurs and two interconvertible isomers are formed which exist in equilibrium with each other is called tautomerism.”

                     O                                               OH             

                       ║                                              │

e.g. (i)   CH₃ – C – CH₃     ⇌            CH₂ = C – CH₃

  Propanone (Keto form)    Propenol ( Enol form)

B)  Stereo Isomerism:

     “The phenomenon of the existence of different compounds with same molecular formula and the same structural formula but differ only in the orientation of atoms or groups in the molecules.”

Types of Stereo Isomerism :

It is further of two types:

(i)   Geometric Isomerism (Cis-trans isomerism):

“The phenomenon of the existence of different compounds with the same molecular formula and the same structural formula but differ only in the orientation of identical nature groups in space around double bond is called geometric isomerism or cis-trans isomerism and the compounds are called geometric isomers or cis-trans isomers.”

Carbon atoms joined by a single bond are capable of free rotation about the bond. But carbon atoms joined by a double bond are not capable of free rotation about a double bond as it needs first to break the pi-bond which requires energy comparable to that which needs in chemical reactions. This lack of rotation about double bonds gives rise to geometric isomerism. When identical nature groups are on the same side of the double bond then it is cis-form and if identical nature groups are on the opposite side of the double bond then it is trans-form.

Geometric isomerism in alkenes is possible only when each double-bonded carbon atom is attached to two different atoms or groups of atoms. 

e.g (I) Cis 2-butene & Trans 2-butene

e.g (ii) Cis 2-pentene & Trans 2-pentene

e.g (iii)Cis-2-bromo-3-chloro-2-pentene  &  Trans-2-bromo-3-chloro-2-pentene

(ii)  Optical Isomerism:

Optical Activity:

“The property of compounds due to which they rotate plane-polarized light either towards the left or towards the right is called optical activity.”

The compounds which have chiral (asymmetric) carbon show optical activity. They either rotate plane polarized light towards the right and are called dextrorotatory or they rotate plane polarized light towards the left and are called laevorotatory. The carbon to which four different atoms or groups are attached is called chiral or asymmetric carbon.

Optical activity is checked by a polarimeter.

Ordinary light is electromagnetic radiation consisting of waves vibrating in different planes and having different wavelengths. The planes are perpendicular to the direction of propagation of waves. When these radiations are passed through a monochromator, the radiation becomes monochromatic i.e. radiations of a single wavelength but still, it has waves in all planes. When these radiations are passed through a polarizer (Nicol prism-1), the radiation (light) vibrates only in one plane and is called plane-polarized light. The plane polarized light is then passed through the sample. After passing through the sample the light is passed through the second Nicol prism-2 (Analyzer). After passing through the second Nicol prism of light is seen to the observer the sample will be optically inactive as it has not rotated plane-polarized light. On another hand, if the light is not seen by the observer then a sample is optically active as it has rotated the plane of polarized light. The second Nicol prism can be rotated. If the light is seen by rotating the second Nicol prism towards the right then the sample is dextrorotatory or (+) isomer and if the light is seen by rotating the second Nicol prism towards the left then the sample is laevorotatory or (-) isomer.

As light is seen so the substance is optically inactive

As light is not seen however light is seen on the rotating Nicol-2 prism to the right so the substance is optically active and is dextrorotatory

As light is not seen however light is seen on rotating Nicol prism-2 to the left so the substance is optically active and is laevorotatory

“The phenomenon of the existence of different compounds with same molecular formula and the same structural formula but differ only in their interaction towards plane polarized light is called optical isomerism and the compounds are called optical isomers or enantiomers of each other.”

A compound having asymmetric or chiral carbon can show optical activity and thus optical isomerism. A chiral molecule has defined as one that is not superposable on its mirror image. Chiral molecules can be represented by two spatial arrangements, such that one is the mirror image of the other, but cannot be superimposed on one another. Such molecules do not have mirror symmetry.    

  Optical isomers are two i.e. dextrorotatory which rotates plane-polarized light towards the right and levorotatory which rotates plane-polarized light towards the left.

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