chemistry

Isomerism and its Types with Examples

Isomerism:

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

The phenomenon of 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             

                                                                                                          │  

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

                             n-butane                                             Isobutane

     n-butane and Isobutane differ in physical properties  

e.g. (ii)                    CH3─CH2─OH    and         CH3─O─CH3 ( differ in both properties)

                                Ethyl alcohol                        Di methyl ether

Types of Isomerism:

There are two major types of isomerism.

A)  Structural Isomerism:

The phenomenon of 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 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.”

e.g (i)                                                                                                      CH3                                                 CH3

                                                                                                                │                                                       │

                CH3─CH2─CH2─CH2─CH3                              CH3─CH─CH2─CH3                          CH3─C─CH3

                n-pentane                                                              Isopentane                                                     │  Neopentane

                                                                                                                                                                          CH3

e.g (ii)                                                                                             CH3                                                      

                                                                                                          │                                           

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

                n-pentyl alcohol                                                   iso pentyl alcohol                                               

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

(ii)   Position Isomerism:

                “The phenomenon of 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                                              2-pentyne

(iii)  Functional Group Isomerism:

                “The phenomenon of 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                        e.g (ii)                     Glucose     and      Fructose

                                Ethyl alcohol                        Dimethyl ether

(iv)    Metamerism

                “The phenomenon of existence of different compounds with same molecular formula but having unequal number of carbons (different alkyl groups) on two side 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                                             Methyl n-propyl ether

e.g (ii)     

                        O                                                                O  

                                       ║                                                                ║                  

                CH3─CH2─C─CH2─CH2─CH3        and         CH3─C─CH2─CH2─CH2─CH3

                                Ethyl n-propyl ketone                                         methyl n-butyl ketone

ðEthers, ketones and amines often show metamerism.

(v)   Tautomerism:

A special type of functional group isomerism in which proton transfer occurs and two inter convertible 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)

                                           O                  O                                                        OH               O

                                           ║                  ║                                                        │                  ║

e.g. (ii)                    CH3 – C – CH2 – C – O – C2H5         ⇌            CH3 – C = CH –  C – O – C2H5

                                                 (Keto form)                                                         ( Enol form)

B)  Stereo Isomerism:

                “The phenomenon of existence of different compounds with same molecular formula and same structural formula but differ only in 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 existence of different compounds with same molecular formula and same structural formula but differ only in 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 single bond are capable of free rotation about the bond. But carbon atoms joined by double bond are not capable of free rotation about double bond as it needs first to break pi-bond which requires energy comparable to that which needs in chemical reactions. This lack of rotation about double bond gives rise to geometric isomerism. When identical nature groups are on same side of double bond then it is cis-form and if identical nature groups are on opposite side of 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 group 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 left or towards right, is called optical activity.”

The compounds which have chiral (asymmetric) carbon show optical activity. They either rotate plane polarized light towards right and are called dextrorotatory or they rotate plane polarized light towards 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 polarimeter.

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

As light is seen so substance is optically inactive

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

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

                “The phenomenon of existence of different compounds with same molecular formula and 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 right and levorotatory which rotates plane polarized light towards left.

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