Isomerism - Structural Isomerisms in Organic compounds

Structural Isomerism in Organic compounds - Chain Isomerism, Positional Isomerism, Functional Isomerism, Metamerism and Tautomerism explained with suitable examples. In Introduction, Definition of Isomerism and Stereoisomerism given simply to understand the difference between Structural and Stereo isomerism.
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1. Isomerism Structural Isomerism in Organic Compounds Prof.Dr.P.Venkatesh Chem Eazy

2. Isomerism Isomers & Isomerism Compounds having the same molecular formula but differ from each other in physical or chemical properties are called Isomers and the phenomenon is termed Isomerism. Since isomers have the same molecular formula, the difference in their properties must be due to different modes of combination or arrangement of atoms within the molecule . Isomerism is of two types : 1. Stereoisomerism 2. Structural isomerism

3. Stereoisomerism When isomerism is caused by the different arrangements of atoms or groups in space , the phenomenon is called stereoisomerism. Stereoisomers have the same structural formulas but differ in the spatial arrangement of atoms or groups in the molecule. Stereoisomerism is exhibited by such compounds which have identical molecular structure but different configurations.

4. Structural isomerism ( Constitutional isomerism) When the isomerism is simply due to difference in the arrangement of atoms within the molecule without any reference to space , the phenomenon is termed structural isomerism. Structural isomers have the same molecular formulas, possess different structural formula . Structural isomerism includes, a. Chain isomerism b. Positional isomerism c. Functional isomerism d. Metamerism e. Tautomerism

5. Chain isomerism [ Skeletal isomerism ] Chain isomers are molecule with the same molecular formula. But different arrangements of the carbon ‘skeleton’. Chain isomerism arises due to different arrangements of carbon atoms leading to linear and branched chains. The chain isomers have almost similar chemical properties but different physical properties. Ex: There are two butanes which have the same molecular formula ( C 4 H 10 ) but differ in the structure of the carbon chains in their molecules.

6. n-butyl alcohol & isobutyl alcohol having the same molecular formula ( C 4 H 9 OH) are chain isomers. Pentane, Isopentane & Neopentane are chain isomers with the molecular formula C 5 H 12 The molecule of chain isomers differ only in respect of the linking of the carbon atoms in the alkanes or in the alkyl radicals present in other compounds.

7. Positional isomerism The positional isomerism arises due to different positions of the side chains, substituents, functional groups, double bonds, triple bonds, etc., on the parent chain. Ex: n-propyl alcohol and isopropyl alcohol possess the same molecular formula ( C 3 H 7 OH) and their molecules are made of a chain of three carbon atoms. In n-propyl alcohol, the –OH group is on the end carbon atom while in isopropyl alcohol it is on the middle carbon atom.

8. 1-chloropropane and 2-chloropropane 1-butene and 2-butene Xylene exists in 3 forms

9. Functional isomerism [Functional group isomerism] When any two compound have the same molecular formula but possess different functional groups , they are called Functional isomers and the phenomenon is termed Functional isomerism. Substances with the same molecular formula but belonging to different classes of compounds exhibit functional isomerism This is possible by rearranging the atoms within the molecule so that they are bonded together in different ways. Ex: Diethyl ether & butyl alcohol both have the molecular formula C 4 H 10 O, but contain different functional groups.

10. Dimethyl ether & ethyl alcohol Acetone & Propionaldehyde Acetic acid & Methyl formate

11. Metamerism Metamerism is due to the unequal distribution of carbon atoms on either side of the functional group in the molecule of compounds belonging to the same class. Metamerism arises when different alkyl groups attached to same functional group. Metamerism is shown by members of classes such as ethers, ketones and amines where the functional group is flanked by two chain. The individual isomers are known as Metamers.

12. Ex: Methyl propyl ether, Diethyl ether & Isopropyl methyl ether Diethylamine , Methyl propylamine & Isopropyl methylamine

13. Tautomerism Tautomers are two isomeric forms having different functional groups which are spontaneously interconvertible and can exist in dynamic equilibrium . All carbonyl compounds – aldehydes, ketones & esters exhibit tautomerism. [Note: Tautomers are not the resonance structures of same compound]

14. A . Keto-enol tautomerism Tautomerism which involves a dynamic equilibrium between keto form & enol form of a compound is termed keto-enol tautomerism. The carbonyl compounds containing at least one α-hydrogen atom exhibit keto-enol tautomerism. The carbonyl group can be converted to enol form due to transfer of one of the α-hydrogen onto oxygen atom.

15. B. The nitro- aci tautomerism It is exhibited by nitro compounds containing at least one α-hydrogen. The hydrogen atom is transferred from one atom to another during the conversion of nitro form to aci form and vice versa. Ex: Nitro- aci tautomerism in nitromethane [ aci form – a tautomeric form of a nitro compound]

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