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. https://youtu.be/a0snq_oz50A

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
(Constitutionalisomerism)
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 (C4H10) but differ in the
structure of the carbon chains in their molecules.
CH3 CH2 CH2 CH3
n-butane
(a linear chain isomer)
CH3 CH CH3
CH3
Isobutane
(a branched isom
er)

6. n-butyl alcohol & isobutyl alcohol having the same molecular formula (C4H9 OH) are
chain isomers.
Pentane, Isopentane & Neopentane are chain isomers with the molecular formula C5H12
 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.
CH3 CH2 CH2 CH2OH
n-butyl alcohol
CH3 CH CH2OH
CH3
Isobutyl alcohol
CH3 CH2 CH2 CH2 CH3
Pentane
CH3 CH CH2
CH3
CH3
Isopentane
CH3 C CH3
CH3
CH3
Neopentane

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 (C3H7 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.
CH3 CH2 CH2
OH
n-propyl alcohol
CH3 CH CH3
OH
Isopropyl alcohol

8. 1-chloropropane and 2-chloropropane
1-butene and 2-butene
Xylene exists in 3 forms
CH3 CH2 CH2
Cl
1-chloropropane
CH3 CH CH3
Cl
2-chloropropane
CH2 CH CH2 CH3
1-butene
CH
CH3 CH CH3
2-butene
CH3
CH3
o-xylene
CH3
CH3
m-xylene
CH 3
CH 3
p-xylene

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 C4H10O, but contain
different functional groups.
O
Diethyl ether
CH3 CH2 CH2 CH3 CH3 CH2 CH2 CH2 OH
Butyl alcohol

10. Dimethyl ether & ethyl alcohol
Acetone & Propionaldehyde
Acetic acid & Methyl formate
O
CH3 CH3
Dimethyl ether
CH3 CH2 OH
Ethyl alcohol
CH3 C CH3
O
Acetone
CH3 CH2 CHO
Propionaldehyde
CH3 C OH
O
Acetic acid
H C OCH 3
O
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
O
CH3 CH2 CH2 CH3
Methyl propyl ether
CH3 CH2 O CH2 CH3
Diethyl ether
O
CH3 CH CH3
CH3
Isopropyl methyl ether
CH3 CH2 NH CH2 CH3
Diethyl amine
CH3 NH CH2 CH2 CH3
Methyl propyl amine
NH
CH3
CH CH3
CH3
Isopropyl methyl amine

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. CH3 C CH2
O
C OC2H5
O
CH3 C CH
OH
C OC2H5
O
(Keto form) (Enol form)
Ethyl acetoacetate
CH3 C CH3
O
CH3 C CH2
OH
(Keto form) (Enol form)
Acetone
OH
O
(Keto form) (Enol form)
Phenol

16. 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]
CH3 N
O
O CH2 N
OH
O
(Nitro form) (aci form)
Nitromethane

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