Isomerism

Ask a Question, Get an Answer!

Type your question here

Hundreds of tutors are online and ready to help you right now!

Compounds having the same molecular formula, but differing in physical and chemical properties are known as isomers. This phenomenon is known as isomerism. The isomers can be identified and distinguished from one another because of difference in their physical and chemical properties.

Types of Isomerism

Isomerism may be classified into two types viz.,

• Structural isomerism

• Stereo isomerism

These are further classified into different classes as follows:

Structural Isomerism

Structural isomerism arises due to the difference in the arrangement of atom in the molecule i.e., the isomers have different structure. There are five different ways in which the structural isomerism arises. These are:

Chain isomerism

This type of isomerism arises from the difference in the nature, structure of the carbon chain. For example, for butane (C₄H₁₀), the following arrangements are possible.

Pentane (C₅H₁₂) exhibits the following three structures.

 

Butanol (butyl alcohol) has the following two structures of the carbon chain.

 

For butanal, (C₃H₇CHO), the following chain isomers are possible.

 

Position isomerism

This isomerism arises due to the difference in the position of the same functional group or the same substituent while the arrangement of carbon atoms remains same. For example,

Chloropropane (C₃H₇Cl) can have two positional isomers given below.

 

 

 

Butene (C₄H₈) can have two positional isomers.

 

 

 

Propanol (C₃H₇OH) has the following positional isomers.

 

 

 

Functional isomerism

The compounds having the same molecular formula but different functional groups are said to exhibit functional isomerism. Such compounds are termed functional isomers. For example, there are two compounds having the molecular formula C₂H₆O viz.,

Ether

 

 

Alcohol

The molecular formula C₃H₆O represents two functional isomers.

Aldehyde

Ketone

Metamerism

This type of isomerism arises from the unequal distribution of carbon atoms on either side of the functional group in the molecule of compounds belonging to the same homologous series. For example, diethyl ether and methylpropyl ether are metamers.

A ketonic compound having the molecular formula C₅H₁₀O, has the following two metamers.

Tautomerism

This is a special case of functional isomerism. Certain substances can

exhibit properties of two functional groups as for instance, acetoacetic ester behaves like a ketone as well as an alcohol. This arises due to the migration of a labile hydrogen atom from one place to another.In acetoacetic ester, the inter-conversion between keto and enolic group takes place as follows.

 

 

In amides, it happens as follows:

 

 

Both these forms of the same molecule exist in equilibrium in a sample of compound. The migration of a labile hydrogen between two polyvalent atoms in the same molecule causes this type of dynamic isomerism. This is termed as tautomerism or keto-enol tautomerisn. Such isomers are termed as tautomers.

Problems

1. An alkane has a molecular mass of 72. Draw its possible chain isomers.

Solution

First of all, we will derive the molecular formula.

Molecular formula of alkane is C_nH_2n+2

As molecular mass = 72,

12n + 2n + 2 = 72

n = 5

The alkane is C₅H₁₂. The possible chain isomers are

 

and

 

2. What are the possible structures for a carboxylic acid with the molecular formula C₄H₈O₂?

Solution

The possible structures for the carboxylic acid are:

3. Write structural formulas for all the isomeric amines with molecular formula C₄H₁₁N.

Solution