Isomerism - Definition, Structure, Applications, Functions, FAQs

Structural isomerism

Compounds having the same molecular formula but different structures (manners in which atoms are linked) are classified as structural isomers. Some typical examples of different types of structural isomerism are given below:

• Chain isomerism: When two or more compounds have similar molecular formula but different carbon skeletons, these are referred to as chain isomers and the phenomenon is termed as chain isomerism. For example,$\mathrm{C}_5 \mathrm{H}_{12}$ represents three compounds as follows:
  • Position isomerism: When two or more compounds differ in the position of substituent atom or functional group on the carbon skeleton, they are called position isomers and this phenomenon is termed as position isomerism. For example, the molecular formula $\mathrm{C}_3 \mathrm{H}_8 \mathrm{O}$ represents two alcohols:
    
    
• Functional group isomerism: Two or more compounds having the same molecular formula but different functional groups are called functional isomers and this phenomenon is termed as functional group isomerism. For example, the molecular formula $\mathrm{C}_3 \mathrm{H}_6 \mathrm{O}$ represents an aldehyde and a ketone as follows:

• Metamerism: It arises due to different alkyl chains on either side of the functional group in the molecule. For example$\mathrm{C}_4 \mathrm{H}_{10} \mathrm{O}$represents methoxypropane $\left(\mathrm{CH}_3 \mathrm{OC}_3 \mathrm{H}_7\right)$ and ethoxyethane $\left(\mathrm{C}_2 \mathrm{H}_5 \mathrm{OC}_2 \mathrm{H}_5\right)$

It is the type of isomerism in which the compounds possessing same molecular formula differ in their properties due to the difference in their geomtery that is, due to the difference in the direction of attachment of same atoms or groups in their molecule. It is not shown by single bonded compounds like(C-C) due to free rotation.
Geometrical isomerism is shown by$[>\mathrm{C}=\mathrm{C}<],[>\mathrm{C}=\mathrm{N}-],\left[-\mathrm{N}=\mathrm{N}_{-}\right]$and cyclo alkanes.

Also read :

• NCERT notes Class 11 Chemistry Chapter 12 Organic chemistry- some basic principles and techniques
• NCERT solutions for Class 11 Chemistry Chapter 12 Organic chemistry- some basic principles and techniques
• NCERT Exemplar Class 11 Chemistry Solutions Chapter 12 Organic chemistry- some basic principles and techniques

Geometrical isomerism in Alkenes and Cyclo Alkanes

• Case I
  These compounds cannot exhibit geometrical isomerism
  
  
  These compounds cannot exhibit geometrical isomerism
  
• Case II
  
  
  
  

Difference between Cis and Trans Forms

Cis

• Cis is more reactive, but less stable form as the same species are on the same side, so steric repulsion increases reactivity and decreases stability.
• The dipole moment of cis is more.
• It has less melting point as same groups are on the same side.
• The boiling point of cis is more.
• The solubility, viscosity, and refractive index of cis is more. Trans

Trans

• It is a more stable, but less reactive form as the same species are on opposite sides.
• The dipole moment is mostly zero due to symmetry in the case of symmetrical alkenes.
  For example,
  
• In case of unsymmetrical alkenes, due to little unsymmetric, there may be some dipole moment value as well
  
  For example, trans-2-pentene has some dipole moment value but is lesser than cis form.
• It has more melting point than cis.
• The boiling point of the trans form is less.
• The solubility, viscosity, and refractive index are less than cis form.

Related Topics link

• Lewis Acids and Bases
• Examples of Bases
• Aromatic compounds

Geometrical isomerism in cycloalkanes

It cannot show geometrical isomerism as one carbon atom has two similar species 'A'.

Geometrical isomerism in Oximes and Azo compounds

• In oximes
  
  
• In azo compounds
  
  Here, the attached groups may also differ, that is, A and B.
• When Ends are Different Number of geometrical isomers = 2ⁿ
  Here, n = number of double bonds
  For example,$\mathrm{CH}_3-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}=\mathrm{CH}-\mathrm{C}$
  Here, n=2
  Number of geometrical isomers $=2^n=2^2=4$
• When Ends are Same
  (i) When n is an even number
  Number of Geometric Isomers $=2^{n-1}+2^{n / 2-1}$
  $\begin{aligned} & \text { For example, } \mathrm{X}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}=\mathrm{CH}-\mathrm{X} \\ & \mathrm{n}=4\end{aligned}$
  Number of geometric isomers $=2^{4-1}+2^{4 / 2-1}$ $=2^3+2^1=10$0
  (ii) When n is an odd number
  Number of geometric isomers $=2^{n-1}+2^{\{(n+1) / 2\}-1}$
  For example, $\mathrm{CH}_3-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_3$
  Here, n=3
  Number of geometric isomersNumber of geometric isomers $=2^{3-1}+2^{(3+1) / 4\}^{-1}}$
  $$
  =2^2+2^1=6
  $$

NCERT Chemistry Notes:

• NCERT Notes Class 11th Chemistry
• NCERT Notes Class 12th Chemistry
• NCERT Notes For All Subjects

Recommended topic video on(Isomerism)

Some Solved Examples

Q.1 The number of chiral centers in penicillin is:

(1) 3

(2) 4

(3) 5

(4) 6

Solution:

As we have learned

An asymmetric carbon atom (chiral carbon) is a carbon atom that is attached to four different types of atoms or groups of atoms.

As shown in the figure below, there are 3 chiral carbons in penicillin.

Therefore, option (1) is correct

Q.2 In the following structure, the double bonds are marked as I, II, III and IV

Geometrical isomerism is not possible at site (s) :

(1) III

(2) I

(3) I and II

(4) III and IV

Solution:

As we have learned

Geometrical isomerism is not possible at Site I as two identical methyl groups are attached to the same carbon bearing the double bond.

Hence, the answer is Option (2)

Q.3 Which of the following compounds will exhibit geometrical isomerism ?

(1) 1 - Phenyl - 2 - butene

(2) 3 - Phenyl - 1 - butene

(3) 2 - Phenyl - 1 - butene

(4) 1, 1 - Diphenyl - prop-1-ene

Solution:

As we have learned

Out of the given structures, only 1 - Phenyl - 2 - butene is capable of showing geometrical isomerism because it will show Cis–trans isomerism. Other structures have alkene at terminal carbon which does not show geometrical isomerism.

Hence, the correct answer is Option (1)

Conclusion

Isomerism is a key concept in chemistry that highlights the diversity and complexity of molecular structures. It occurs when compounds share the same molecular formula but differ in atom arrangement.

Isomerism arises from the fact that the atoms in a molecular formula can be organized in multiple ways to produce compounds with varied physical and chemical properties, known as isomers. Two main forms of isomerism are structural or constitutional isomerism, in which bonds between the atoms differ; and stereoisomerism or spatial isomerism, in which the bonds are the same but the relative positions of the atoms differ.