Structural Isomerism | Organic Chemistry Class 11

Structural Isomerism

Chain, Position, Ring-Chain & Metamerism

1. What is Structural Isomerism?

Compounds having the same molecular formula but different structural arrangements of atoms are called structural isomers.

2. Chain Isomerism

Definition: Isomers that have the same molecular formula but different arrangements of the Carbon skeleton (parent chain).

Also called Nuclear or Skeletal isomerism.

• Example ($C_4H_{10}$):
  • n-Butane (Straight chain)
  • Isobutane (2-Methylpropane, Branched)
• Condition: Minimum 4 carbons required for alkanes.

3. Position Isomerism

Definition: Isomers that have the same carbon skeleton but differ in the position of the functional group, substituent, or multiple bond.

• Example ($C_4H_8$):
  • But-1-ene (Double bond at C1)
  • But-2-ene (Double bond at C2)
• Example ($C_3H_8O$): Propan-1-ol vs Propan-2-ol.

4. Ring-Chain Isomerism

Definition: Isomers where one has an open-chain structure and the other has a cyclic (ring) structure.

This is a specific type of functional isomerism where the unsaturation degree allows for a ring.

• Example ($C_3H_6$):
  • Propene ($CH_3-CH=CH_2$)
  • Cyclopropane (Triangle ring)
• Example ($C_4H_8$): But-1-ene vs Cyclobutane vs Methylcyclopropane.

5. Metamerism

Definition: Isomers having the same functional group but different alkyl chains attached to the polyvalent functional group (like -O-, -S-, -NH-, -CO-).

• Ethers ($C_4H_{10}O$):
  • Diethyl ether ($C_2H_5 - O - C_2H_5$)
  • Methyl propyl ether ($CH_3 - O - C_3H_7$)
• Ketones ($C_5H_{10}O$):
  • Pentan-2-one ($CH_3-CO-C_3H_7$)
  • Pentan-3-one ($C_2H_5-CO-C_2H_5$)

6. Summary & Priority

When identifying isomers, check in this order:

Ring-Chain > Functional > Metamerism > Chain > Position

Practice Quiz

Test your ability to identify Isomers.

Your Score: 0 / 10

© 2026 chemca-chemistry made easy

Quiz Revision Notes Downloads Class Notes Video Lectures