HSC Chemistry

Module: Module 7: Organic Chemistry

Understanding Molecular Isomers: Same Formula, Different Structures

Understanding Structural Isomers in Organic Chemistry

Expert reviewed • 04 March 2025 • 4 minute read

Structural isomers are molecules that share the same molecular formula but have different arrangements of atoms. This fundamental concept in organic chemistry helps us understand how molecular structure influences chemical properties and behavior.

Types of Structural Isomers

1. Chain Isomers

Chain isomers differ in how their carbon atoms are connected in the main chain. They can form:

Straight chains
Branched chains

Example: Pentane $({C5H12})$ can exist as:

n-pentane (straight chain)
2-methylbutane (branched chain)
2,2-dimethylpropane (branched chain)

2. Position Isomers

Position isomers have the same carbon chain but differ in where functional groups or double/triple bonds are located.

Example: But-1-ene and but-2-ene $({C4H8})$ differ in the position of their double bond:

CH₂=CH-CH₂-CH₃ (but-1-ene)
CH₃-CH=CH-CH₃ (but-2-ene)

3. Functional Group Isomers

These isomers contain different functional groups while maintaining the same molecular formula.

Example: ${C4H8O}$ can exist as:

Butanal (aldehyde)
Butanone (ketone)

How to Identify and Draw Isomers

Step-by-Step Process:

Start with the molecular formula
Calculate the number of carbons and degree of unsaturation
Consider all possible:
- Carbon chain arrangements
- Functional group positions
- Types of functional groups

Practice Problems:

Draw all possible isomers for ${C5H12}$
Identify the type of isomerism between:
- 1-chloropropane and 2-chloropropane
- Propene and cyclopropane
- Butanal and butan-2-ol