Define structural isomers as compounds with the same molecular formula, but different structural formulae, including $mathrm{C}_4 mathrm{H}_{10}$ as $mathrm{CH}_3 mathrm{CH}_2 mathrm{CH}_2 mathrm{CH}_3$ and $mathrm{CH}_3 mathrm{CH}left(mathrm{CH}_3 i

Organic Chemistry: Formulae, Functional Groups & Terminology

What are Structural Isomers? 🧩

Structural isomers are compounds that share the same molecular formula but differ in the way their atoms are connected – their structural formulae. Think of them like different ways to arrange the same set of Lego bricks: the bricks are identical, but the final model looks different.

In IGCSE, you’ll often see isomers of simple alkanes and alkenes. Below are classic examples.

Example 1: The C4H10 Family (Alkanes)

IsomerStructural FormulaCommon Name
1

CH3–CH2–CH2–CH3

n‑Butane
2

CH3–C(CH3)2–CH3

Isobutane (methylpropane)

Example 2: The C4H8 Family (Alkenes)

IsomerStructural FormulaCommon Name
1

CH3–CH2–CH=CH2

1‑Butene
2

CH3–CH=CH–CH3

2‑Butene (cis/trans)

Exam Tip: How to Spot Isomers Quickly

  1. Check the molecular formula first. If two compounds share it, they might be isomers.

  2. Look for different bonding patterns – single bonds vs double bonds, branching, or ring structures.

  3. Write both the structural formula and the IUPAC name in your answer; this shows you understand the relationship.

  4. Remember that functional groups (e.g., OH, COOH) also create isomeric differences.

Analogy Time! 🔬

Imagine you have a set of four identical Lego bricks. You can build a straight line (n‑butane) or a T‑shape (isobutane). Both use the same bricks, but the final structure is different. That’s exactly what structural isomers do with atoms.

Keep practicing drawing structures and naming them – the more you do it, the easier it becomes to spot isomeric relationships in exam questions. Good luck! 🚀