| Type | What Changes | Example (C₄H₁₀) |
|---|---|---|
| Chain | Arrangement of the carbon skeleton (straight vs branched) | Butane vs 2-methylpropane |
| Positional | Position of the functional group on the same chain | Butan-1-ol vs Butan-2-ol |
| Functional group | The functional group itself is different | Ethanol (C₂H₅OH) vs Methoxymethane (CH₃OCH₃) |
🔑 Consequences
Structural isomers have the same molecular formula but different physical properties (different BPs, solubilities) and may have different chemical properties (if the functional group changes). Branched isomers typically have lower boiling points due to smaller surface area and weaker LDFs.
⚠️ "Bending" Is Not Isomerism
Drawing a straight-chain molecule with a bend at 90° does not create an isomer. Test: name both structures using IUPAC rules. If they have the same name, they are the same molecule, not isomers.
⚠️ 1° / 2° / 3° Classification
Alcohols & halogenoalkanes: classify by how many carbons are attached to the alpha-carbon (the C bonded to –OH or –X).
Amines & amides: classify by how many carbons are attached directly to the nitrogen atom.
Don't mix these rules up. The IB tests this distinction!