A structural isomer is a compound with the same molecular formula as another compound but a different connectivity of atoms. Structural isomerism is part of Structure 3.2 and is assessed at both IB Chemistry SL and HL.
How Structural Isomerism Works
The molecular formula states the number of atoms of each element, but it does not show how those atoms are connected. Different arrangements of the same atoms can therefore produce different compounds.
For example, the molecular formula represents two structural isomers:
- Butane:
- 2-methylpropane:
Both contain four carbon atoms and ten hydrogen atoms, but their carbon atoms have different connectivities. They are therefore different compounds and may have different physical properties, such as boiling points.
Structural isomers may differ in several ways:
| Type of difference | Example |
|---|---|
| Carbon skeleton | Butane and 2-methylpropane |
| Position of a functional group | Propan-1-ol and propan-2-ol |
| Functional group present | Ethanol and methoxymethane |
A key distinction is between structural isomers and stereoisomers. Structural isomers have different atom-to-atom connectivity, whereas stereoisomers have the same connectivity but different spatial arrangements. Stereoisomerism is additional higher level content, although the definition of structural isomerism applies at both levels.
A common misconception is that rotating a molecule or drawing its carbon chain in a different direction creates another structural isomer. It does not: if every atom remains bonded to the same atoms, the drawings represent the same compound.
Exam Technique
When asked to deduce or draw structural isomers, first confirm that every structure has the same molecular formula. Then check that each has genuinely different connectivity, apply correct IUPAC names, and avoid counting the same structure twice because it has been rotated or redrawn.