Structural isomers are molecules that share the same molecular formula but have different structural arrangements of atoms. Butane and methylpropane both have the formula C₄H₁₀, yet butane is a straight chain while methylpropane has a branched chain — giving them different boiling points and slightly different properties despite being built from identical atoms.
What are structural isomers?
Structural isomers (also called constitutional isomers) are compounds with:
- The same molecular formula — the same number and type of each atom.
- A different structural formula — the atoms are bonded in a different order or arrangement.
Because the bonding arrangement differs, the molecules have different shapes, different intermolecular forces, and therefore different physical properties (boiling point, melting point, density). Their chemical properties may also differ.
Structural isomerism arises from the carbon skeleton's ability to form straight chains, branched chains, or rings. It is distinct from stereoisomerism (same connectivity but different spatial arrangement of atoms), which is an A-level topic.
How do you identify and draw structural isomers of alkanes?
For a molecular formula, systematically build every possible carbon skeleton:
Worked example — isomers of C₄H₁₀:
Step 1: Draw the straight-chain structure.
- Butane: C–C–C–C (four carbons in a row)
- Full name: butane; boiling point: −0.5 °C
Step 2: Introduce one branch.
- Move one carbon off the main chain to create a three-carbon chain with a methyl group attached to carbon 2.
- Full name: 2-methylpropane (also called methylpropane); boiling point: −11.7 °C
There are no other distinct arrangements for C₄H₁₀ — only two structural isomers exist.
| Isomer | IUPAC name | Carbon skeleton | Boiling point |
|---|---|---|---|
| CH₃CH₂CH₂CH₃ | Butane | Straight chain: C-C-C-C | −0.5 °C |
| (CH₃)₂CHCH₃ | 2-Methylpropane | Branched: C with CH₃ on C2 | −11.7 °C |
How many structural isomers does C₅H₁₂ have?
C₅H₁₂ has three structural isomers:
| Isomer | IUPAC name | Description |
|---|---|---|
| CH₃CH₂CH₂CH₂CH₃ | Pentane | Straight chain of 5 carbons |
| CH₃CH(CH₃)CH₂CH₃ | 2-Methylbutane | Methyl branch on C2 of a 4-carbon chain |
| C(CH₃)₄ | 2,2-Dimethylpropane | Two methyl branches on the central carbon of a 3-carbon chain |
The number of possible structural isomers increases dramatically with carbon number: C₆H₁₄ has 5 isomers; C₁₀H₂₂ has 75.
Why do structural isomers have different boiling points?
Boiling point is determined by the strength of the intermolecular forces between molecules. For alkane isomers, the dominant forces are London dispersion forces (temporary dipole–induced dipole attractions).
The surface area of a molecule determines how strong these forces are. A straight-chain molecule has a larger surface area (more contact between adjacent molecules) than a branched molecule of the same formula. Therefore:
- Straight-chain alkanes have stronger intermolecular forces → higher boiling points.
- Branched alkanes are more compact (roughly spherical) → weaker forces → lower boiling points.
This explains why butane (−0.5 °C) has a higher boiling point than 2-methylpropane (−11.7 °C), even though both are C₄H₁₀.
Do structural isomers exist for other homologous series?
Yes — structural isomers appear across all organic compound classes:
| Molecular formula | Possible structural isomers | Example |
|---|---|---|
| C₃H₇OH | Two alcohols | Propan-1-ol (OH on end carbon) and propan-2-ol (OH on middle carbon) |
| C₄H₈ | Multiple alkenes and a cycloalkane | But-1-ene, but-2-ene, 2-methylpropene, cyclobutane |
| C₂H₆O | Two functional-group isomers | Ethanol (an alcohol) and methoxymethane (an ether) |
The last example — where isomers have different functional groups entirely — shows that structural isomers can have very different chemical properties, not just different physical properties.
How do you count structural isomers systematically?
- Start with the longest possible carbon chain.
- Draw the straight-chain isomer.
- Reduce the main chain by one carbon and attach the removed carbon as a methyl branch; place the branch on carbon 2, then carbon 3, etc. (checking each result is genuinely unique — not a redrawn version of one already drawn).
- Reduce the main chain by a further carbon and repeat, now placing ethyl or two methyl branches.
- Stop when placing branches no longer produces new distinct structures.
A common error is counting the same structure twice because it was drawn from different ends. Always check both ends give the same name.
Frequently asked questions
Why do butane and 2-methylpropane have different boiling points if they contain exactly the same atoms?
Boiling point depends on intermolecular forces, which depend on molecular shape and surface area rather than on the number of atoms. Butane's straight chain maximises surface contact with neighbouring molecules, creating stronger temporary dipole interactions. 2-Methylpropane is more compact and spherical, reducing surface contact and weakening the attractions. Because less energy is needed to separate 2-methylpropane molecules, it boils at a lower temperature.
Are structural isomers the same compound or different compounds?
They are different compounds — they have different structural formulae, different IUPAC names, different physical properties and (sometimes) different chemical properties. Sharing a molecular formula does not make two substances the same any more than two different words being the same length makes them the same word. Each isomer can be isolated as a pure, distinct substance with its own data sheet of properties.
How do structural isomers of alcohols differ from each other?
Alcohols are structural isomers of each other when the hydroxyl group (–OH) is on different carbons. Propan-1-ol has –OH on the end carbon; propan-2-ol has –OH on the middle carbon. This matters chemically: primary alcohols (–OH on a terminal carbon) can be oxidised first to aldehydes and then to carboxylic acids; secondary alcohols (–OH on an internal carbon) are oxidised only to ketones; tertiary alcohols (–OH on a carbon with three alkyl groups attached) resist oxidation under mild conditions. Position of the functional group therefore directly controls reactivity.
Does isomerism exist beyond the GCSE syllabus?
Yes — at A-level, stereoisomerism is introduced: molecules with the same structural formula but different three-dimensional arrangements of atoms. Geometric (cis–trans) isomerism arises from restricted rotation around a C=C double bond; optical isomerism arises when a carbon is bonded to four different groups, creating non-superimposable mirror images. Stereoisomers can have dramatically different biological activities: one enantiomer of a drug may be therapeutic while the other is inactive or harmful. GCSE covers only structural isomers; stereoisomers are beyond the current specification.
For Socratic GCSE chemistry with Professor Curie — building structural isomers from molecular formula through particle geometry to physical properties — visit aitutors.me.