# Constitutional isomers

Constitutional (structural) isomers have the same molecular formula but different atom connections. They have different physical and often chemical properties. The number of isomers grows rapidly with chain length.

Chemistry (university) · Part 09: Organic structure and stereochemistry · Card 3 of 9 · Last updated 2026-10-19

## Analogy

The same Lego bricks built into different models.

## Example

**C₅H₁₂ has 3 isomers**

pentane (straight chain)
2-methylbutane (one branch)
2,2-dimethylpropane (two branches)
Boiling points: 36, 28, 10 °C

## In practice

Ethanol and dimethyl ether are both C₂H₆O, but ethanol is a liquid alcohol and dimethyl ether is a gas.

## Practice

### Flashcards

- **What are constitutional isomers?** Same molecular formula, different connections of atoms.

### Interview questions

**Draw and name the isomers of C₅H₁₂ and explain how branching affects boiling point.**

- Three isomers: pentane, 2-methylbutane, 2,2-dimethylpropane
- More branching, more compact shape
- Less surface contact, weaker London forces
- Boiling points fall as branching increases

## Questions people ask

### Do isomers have the same properties?

No. Isomers share a molecular formula but have different bonding, so their properties differ. Branching lowers the boiling point, since a compact shape has less surface contact and weaker London forces. Different functional groups can make physical and chemical behaviour very different.

### How do I draw all the isomers of a formula?

Start with the longest straight chain, then shorten it by one carbon and add that carbon as a branch in each possible position. Repeat with two branches. Check each drawing for duplicates by naming it. If two have the same IUPAC name, they are the same molecule.

## Related concepts

- [IUPAC naming of alkanes](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-9/chemistry-university-part-9-concept-2)
- [E/Z isomerism](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-9/chemistry-university-part-9-concept-6)

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