Which of the following compounds has the lowest boiling point?

Analyze the question: We need the compound with the weakest intermolecular forces, since that gives the lowest boiling point.

Concept: Boiling point depends on the strength of the forces between molecules:

• Hydrogen bonding is strong, so these compounds have high boiling points.
• Van der Waals (London dispersion) forces are weak, so these compounds have low boiling points.

Solution: Look at each option.

| Option | Compound | Main intermolecular force | Boiling point | |---|---|---|---| | A | C₄H₁₀ (butane) | Weak van der Waals only (non-polar alkane) | about 0 °C | | B | CH₃COOH (ethanoic acid) | Strong H-bonding, and it forms dimers | about 118 °C | | C | H₂O (water) | Extensive H-bonding | 100 °C | | D | C₂H₅OH (ethanol) | H-bonding | about 78 °C |

Butane is a non-polar hydrocarbon. It has no -OH group, so it cannot form hydrogen bonds. Its molecules are held together only by weak van der Waals forces, so little energy is needed to separate them.

The other three compounds all contain an -OH group and form hydrogen bonds, so they boil at much higher temperatures.

Answer: A. C₄H₁₀

Exam tip: In boiling-point questions, check for hydrogen bonding first (-OH, -COOH, -NH₂, or H bonded to F, O or N). A compound with none of these, such as an alkane, will usually have the lowest boiling point. Order of strength: H-bonding > dipole-dipole > van der Waals.