Question

Boiling point of propanol is greater than butane. Explain.

Hint:

Always compare the type of intermolecular forces when explaining differences in boiling points: hydrogen bonding $>$ dipole–dipole interactions $>$ Van der Waals forces.

Answer 1

Step 1: Nature of forces in butane.
Butane (C$_4$H$_{10}$) is a hydrocarbon and non-polar in nature. The only intermolecular forces present in butane are weak Van der Waals (London dispersion) forces. These forces require relatively little energy to overcome, so butane has a low boiling point.

Step 2: Nature of forces in propanol.
Propanol (C$_3$H$_7$OH) is an alcohol containing an –OH group. The hydroxyl group allows molecules of propanol to form intermolecular hydrogen bonds. Hydrogen bonds are much stronger than Van der Waals forces. Breaking these bonds requires more energy, which raises the boiling point of propanol significantly.

Step 3: Comparison.
Because propanol forms hydrogen bonds while butane does not, the boiling point of propanol is much greater than that of butane.
\[ \boxed{\text{Propanol has a higher boiling point due to hydrogen bonding.}} \]