Which of the following has the highest boiling point?

The boiling point of a substance depends on the strength of its intermolecular forces and, to a lesser extent, its molecular mass. The key intermolecular forces in order of strength are: hydrogen bonding > dipole-dipole > van der Waals (London dispersion).

Consider the four compounds:

• CH₃CH₂OH (ethanol, M_r = 46): contains an -OH group, so it forms hydrogen bonds. Boiling point approximately 78 °C.
• CH₃CH₂CH₂Cl (1-chloropropane, M_r = 78.5): polar C-Cl bond gives dipole-dipole interactions but no hydrogen bonding. Boiling point approximately 47 °C.
• CH₃CH₂CH₂CH₂CH₂CH₃ (hexane, M_r = 86): non-polar, only van der Waals forces. Boiling point approximately 69 °C.
• CH₃CH₂CH₂OH (propan-1-ol, M_r = 60): contains an -OH group, so it forms hydrogen bonds, and has a higher molecular mass than ethanol. Boiling point approximately 97 °C.

Propan-1-ol (CH₃CH₂CH₂OH) has the highest boiling point. It combines hydrogen bonding (the strongest intermolecular force among these molecules) with a greater molecular mass than ethanol, giving it stronger overall intermolecular attractions.