How does ethanol differ from propane in boiling point despite their similar molecular weights?

Ethanol and propane have similar molecular weights—46 and 44, respectively—yet their boiling points differ significantly: ethanol boils at 78 °C (173 °F), while propane boils at −42 °C (−44 °F).1

This large discrepancy is primarily due to the ability of ethanol molecules to form intermolecular hydrogen bonds, a type of strong attraction that does not occur between propane molecules. In ethanol, the oxygen atom of the strongly polarized O–H bond pulls electron density away from the hydrogen atom, leaving it with a partial positive charge. This allows the hydrogen atom to form a bond with a pair of nonbonding electrons on the oxygen atom of a neighboring ethanol molecule.1

Because these hydrogen bonds must be broken for the liquid to vaporize, significantly more thermal energy is required to transition ethanol into the gas phase compared to propane, which lacks such strong intermolecular associations.12