Boiling point generally rises with increasing molar mass because larger molecules possess stronger London dispersion forces. However, when comparing molecules of nearly identical molar mass, structural differences can dominate.
Alcohols contain a hydroxyl group (–OH) attached to a carbon atom, whereas alkanes consist solely of carbon–hydrogen bonds. The hydroxyl group introduces polarity and the ability to form hydrogen bonds, a feature absent in alkanes.
The hierarchy of intermolecular attractions from strongest to weakest is: ionic > hydrogen bonding > dipole–dipole > London dispersion. Alcohols benefit from hydrogen bonding, while alkanes rely only on dispersion forces.
Boiling occurs when kinetic energy overcomes these intermolecular forces. Hydrogen bonds require significantly more energy to break than dispersion forces, so alcohols with the same molar mass as alkanes will have higher boiling points. For example, ethanol (C₂H₅OH) boils at 78 °C, whereas the isomeric alkane butane (C₄H₁₀) boils at –0.5 °C, despite similar molecular weights.
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