Which statement best explains why propanoic acid boils at a much higher temperature than hexane?
The correct answer and explanation is:
Correct Answer:
Propanoic acid can form hydrogen bonds, whereas hexane cannot.
Explanation:
Propanoic acid (CH₃CH₂COOH) and hexane (C₆H₁₄) are both organic compounds, but they have very different boiling points due to the types of intermolecular forces they exhibit. The key difference between the two lies in the ability of propanoic acid to form hydrogen bonds, while hexane cannot.
Hydrogen bonding is a strong type of dipole-dipole interaction that occurs when a hydrogen atom is directly bonded to a highly electronegative atom like oxygen, nitrogen, or fluorine. In propanoic acid, the –COOH (carboxylic acid) group contains both a hydrogen bonded to an oxygen atom and a lone pair on another oxygen. This structure enables molecules of propanoic acid to form strong hydrogen bonds with each other. These bonds require a significant amount of energy to break, which increases the temperature needed for the substance to transition from a liquid to a gas. Therefore, propanoic acid has a much higher boiling point.
On the other hand, hexane is a straight-chain alkane composed entirely of carbon and hydrogen atoms. Its molecules are non-polar, and the only intermolecular forces it can form are London dispersion forces (also called van der Waals forces), which are much weaker than hydrogen bonds. These forces arise due to temporary dipoles created by the movement of electrons. Because of the weak intermolecular forces, hexane molecules require less energy to escape the liquid phase and vaporize, resulting in a much lower boiling point.
In summary, the ability of propanoic acid to form strong hydrogen bonds greatly increases its boiling point compared to hexane, which relies only on weak London dispersion forces.