Propanal (bp 48°C) and propanol (97°C) have very similar surface areas and dipole moments. Explain the large differences in boiling points between the two. Consider the intermolecular forces present.
The Correct Answer and Explanation is:
Correct Answer:
The large difference in boiling points between propanal (48°C) and propanol (97°C) is due to the ability of propanol to form strong intermolecular hydrogen bonds, while propanal cannot.
Explanation
Boiling point is a physical property that reflects the strength of the intermolecular forces (IMFs) holding the molecules of a substance together in a liquid state. To boil a liquid, sufficient energy must be supplied to overcome these forces. The stronger the IMFs, the more energy is required, and the higher the boiling point.
Both propanal and propanol are three-carbon molecules with similar molar masses and, as stated in the problem, similar surface areas. This means that the strength of the London dispersion forces—temporary dipoles induced in all molecules—is comparable for both compounds. Both molecules are also polar and have similar dipole moments, meaning the strength of their standard dipole-dipole interactions is also comparable.
The critical difference lies in the functional groups and the strongest type of IMF each can exhibit.
- Propanal (an aldehyde): Propanal possesses a polar carbonyl group (C=O). This creates a permanent dipole, allowing for dipole-dipole interactions between molecules. However, propanal does not have a hydrogen atom directly bonded to a highly electronegative atom like oxygen, nitrogen, or fluorine. Therefore, propanal molecules cannot form hydrogen bonds with each other. Their primary IMFs are London dispersion forces and dipole-dipole interactions.
- Propanol (an alcohol): Propanol possesses a hydroxyl group (-OH). In this group, a hydrogen atom is covalently bonded to a highly electronegative oxygen atom. This creates a very strong partial positive charge (δ+) on the hydrogen and a partial negative charge (δ-) on the oxygen. This arrangement allows propanol molecules to form hydrogen bonds with each other. A hydrogen bond is a particularly strong type of dipole-dipole attraction between the partially positive hydrogen of one molecule and the lone pair of electrons on the oxygen of a neighboring molecule.
Because hydrogen bonds are significantly stronger than standard dipole-dipole interactions, much more energy is required to separate propanol molecules from each other than to separate propanal molecules. This accounts for propanol’s substantially higher boiling point (97°C) compared to that of propanal (48°C).
