Dimethylether (CH3OCH3) acetic acid (CH3COOH)
Which of the following molecules can form hydrogen bonds? methanol (CH3OH) ethane (CH3CH3) dimethylether (CH3OCH3) acetic acid (CH3COOH)
The Correct Answer and Explanation is:
Correct Answer: Acetic acid (CH3COOH)
Explanation
The physical properties of molecular substances, such as boiling point, are primarily determined by the strength of their intermolecular forces (IMFs). These are the attractive forces that exist between molecules. To determine which of the given compounds has the strongest IMFs (and thus the highest boiling point), an analysis of the forces present in each is required.
Ethane (CH3CH3) is a nonpolar hydrocarbon. The only intermolecular forces present between ethane molecules are weak London dispersion forces. These temporary, induced dipoles require minimal thermal energy to overcome, resulting in a very low boiling point (-89 °C).
Dimethylether (CH3OCH3) is a polar molecule due to the bent geometry around the central, electronegative oxygen atom. It therefore exhibits permanent dipole-dipole interactions, which are stronger than London dispersion forces. However, dimethylether lacks a hydrogen atom bonded to the highly electronegative oxygen, so it cannot form hydrogen bonds with itself. Its boiling point (-24 °C) is higher than ethane’s but still relatively low.
Methanol (CH3OH) contains a hydroxyl (-OH) group. The presence of a hydrogen atom bonded to a highly electronegative oxygen atom allows methanol molecules to engage in hydrogen bonding. This is a particularly strong type of dipole-dipole interaction where one molecule’s hydrogen atom is strongly attracted to the lone pair of electrons on an adjacent molecule’s oxygen atom. These strong forces require significantly more energy to break, giving methanol a much higher boiling point (65 °C).
Acetic Acid (CH3COOH) possesses a carboxyl group (-COOH), which contains both a hydroxyl group and a carbonyl group (C=O). This structure allows for exceptionally strong and unique hydrogen bonding. Two acetic acid molecules can form a stable cyclic dimer, linked by two hydrogen bonds simultaneously. This effectively doubles the size of the particle that must be separated to enter the gas phase, requiring a substantial amount of energy. Consequently, acetic acid has the strongest intermolecular forces of the group and an exceptionally high boiling point (118 °C).
In summary, the strength of the intermolecular forces, and therefore the boiling points, follows the order: Acetic Acid > Methanol > Dimethylether > Ethane.
