A chemist is studying the properties of different types of intermolecular forces. Which of the following compounds is expected to have the highest boiling point?
- ACH₃CH₂OH (Ethanol)
- BCH₃CH₂CH₃ (Propane)
- CCH₄ (Methane)
- DCH₃OCH₃ (Dimethyl Ether)
Show answer & explanationAnswer & explanation
Correct answer: A. CH₃CH₂OH (Ethanol)
Boiling point is directly related to the strength of intermolecular forces. Methane and propane are nonpolar hydrocarbons and only exhibit weak London Dispersion Forces (LDFs). Propane has stronger LDFs than methane due to its larger size. Dimethyl ether is a polar molecule and exhibits dipole-dipole interactions, in addition to LDFs. Ethanol is also a polar molecule, but critically, it has a hydrogen atom directly bonded to an oxygen atom (-OH group), allowing it to form strong hydrogen bonds. Hydrogen bonding is generally the strongest type of intermolecular force among these options, leading to the highest boiling point for ethanol.
Why the other options are wrong
- B. Propane is nonpolar with LDFs, stronger than methane's but weaker than dipole-dipole or hydrogen bonding.
- C. Methane is nonpolar with only weak London Dispersion Forces (LDFs), leading to a very low boiling point.
- D. Dimethyl ether is polar and exhibits dipole-dipole interactions, which are stronger than LDFs, but it cannot form hydrogen bonds.
Intermolecular Forces and Boiling Point
Intermolecular forces (IMFs) are attractive forces between molecules. The stronger the IMFs, the more energy is required to overcome them, resulting in a higher boiling point.
- Hydrogen bonding > Dipole-dipole > London Dispersion Forces (LDFs).
- LDFs increase with molecular size/surface area.
- Hydrogen bonding requires H bonded to N, O, or F.
Memory trick: Stronger Forces, Higher Boil.