Question

Which of the following organic compounds is expected to have the highest boiling point?

• A. $\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{OH}$
• B. $\text{CH}_3\text{CH}_2\text{CH}_2\text{CHO}$
• C. $\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{CH}_3$
• D. $\text{CH}_3\text{CH}_2\text{OCH}_2\text{CH}_3$

Hint:

When molecular masses are similar, alcohols will almost always outboil aldehydes, ethers, and hydrocarbons due to extensive hydrogen bonding.

Answer 1

The Correct Option is A

Concept: The boiling point of volatile organic molecules reflects the quantity of kinetic thermal energy needed to completely separate condensed molecules into the gas phase. This physical property is governed by the structural type and cumulative strength of intermolecular forces holding the liquid phase together. When comparing organic molecules with highly similar molecular weights, the relative hierarchy of boiling points follows the sequence of baseline intermolecular forces: \[ \text{Intermolecular Hydrogen Bonding} > \text{Dipole-Dipole Interaction} > \text{London Dispersion Forces} \]

Step 1: Identifying the active functional groups and matching their intermolecular forces.
We analyze the structural features of each molecular choice:
• Butan-1-ol ($\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{OH}$): Contains a strongly polarized hydroxyl ($-\text{OH}$) functional group. The presence of a hydrogen atom covalently attached to highly electronegative oxygen permits extensive intermolecular hydrogen bonding.
• Butanal ($\text{CH}_3\text{CH}_2\text{CH}_2\text{CHO}$): Features a polar carbonyl group ($-\text{C}=\text{O}$). This structure gives rise to fixed molecular dipoles that interact via dipole-dipole attractions, which are weaker than hydrogen bonds.
• Ethoxyethane ($\text{CH}_3\text{CH}_2\text{OCH}_2\text{CH}_3$): An asymmetrical ether containing a polar C-O-C framework. It exhibits mild dipole-dipole interactions but lacks any hydrogen atoms bonded to oxygen to support self-hydrogen bonding.
• Pentane ($\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{CH}_3$): A completely nonpolar saturated hydrocarbon with a uniform electron cloud, operating strictly via temporary London dispersion forces.

Step 2: Evaluating force magnitudes to isolate the compound with the maximum boiling point.
Intermolecular hydrogen bonds are significantly stronger than basic dipole-dipole attractions or weak dispersion forces. Because Butan-1-ol possesses the capability to form a network of hydrogen bonds between its molecules, it requires a much higher temperature to overcome these cohesive forces and transition into the gas phase. Therefore, Butan-1-ol has the highest boiling point among the given options.