Concept:

The physical properties of organic compounds are dominated by the type and strength of intermolecular forces available to their specific functional groups.

Formula:

$$ \text{B.P.} \propto \text{Strength of IMF} $$

Solution:

• Alcohols (\( \text{R-OH} \)) and Phenols (\( \text{Ar-OH} \)) contain a highly polar hydroxyl group. The presence of hydrogen covalently bonded directly to oxygen allows them to form strong, extensive Hydrogen bond networks with adjacent molecules.



• Ethers (\( \text{R-O-R'} \)) have an oxygen atom, but it is bound strictly to carbon atoms, not hydrogen. Therefore, ethers cannot act as hydrogen bond donors to themselves.



• Because ethers only experience much weaker dipole-dipole interactions and London dispersion forces, their molecules are much easier to separate, resulting in a significantly lower boiling point.

Why other options are incorrect:

While the functional group dictates the forces, "Intermolecular forces" is the direct physical and thermodynamic cause of the boiling point difference. Alkyl nature and isomerism do not overcome the massive energy gap created by the presence or absence of hydrogen bonding.