Permanent dipole-dipole forces in CH₃F

Steps:

• Both molecules have 18 electrons, leading to similar London dispersion forces.
• CH₃CH₃ is nonpolar (symmetric, no dipole moment).
• CH₃F is polar due to electronegativity difference between C and F, creating a permanent dipole.
• Dipole-dipole interactions in CH₃F require more energy to overcome, raising its boiling point.

Why B is correct:

• Permanent dipoles enable dipole-dipole forces (intermolecular attractions between opposite partial charges), stronger than dispersion forces alone per intermolecular force hierarchies.

Why the others are wrong:

• A: CH₃F has higher molar mass (34 g/mol vs. 30 g/mol), but this minor difference alone doesn't override similar electron counts for dispersion forces.
• C: C-F bond strength is intramolecular and does not influence intermolecular attractions affecting boiling points.
• D: Hydrogen bonding requires H directly bonded to N, O, or F; CH₃F has C-H bonds, not H-F.

Final answer: B