Day 19 MCAT Practice Question

Correct answer: B. Intermolecular forces of attraction largely govern

a compound’s boiling point; a substance with fewer cohesive forces between

particles can escape into the gas phase easier. Hydrogen bonds tend to be the

strongest intermolecular force of attraction. Thus, we would expect that the

substance with the most extensive hydrogen bonding would experience the largest

boiling point - benzoic anhydride and benzamide fits the bill. That eliminates

choices A since it does not list either as having the highest boiling point.

Another important consideration when it comes to boiling point is molecular weight.

Benzoic anhydride is much heavier than benzamide (the formation of an anhydride

from benzoic acid essentially involves combining two benzoic acid molecules

following dehydration). That means that benzoic anhydride has the higher boiling

point, eliminating choice D.

Neither methyl benzoate nor benzoyl chloride have the capacity to form hydrogen

bonds (neither has a hydrogen that is bonded to an oxygen, nitrogen, or fluorine).

That means that benzamide (because of its hydrogen bonding) must have a higher

boiling point than both methyl benzoate and benzoyl chloride, eliminating choice C.

Thus, Choice B must be the correct answer.