MCAT Organic Chemistry 5

Question 1

Which of the following best explains why carboxylic acids have higher boiling points than alcohols of similar molecular weight?

Accepted Answer

Carboxylic acids form stable dimers via two hydrogen bonds

Answer

Carboxylic acids dimerize through two intermolecular hydrogen bonds between the carbonyl oxygen and O–H groups, effectively doubling the interaction energy and raising boiling points.

Question 2

What is the product when an ester reacts with excess LiAlH4 followed by aqueous workup?

Accepted Answer

Two alcohols

Answer

LiAlH4 reduces esters all the way to two alcohols: one from the acyl portion and one from the alkoxide leaving group (both end up as primary alcohols).

Question 3

Which of the following pairs are diastereomers?

Accepted Answer

(2R,3S)-tartaric acid and (2R,3R)-tartaric acid

Answer

Diastereomers are stereoisomers that are not mirror images; (2R,3S)-tartaric acid and (2R,3R)-tartaric acid differ at only one chiral center and are therefore diastereomers.

Question 4

In an aldol condensation reaction, what type of bond is formed in the beta-hydroxy carbonyl intermediate?

Accepted Answer

C–C bond between the alpha carbon of one carbonyl and the carbonyl carbon of another

Answer

In aldol addition, the enolate (alpha carbon nucleophile) of one molecule attacks the electrophilic carbonyl carbon of another, forming a new C–C bond to give a beta-hydroxy carbonyl compound.

Question 5

Which protecting group strategy would allow selective acylation of an amine in the presence of an alcohol?

Accepted Answer

Protect the alcohol as a TBS ether, then acylate the free amine

Answer

Converting the alcohol to a silyl ether (TBS) blocks the OH from reacting, leaving the amine free for selective acylation under mild conditions.

MCAT - Medical College Admission Test Practice Test

MCAT - Medical College Admission Test Practice Test

MCAT Organic Chemistry 5