MCAT Organic Chemistry 5 — Questions and Answers
Question 1: Which of the following best explains why carboxylic acids have higher boiling points than alcohols of similar molecular weight?
- Carboxylic acids are more nonpolar
- Carboxylic acids form stable dimers via two hydrogen bonds (Correct answer)
- Carboxylic acids have lower molecular weight
- Carboxylic acids undergo ionic bonding
Correct answer: Carboxylic acids form stable dimers via two hydrogen bonds
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?
- Carboxylic acid + alcohol
- Two alcohols (Correct answer)
- Aldehyde + alcohol
- Ketone + alcohol
Correct answer: Two alcohols
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?
- (R)-2-bromobutane and (S)-2-bromobutane
- (2R,3R)-tartaric acid and (2S,3S)-tartaric acid
- (2R,3S)-tartaric acid and (2R,3R)-tartaric acid (Correct answer)
- Propane and butane
Correct answer: (2R,3S)-tartaric acid and (2R,3R)-tartaric acid
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?
- C–N bond at the alpha carbon
- C–C bond between the alpha carbon of one carbonyl and the carbonyl carbon of another (Correct answer)
- C–O bond between two hydroxyl groups
- C–H bond at the carbonyl carbon
Correct answer: C–C bond between the alpha carbon of one carbonyl and the carbonyl carbon of another
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?
- Protect the amine as a TMS ether
- Protect the alcohol as a Boc carbamate
- Protect the alcohol as a TBS ether, then acylate the free amine (Correct answer)
- Protect the amine as an acetate ester
Correct answer: Protect the alcohol as a TBS ether, then acylate the free amine
Converting the alcohol to a silyl ether (TBS) blocks the OH from reacting, leaving the amine free for selective acylation under mild conditions.
Question 6: What is the correct order of decreasing reactivity toward electrophilic aromatic substitution (EAS)?
- Nitrobenzene > benzene > aniline
- Aniline > benzene > nitrobenzene (Correct answer)
- Benzene > aniline > nitrobenzene
- Nitrobenzene > aniline > benzene
Correct answer: Aniline > benzene > nitrobenzene
Aniline (–NH2 is a strong activator) > benzene (no substituent) > nitrobenzene (–NO2 is a strong deactivator), reflecting electron density on the ring.
Question 7: Which of the following correctly describes the Diels-Alder reaction?
- A radical chain reaction between a diene and a dienophile
- A concerted [4+2] cycloaddition between a conjugated diene and a dienophile (Correct answer)
- An ionic substitution reaction forming a six-membered ring
- A [2+2] photocycloaddition forming a four-membered ring
Correct answer: A concerted [4+2] cycloaddition between a conjugated diene and a dienophile
The Diels-Alder reaction is a concerted, pericyclic [4+2] cycloaddition that proceeds through a six-membered cyclic transition state, forming a cyclohexene product stereospecifically.
MCAT Organic Chemistry 5
Which of the following best explains why carboxylic acids have higher boiling points than alcohols of similar molecular weight?