PCAT Organic Chemistry 3 — Questions and Answers
Question 1: Which of the following best describes the Diels-Alder reaction?
- Radical addition of HBr to an alkene
- Concerted [4+2] cycloaddition between a diene and a dienophile (Correct answer)
- Acid-catalyzed addition of water to an alkyne
- Nucleophilic substitution at a carbonyl
Correct answer: Concerted [4+2] cycloaddition between a diene and a dienophile
The Diels-Alder reaction is a pericyclic [4+2] cycloaddition occurring in a single concerted step between a conjugated diene and a dienophile.
Question 2: What is the product of treating a ketone with NaBH4 followed by aqueous workup?
- A primary alcohol
- A secondary alcohol (Correct answer)
- An aldehyde
- A carboxylic acid
Correct answer: A secondary alcohol
NaBH4 reduces a ketone (R2C=O) by delivering hydride to the carbonyl carbon, yielding a secondary alcohol after protonation.
Question 3: What does the term 'leaving group ability' depend on most directly?
- Electronegativity of the atom
- Stability of the leaving group as an anion or neutral molecule (Correct answer)
- Molecular weight of the leaving group
- Boiling point of the leaving group
Correct answer: Stability of the leaving group as an anion or neutral molecule
A good leaving group departs as a stable species (weak base/stable anion), so leaving group ability correlates with stability after departure.
Question 4: Which functional group is produced by the reaction of a carboxylic acid with an amine?
- Ester
- Anhydride
- Amide (Correct answer)
- Acyl chloride
Correct answer: Amide
A carboxylic acid reacts with an amine (with heat or activation) to form an amide (R-CO-NR2) plus water.
Question 5: In NMR spectroscopy, chemically equivalent protons show how many signals?
- One signal per equivalent set (Correct answer)
- Separate signals for each proton
- No signal
- Signals proportional to molecular weight
Correct answer: One signal per equivalent set
Chemically equivalent protons are indistinguishable and appear as a single NMR signal regardless of how many protons are in that set.
Question 6: Which of the following is an example of electrophilic aromatic substitution (EAS)?
- Hydrogenation of benzene
- Nitration of benzene with HNO3/H2SO4 (Correct answer)
- Oxidation of toluene to benzoic acid
- Birch reduction of benzene
Correct answer: Nitration of benzene with HNO3/H2SO4
Nitration of benzene is a classic EAS reaction where the nitronium ion (NO2+) acts as the electrophile and substitutes a ring hydrogen.
Question 7: What is the IUPAC name of CH3CH2CHO?
- Propanal (Correct answer)
- Propanone
- Butanal
- Propanol
Correct answer: Propanal
CH3CH2CHO is a three-carbon aldehyde; the IUPAC suffix for aldehydes is '-al,' so the name is propanal.
PCAT Organic Chemistry 3
Which of the following best describes the Diels-Alder reaction?