PCAT Organic Chemistry 5 — Questions and Answers
Question 1: Which of the following correctly ranks carboxylic acid derivatives from most to least reactive toward nucleophilic acyl substitution?
- Amide > ester > anhydride > acyl chloride
- Acyl chloride > anhydride > ester > amide (Correct answer)
- Ester > amide > acyl chloride > anhydride
- Anhydride > amide > ester > acyl chloride
Correct answer: Acyl chloride > anhydride > ester > amide
Reactivity decreases as the leaving group becomes a worse leaving group: Cl⁻ (best) > RCOO⁻ > RO⁻ > NH2⁻ (worst), giving acyl chloride > anhydride > ester > amide.
Question 2: What is the product when an alkene reacts with bromine (Br2) in CCl4?
- A vicinal dibromide via anti addition (Correct answer)
- A geminal dibromide
- An allylic bromide
- A bromohydrin
Correct answer: A vicinal dibromide via anti addition
Br2 adds to alkenes via a bromonium ion intermediate, giving anti addition of two bromine atoms across the double bond to form a vicinal (1,2) dibromide.
Question 3: Which of the following describes the relationship between chair conformers of cyclohexane?
- Constitutional isomers
- Enantiomers
- Conformational isomers (Correct answer)
- Diastereomers
Correct answer: Conformational isomers
The two chair conformers of cyclohexane interconvert rapidly by ring flipping and differ only in conformation, not connectivity, making them conformational isomers.
Question 4: Grignard reagents (RMgX) act as which type of species in organic reactions?
- Electrophiles
- Oxidants
- Nucleophiles and strong bases (Correct answer)
- Radical intermediates
Correct answer: Nucleophiles and strong bases
Grignard reagents are carbanion equivalents that act as strong nucleophiles and bases due to the highly polarized C–Mg bond.
Question 5: What is the role of H2SO4 in the nitration of benzene?
- It is the nucleophile
- It generates the nitronium ion (NO2+) electrophile from HNO3 (Correct answer)
- It reduces the nitro group
- It acts as the solvent only
Correct answer: It generates the nitronium ion (NO2+) electrophile from HNO3
H2SO4 protonates HNO3, leading to loss of water and formation of the nitronium ion (NO2+), the active electrophile in EAS nitration.
Question 6: Which of the following functional groups is correctly identified?
- R-O-R' = ester
- R-CO-NH2 = primary amide (Correct answer)
- R-CO-O-CO-R' = hemiacetal
- R-CHO = ketone
Correct answer: R-CO-NH2 = primary amide
R-CO-NH2 is a primary amide because the nitrogen bears no additional carbon substituents.
Question 7: The Markovnikov product of HBr addition to propene is:
- 1-bromopropane
- 2-bromopropane (Correct answer)
- Propan-1-ol
- Allyl bromide
Correct answer: 2-bromopropane
Markovnikov's rule states that H adds to the carbon with more hydrogens (C1), placing Br on C2 (more substituted), giving 2-bromopropane.
PCAT Organic Chemistry 5
Which of the following correctly ranks carboxylic acid derivatives from most to least reactive toward nucleophilic acyl substitution?