MCAT Organic Chemistry 3 — Questions and Answers
Question 1: Which of the following reactions proceeds via a carbocation intermediate?
- SN2 reaction of CH3Br with OH⁻
- E2 elimination of 2-bromobutane
- SN1 solvolysis of tert-butyl chloride in water (Correct answer)
- Radical bromination of methane
Correct answer: SN1 solvolysis of tert-butyl chloride in water
SN1 reactions proceed by unimolecular ionization to form a carbocation intermediate, favored by tertiary substrates in polar protic solvents.
Question 2: What is the IUPAC name for CH3CH(OH)CH2CH3?
- 1-butanol
- 2-butanol (Correct answer)
- 3-butanol
- tert-butanol
Correct answer: 2-butanol
The hydroxyl group is on C2 of the four-carbon chain, giving 2-butanol by IUPAC nomenclature.
Question 3: In the Fischer projection of D-glucose, which carbon bears the reference hydroxyl group that defines the D-configuration?
- C1
- C2
- C5 (Correct answer)
- C6
Correct answer: C5
In D-glucose, the highest-numbered chiral center (C5) has its –OH on the right in the Fischer projection, defining D-configuration by analogy to D-glyceraldehyde.
Question 4: Which reagent would best convert a carboxylic acid to a primary amine via a Curtius-type rearrangement?
- LiAlH4
- NaBH4
- DPPA (diphenylphosphoryl azide) followed by heat and hydrolysis (Correct answer)
- SOCl2 then NH3
Correct answer: DPPA (diphenylphosphoryl azide) followed by heat and hydrolysis
DPPA converts carboxylic acids to acyl azides, which rearrange on heating (Curtius rearrangement) to isocyanates; hydrolysis then gives primary amines.
Question 5: Which of the following correctly describes a nucleophilic addition reaction to a carbonyl compound?
- The nucleophile attacks the carbonyl oxygen
- The nucleophile attacks the electrophilic carbonyl carbon (Correct answer)
- The electrophile attacks the carbonyl oxygen first
- The reaction proceeds via an E1cb mechanism
Correct answer: The nucleophile attacks the electrophilic carbonyl carbon
Nucleophilic addition to carbonyls occurs because the carbonyl carbon is electrophilic due to the electronegative oxygen withdrawing electron density.
Question 6: A student runs a reaction of 1-bromopropane with NaCN in DMSO. What is the major product?
- Propanol
- Propanenitrile (butanenitrile) (Correct answer)
- Propyne
- Propanamine
Correct answer: Propanenitrile (butanenitrile)
CN⁻ is a strong nucleophile that attacks the primary alkyl bromide via SN2, forming butanenitrile (one carbon longer than the substrate).
Question 7: What characterizes an aromatic compound according to Hückel's rule?
- It must contain only carbon and hydrogen
- It must be planar, cyclic, fully conjugated, and have 4n+2 π electrons (Correct answer)
- It must have alternating single and double bonds only
- It must have 4n π electrons
Correct answer: It must be planar, cyclic, fully conjugated, and have 4n+2 π electrons
Hückel's rule states that aromatic compounds are cyclic, planar, fully conjugated, and contain 4n+2 π electrons (where n is a non-negative integer).
MCAT Organic Chemistry 3
Which of the following reactions proceeds via a carbocation intermediate?