MCAT Organic Chemistry 4 β Questions and Answers
Question 1: Which amino acid side chain contains a thiol group that can form disulfide bonds?
- Serine
- Threonine
- Cysteine (Correct answer)
- Methionine
Correct answer: Cysteine
Cysteine contains a βSH (thiol) group that can be oxidized to form disulfide bonds (βSβSβ) important for protein tertiary and quaternary structure.
Question 2: When 2-methylpropene reacts with HBr, the major product is 2-bromo-2-methylpropane rather than 1-bromo-2-methylpropane. This illustrates:
- Zaitsev's rule
- Markovnikov's rule (Correct answer)
- Anti-Markovnikov addition
- Hofmann's rule
Correct answer: Markovnikov's rule
Markovnikov's rule predicts that the electrophile (HβΊ) adds to the less substituted carbon, placing the nucleophile (Brβ») on the more substituted (more stable carbocation) carbon.
Question 3: What is the stereochemical outcome of an SN2 reaction?
- Retention of configuration
- Racemization
- Inversion of configuration (Walden inversion) (Correct answer)
- Random mixture of products
Correct answer: Inversion of configuration (Walden inversion)
SN2 proceeds via a backside attack transition state, resulting in inversion of configuration at the chiral center (Walden inversion).
Question 4: Which of the following is the strongest acid?
- CH3CH2OH (pKa β 16)
- CH3COOH (pKa β 4.75) (Correct answer)
- H2CO3 (pKa β 6.35)
- PhOH (pKa β 10)
Correct answer: CH3COOH (pKa β 4.75)
Acetic acid (pKa β 4.75) is the strongest acid listed because resonance stabilization of the acetate anion greatly lowers the pKa compared to alcohols or phenols.
Question 5: A compound gives a positive Tollens' test (silver mirror) but does not react with Fehling's solution. Which compound is most likely?
- Acetone
- Benzaldehyde (Correct answer)
- Glucose
- Fructose
Correct answer: Benzaldehyde
Benzaldehyde is oxidized by Tollens' reagent (silver mirror) but its product is insoluble in Fehling's solution (copper-based), which requires water-soluble reducing sugars.
Question 6: In NMR spectroscopy, which proton environment would appear at the highest chemical shift (most downfield)?
- βCH3 in a simple alkane
- βCH2β adjacent to a carbonyl
- βCHO (aldehyde proton) (Correct answer)
- Vinyl proton (=CHβ)
Correct answer: βCHO (aldehyde proton)
Aldehyde protons (βCHO) resonate at Ξ΄ 9β10 ppm, far downfield due to both deshielding by the electronegative oxygen and the anisotropy of the carbonyl.
Question 7: Which reaction mechanism involves a radical chain process initiated by heat or light?
- Nucleophilic aromatic substitution
- Electrophilic addition to alkenes
- Free radical halogenation of alkanes (Correct answer)
- Friedel-Crafts acylation
Correct answer: Free radical halogenation of alkanes
Free radical halogenation involves initiation (homolytic cleavage by heat or light), propagation (chain steps), and termination, characteristic of radical chain mechanisms.
MCAT Organic Chemistry 4
Which amino acid side chain contains a thiol group that can form disulfide bonds?