MCAT Organic Chemistry 4

Question 1

Which amino acid side chain contains a thiol group that can form disulfide bonds?

Accepted Answer

Cysteine

Answer

Cysteine contains a –SH (thiol) group that can be oxidized to form disulfide bonds (–S–S–) 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:

Accepted Answer

Markovnikov's rule

Answer

Markovnikov's rule predicts that the electrophile (H⁺) adds to the less substituted carbon, placing the nucleophile (Br⁻) on the more substituted (more stable carbocation) carbon.

Question 3

What is the stereochemical outcome of an SN2 reaction?

Accepted Answer

Inversion of configuration (Walden inversion)

Answer

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?

Accepted Answer

CH3COOH (pKa ≈ 4.75)

Answer

Acetic acid (pKa ≈ 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?

Accepted Answer

Benzaldehyde

Answer

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.

MCAT - Medical College Admission Test Practice Test

MCAT - Medical College Admission Test Practice Test

MCAT Organic Chemistry 4