MChem Master of Chemistry Master of Chemistry: Organic Chemistry 3 — Questions and Answers
Question 1: Which of the following best explains why aromatic compounds undergo electrophilic substitution rather than addition?
- Addition products are thermodynamically more stable
- Substitution restores the aromatic ring's stability after electrophile attack (Correct answer)
- Electrophilic addition is kinetically too slow for benzene
- Aromatic rings cannot form sigma complexes with electrophiles
Correct answer: Substitution restores the aromatic ring's stability after electrophile attack
After the electrophile forms a sigma complex (arenium ion), loss of a proton restores aromaticity, making substitution the thermodynamically favored pathway.
Question 2: Which stereospecific reaction converts an alkene to a vicinal diol with retention of relative configuration (syn addition)?
- OsO4 dihydroxylation (Correct answer)
- mCPBA followed by acid hydrolysis
- Br2 in water
- Ozonolysis followed by NaBH4 reduction
Correct answer: OsO4 dihydroxylation
OsO4 adds to both faces of the double bond in a concerted fashion, delivering both hydroxyl groups from the same face (syn addition) to give a syn-diol.
Question 3: In the E2 elimination mechanism, what geometric requirement must the leaving group and β-hydrogen satisfy?
- They must be gauche (60°) to each other
- They must be anti-periplanar (180°) to each other (Correct answer)
- They must be syn-periplanar (0°) to each other
- No specific dihedral angle is required
Correct answer: They must be anti-periplanar (180°) to each other
E2 is a concerted, stereospecific mechanism requiring anti-periplanar alignment (180°) of the H and the leaving group for proper orbital overlap.
Question 4: What is the product of treating an ester with excess LiAlH4 followed by aqueous workup?
- A carboxylic acid
- A primary alcohol (Correct answer)
- A secondary alcohol
- An aldehyde
Correct answer: A primary alcohol
LiAlH4 reduces esters fully to primary alcohols; the intermediate aldehyde is not isolated because it is reduced further under the reaction conditions.
Question 5: Which of the following is an example of a [3,3]-sigmatropic rearrangement?
- Cope rearrangement (Correct answer)
- Wagner–Meerwein rearrangement
- Pinacol rearrangement
- Beckmann rearrangement
Correct answer: Cope rearrangement
The Cope rearrangement is a thermal, concerted [3,3]-sigmatropic rearrangement of a 1,5-diene via a chair-like transition state.
Question 6: In NMR spectroscopy, which concept explains why axial and equatorial protons in cyclohexane have different chemical shifts?
- They have different hybridization states
- They experience different anisotropic and steric shielding environments
- They are diastereotopic and in different electronic environments (Correct answer)
- They differ in their coupling constants only, not in chemical shift
Correct answer: They are diastereotopic and in different electronic environments
Axial and equatorial protons are diastereotopic—they cannot be interconverted by any symmetry operation—so they appear at different chemical shifts in the NMR spectrum.
Question 7: What distinguishes a kinetic enolate from a thermodynamic enolate?
- Kinetic enolates are formed at high temperature using weak bases
- Kinetic enolates are the less substituted and form faster under low-temperature, strong-base conditions (Correct answer)
- Thermodynamic enolates form faster because the less substituted alkene is less hindered
- Kinetic enolates are always formed from symmetrical ketones
Correct answer: Kinetic enolates are the less substituted and form faster under low-temperature, strong-base conditions
Under low temperature and strong, hindered base (e.g., LDA), deprotonation of the more accessible (less substituted) position is faster, giving the kinetic enolate.
MChem Master of Chemistry Master of Chemistry: Organic Chemistry 3
Which of the following best explains why aromatic compounds undergo electrophilic substitution rather than addition?