MChem Master of Chemistry Master of Chemistry: Organic Chemistry 2 — Questions and Answers
Question 1: Which reagent converts a primary alcohol to an aldehyde without over-oxidizing to a carboxylic acid?
- KMnO4 in acidic solution
- PCC (pyridinium chlorochromate) (Correct answer)
- Jones reagent (CrO3/H2SO4)
- K2Cr2O7 in H2SO4
Correct answer: PCC (pyridinium chlorochromate)
PCC is a mild oxidant that stops at the aldehyde stage because it is used in anhydrous (non-aqueous) conditions, preventing further oxidation.
Question 2: In the Diels-Alder reaction, which diene conformation is required for the reaction to proceed?
- s-trans
- s-cis (Correct answer)
- anti-periplanar
- gauche
Correct answer: s-cis
The diene must adopt the s-cis conformation so that both terminal carbons can interact with the dienophile in a concerted [4+2] cycloaddition.
Question 3: Which of the following best describes a 1,2-hydride shift in carbocation rearrangements?
- A hydride migrates from an adjacent carbon to the carbocation center (Correct answer)
- A proton is lost from the carbocation to form an alkene
- A hydride attacks from the nucleophile to quench the carbocation
- A hydride migrates from the solvent to the carbocation
Correct answer: A hydride migrates from an adjacent carbon to the carbocation center
In a 1,2-hydride shift, a hydride (H⁻) migrates from the carbon adjacent to the carbocation, generating a new (typically more stable) carbocation at the original position.
Question 4: What is the major product when 2-methylpropene reacts with HBr via Markovnikov addition?
- 1-bromo-2-methylpropane
- 2-bromo-2-methylpropane (Correct answer)
- 1-bromo-1-methylpropane
- 2-bromo-1-methylpropane
Correct answer: 2-bromo-2-methylpropane
Markovnikov's rule directs the proton to the less-substituted carbon, placing the bromine on the more-substituted (tertiary) carbon to give 2-bromo-2-methylpropane.
Question 5: Which spectroscopic feature is most diagnostic for a carbonyl group (C=O) in IR spectroscopy?
- Broad absorption around 3200–3600 cm⁻¹
- Strong absorption around 1630–1850 cm⁻¹ (Correct answer)
- Medium absorption around 2100–2260 cm⁻¹
- Weak absorption around 1450–1600 cm⁻¹
Correct answer: Strong absorption around 1630–1850 cm⁻¹
The C=O stretch produces a strong, sharp absorption in the 1630–1850 cm⁻¹ region; the exact position depends on the type of carbonyl compound.
Question 6: In a Claisen condensation, what acts as the leaving group from the tetrahedral intermediate?
- A hydroxide ion
- An alkoxide ion (Correct answer)
- A hydride ion
- A carbanion
Correct answer: An alkoxide ion
The alkoxide ion (–OR) leaves from the tetrahedral intermediate, regenerating the carbonyl and forming the β-ketoester product.
Question 7: What is the role of the Grignard reagent in a synthesis?
- It acts as a nucleophile by providing a carbanion equivalent (Correct answer)
- It acts as an electrophile attacking electron-rich centers
- It functions as an oxidizing agent toward ketones
- It serves as a base only, not a nucleophile
Correct answer: It acts as a nucleophile by providing a carbanion equivalent
The highly polarized C–Mg bond makes the carbon nucleophilic (carbanion-like), allowing it to attack electrophilic carbons such as carbonyl groups.
MChem Master of Chemistry Master of Chemistry: Organic Chemistry 2
Which reagent converts a primary alcohol to an aldehyde without over-oxidizing to a carboxylic acid?