MChem Master of Chemistry Master of Chemistry: Organic Chemistry 5 â Questions and Answers
Question 1: Which of the following correctly describes the Mitsunobu reaction?
- It converts an alcohol to an alkyl halide with inversion via an SN2 mechanism using DIAD and PPh3 (Correct answer)
- It oxidizes a secondary alcohol to a ketone using DIAD as oxidant
- It replaces a hydroxyl group with an amine with retention of configuration
- It forms an ether between two primary alcohols under acidic conditions
Correct answer: It converts an alcohol to an alkyl halide with inversion via an SN2 mechanism using DIAD and PPh3
The Mitsunobu reaction uses DIAD (or DEAD) and PPh3 to activate the alcohol, and an acidic pronucleophile (HNu) then attacks with inversion of configuration.
Question 2: In mass spectrometry, what is the base peak?
- The peak corresponding to the molecular ion (M+)
- The most abundant ion fragment peak, set to 100% relative abundance (Correct answer)
- The peak with the lowest m/z value in the spectrum
- The peak produced by loss of a methyl radical from the molecular ion
Correct answer: The most abundant ion fragment peak, set to 100% relative abundance
The base peak is the most intense peak in the mass spectrum and is assigned a relative abundance of 100%; it may or may not correspond to the molecular ion.
Question 3: Which reaction converts a terminal alkyne to a methyl ketone via Markovnikov hydration?
- Hydroborationâoxidation with 9-BBN
- Acid-catalyzed hydration with H2SO4/HgSO4 (Correct answer)
- Ozonolysis followed by oxidative workup
- Hydration with mCPBA then acid
Correct answer: Acid-catalyzed hydration with H2SO4/HgSO4
Mercury(II)-catalyzed hydration follows Markovnikov's rule and generates a vinyl alcohol (enol) intermediate that tautomerizes to the methyl ketone.
Question 4: What does a positive Tollens' test (silver mirror) indicate about a compound?
- The compound contains a ketone
- The compound contains an aldehyde (or alpha-hydroxy ketone) (Correct answer)
- The compound contains a terminal alkene
- The compound contains a phenol group
Correct answer: The compound contains an aldehyde (or alpha-hydroxy ketone)
Tollens' reagent ([Ag(NH3)2]+) oxidizes aldehydes (and some α-hydroxy ketones such as fructose) to carboxylates while reducing silver ions to metallic silver.
Question 5: In the Robinson annulation, which two reactions are combined to build a six-membered ring?
- Michael addition followed by intramolecular aldol condensation (Correct answer)
- Diels-Alder cycloaddition followed by retro-ene elimination
- Aldol addition followed by Claisen condensation
- 1,2-addition to an enone followed by E2 elimination
Correct answer: Michael addition followed by intramolecular aldol condensation
The Robinson annulation consists of a conjugate (Michael) addition of an enolate to an enone, followed by an intramolecular aldol condensation and dehydration to give a cyclohexenone.
Question 6: Which of the following factors most destabilizes a cyclopropane ring relative to cyclohexane?
- Transannular strain from non-bonded interactions across the ring
- Angle strain from compressed CâCâC bond angles (~60°) and torsional strain from eclipsed CâH bonds (Correct answer)
- Large gauche interactions between substituents
- Steric strain from axial substituents
Correct answer: Angle strain from compressed CâCâC bond angles (~60°) and torsional strain from eclipsed CâH bonds
Cyclopropane suffers severe angle strain (ideal spÂł angle is 109.5° vs. actual ~60°) and torsional strain because all adjacent CâH bonds are forced into an eclipsed conformation.
Question 7: What is the significance of the Woodward-Hoffmann rules in pericyclic reactions?
- They predict the regiochemistry of radical additions to alkenes
- They use frontier molecular orbital (FMO) symmetry to determine if a pericyclic reaction is thermally or photochemically allowed (Correct answer)
- They quantify the activation energies of electrophilic aromatic substitution
- They describe the stereoselectivity of nucleophilic addition to carbonyls
Correct answer: They use frontier molecular orbital (FMO) symmetry to determine if a pericyclic reaction is thermally or photochemically allowed
Woodward-Hoffmann rules state that pericyclic reactions are thermally allowed when the orbital symmetry of the HOMO and LUMO of the reacting components permits a continuous, bonding overlap in the transition state.
MChem Master of Chemistry Master of Chemistry: Organic Chemistry 5
Which of the following correctly describes the Mitsunobu reaction?