MChem Master of Chemistry Master of Chemistry: Organic Chemistry 4 — Questions and Answers
Question 1: Which reaction mechanism is responsible for the polymerization of ethylene using a Ziegler-Natta catalyst?
- Radical chain-growth polymerization
- Anionic coordination-insertion polymerization (Correct answer)
- Cationic ring-opening polymerization
- Step-growth (condensation) polymerization
Correct answer: Anionic coordination-insertion polymerization
Ziegler-Natta catalysts operate via a coordination-insertion mechanism in which the alkene coordinates to the transition metal and inserts into the metal–carbon bond.
Question 2: What does the term 'retrosynthetic analysis' mean in organic synthesis?
- Analyzing the products of a failed synthesis
- Systematically deconstructing a target molecule into simpler precursors by breaking strategic bonds (Correct answer)
- Repeating a synthesis in reverse order to verify the mechanism
- Running a reaction at lower temperature to observe intermediates
Correct answer: Systematically deconstructing a target molecule into simpler precursors by breaking strategic bonds
Retrosynthetic analysis (Corey's disconnection approach) works backward from the target molecule, identifying bond disconnections that map to known forward reactions.
Question 3: Which of the following substituents is meta-directing in electrophilic aromatic substitution?
- –OH
- –NH2
- –NO2 (Correct answer)
- –OCH3
Correct answer: –NO2
Electron-withdrawing groups such as –NO2 destabilize the carbocation intermediate at ortho/para positions more than at meta, making meta the preferred site of attack.
Question 4: In the aldol condensation, what type of intermediate is initially formed before dehydration?
- A β-hydroxy carbonyl compound (Correct answer)
- A hemiketal
- An enol ether
- A carboxylic acid anhydride
Correct answer: A β-hydroxy carbonyl compound
The aldol addition step produces a β-hydroxy carbonyl compound; subsequent base- or acid-catalyzed dehydration gives the α,β-unsaturated carbonyl product.
Question 5: How does the Wittig reaction construct a carbon–carbon double bond?
- Via an SN2 displacement of a halide with a phosphorus ylide
- By a [2+2] cycloaddition of an alkene to a phosphonium salt
- Through a betaine/oxaphosphetane intermediate that decomposes to give an alkene and Ph3P=O (Correct answer)
- By oxidation of a geminal diol with a phosphine oxide
Correct answer: Through a betaine/oxaphosphetane intermediate that decomposes to give an alkene and Ph3P=O
The phosphorus ylide reacts with the carbonyl to form a four-membered oxaphosphetane ring, which undergoes retrocycloaddition to give the alkene and triphenylphosphine oxide.
Question 6: Which type of isomerism is exhibited by (R)-lactic acid and (S)-lactic acid?
- Constitutional isomerism
- Diastereomerism
- Conformational isomerism
- Enantiomerism (Correct answer)
Correct answer: Enantiomerism
Enantiomers are non-superimposable mirror images that differ only in the configuration at one or more stereocenters, and (R)- and (S)-lactic acid are a classic example.
Question 7: What is the purpose of using a protecting group for an alcohol in multi-step synthesis?
- To increase the alcohol's nucleophilicity in the next step
- To temporarily mask the alcohol's reactivity so other functional groups can be transformed selectively (Correct answer)
- To convert the alcohol into a better leaving group for elimination
- To permanently change the alcohol's oxidation state
Correct answer: To temporarily mask the alcohol's reactivity so other functional groups can be transformed selectively
Protecting groups (e.g., TBS ethers or THP acetals) block the alcohol from reacting under conditions intended for another part of the molecule, then are removed selectively later.
MChem Master of Chemistry Master of Chemistry: Organic Chemistry 4
Which reaction mechanism is responsible for the polymerization of ethylene using a Ziegler-Natta catalyst?