Chapter 8 Practice Problems
Chapter 8 Practice Problems
Work through these carbonyl reaction problems. Reveal answers after you’ve tried each.
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Imine mechanism: Draw the mechanism for acetone + ethylamine → imine + H₂O. Show carbinolamine formation, proton transfers, water loss, and iminium deprotonation. Why pH ~5?
Answer
Mild acid activates C=O and protonates –OH to leave as water, but keeps some amine unprotonated for attack; too acidic blocks the amine, too basic slows –OH departure. -
Imine vs enamine: Cyclopentanone + (a) excess methylamine, trace acid; (b) excess dimethylamine, trace acid. Predict the products.
Answer
(a) Imine (Schiff base) from 1° amine. (b) Enamine from 2° amine (C=C adjacent to N). -
Reversibility checks: (a) What happens to an imine in aqueous acid? (b) Cyanohydrin in base? (c) How to deprotect an acetal?
Answer
(a) Hydrolyzes to carbonyl + amine. (b) Reverts to carbonyl + CN⁻. (c) Aqueous acid (H₃O⁺) hydrolyzes acetal to carbonyl + alcohols. -
Grignard target: Convert 2-butanone to 2-methyl-2-butanol. Provide reagents.
Answer
Add CH₃MgBr (ether), then aqueous workup → tertiary alcohol 2-methyl-2-butanol. -
Unknown ID: C₄H₈O gives yellow 2,4-DNP and positive Tollens. What functional group, and give one likely structure.
Answer
Aldehyde (Tollens-positive). Likely butanal or 2-methylpropanal. -
Protecting strategy: You must reduce an ester with LiAlH₄ in a molecule that also has a ketone. How to spare the ketone?
Answer
Protect the ketone as an acetal (e.g., ethylene glycol + acid), do LiAlH₄, then hydrolyze the acetal with aqueous acid. -
Enamine alkylation (challenge): Outline an enamine route to α-benzylate cyclohexanone with benzyl bromide.
Answer
Form the enamine of cyclohexanone (e.g., with pyrrolidine + acid), alkylate with PhCH₂Br at the β-carbon of the enamine, then acidic hydrolysis to regenerate the ketone → α-benzylated cyclohexanone.