Acetals, Imines and Protecting Groups · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Hiding a carbonyl in plain sight

Science · Chemistry · ages 20-21
Name ______________________   Date ____________

  1. How does a hemiacetal differ from an acetal?

    • A hemiacetal holds two OR groups and no OH
    • They are identical names for one compound
    • A hemiacetal holds one OR and one OH on the same carbon

  2. What partners and catalyst build an acetal?

    • Carbonyl plus two alcohols under acid catalysis
    • Carbonyl plus water under base catalysis
    • Two carbonyls plus one alcohol with no catalyst

  3. Removing water pushes acetal formation forward.

    Circle one:   True   False

  4. One carbonyl meets a primary amine, another meets a secondary amine. What forms?

    • Both give an imine with C=N
    • The first gives an imine, the second an enamine
    • Both give an enamine with C=C-N

  5. How do you unmask an acetal back to the carbonyl?

    • Add aqueous acid to hydrolyse it
    • Add strong base and heat hard
    • Add more dry alcohol

  6. Why is an acetal a good mask during a Grignard step?

    • It survives base and nucleophiles but falls to aqueous acid
    • It reacts with Grignard reagents before the target does
    • It cannot be removed once it has formed

  7. A student wonders why acid is needed twice in acetal formation. How do you answer?

    • Acid protonates the carbonyl at both stages, enabling attack and water loss
    • Acid is only needed to colour the mixture
    • One dose of acid always survives the whole reaction untouched

  8. You must reduce an ester in a molecule that also carries a ketone. What is the plan?

    • Reduce directly, since ketones ignore hydrides
    • Protect the ketone as an acetal, reduce, then unmask with aqueous acid
    • Protect the ester instead and reduce the ketone
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Hiding a carbonyl in plain sight W1-mt_3dB99bKrMf-s1

  1. A hemiacetal holds one OR and one OH on the same carbon · The hemiacetal is the halfway intermediate; the acetal is the finished mask.
  2. Carbonyl plus two alcohols under acid catalysis · Two alcohol equivalents add and water leaves, with acid at both stages.
  3. True · Draining a product drags the equilibrium toward the acetal.
  4. The first gives an imine, the second an enamine · Two N-H protons reach C=N; one proton strands the double bond between carbons.
  5. Add aqueous acid to hydrolyse it · Water plus acid reverses the equilibrium toward the carbonyl.
  6. It survives base and nucleophiles but falls to aqueous acid · The mask must endure the planned step yet leave on demand.
  7. Acid protonates the carbonyl at both stages, enabling attack and water loss · Each stage needs a protonated carbonyl before it can move.
  8. Protect the ketone as an acetal, reduce, then unmask with aqueous acid · The mask keeps the ketone out of the fight until the ester is reduced.
Worksheet · LightMySky