Enols, Enolates and Alpha Substitution · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

The carbon next door fights back

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

  1. Why are alpha hydrogens far more acidic than ordinary C-H bonds?

    • Because the carbonyl carbon is positively charged forever
    • Because the resulting anion delocalises onto oxygen
    • Because alpha carbons carry extra hydrogens

  2. What is an enolate?

    • An anion with its charge shared between carbon and oxygen
    • A neutral molecule with two carbonyl groups and no charge at all
    • A cation sitting on the alpha carbon

  3. For a simple ketone, the keto form dominates the enol by a huge margin.

    Circle one:   True   False

  4. You need the ketone converted to enolate as fully as equilibrium allows before the next step. Which base do you pick?

    • A weak base that sets a small equilibrium
    • No base at all, since enols form alone
    • A strong hindered base whose conjugate acid has the far larger pKa

  5. Why does a 1,3-diketone hold far more enol than a simple ketone?

    • Because it has no alpha hydrogens at all
    • Conjugation with the second carbonyl steadies the enol
    • Because diketones cannot form keto forms under any conditions

  6. An unsymmetrical ketone meets a bulky strong base under fast irreversible conditions. Which hydrogen is grabbed?

    • The more exposed one
    • The most buried one
    • A hydrogen on the oxygen

  7. A student uses a weak base but plans as if all ketone became enolate. What goes wrong?

    • Nothing, since weak bases always finish deprotonation
    • Only a small equilibrium concentration forms, so the product plan fails
    • The ketone turns entirely into enol instead

  8. You want the more substituted enolate of an unsymmetrical ketone. What conditions do you choose?

    • A bulky base for a fast grab
    • A small base with time to equilibrate
    • A strong acid instead of any base
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Answer key

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

The carbon next door fights back W1-mt_EltcLAjnLC-s1

  1. Because the resulting anion delocalises onto oxygen · Delocalisation steadies the anion that deprotonation leaves.
  2. An anion with its charge shared between carbon and oxygen · Removing an alpha proton leaves a delocalised anion.
  3. True · Only a tiny fraction sits as enol at any moment.
  4. A strong hindered base whose conjugate acid has the far larger pKa · Near-complete conversion needs a base whose conjugate acid has the far larger pKa.
  5. Conjugation with the second carbonyl steadies the enol · Conjugation plus an internal hydrogen bond steadies the enol side of the balance.
  6. The more exposed one · Under fast conditions bulk cannot reach crowded sites, so it takes the open path.
  7. Only a small equilibrium concentration forms, so the product plan fails · Planning for full enolate with a weak base overcounts the real nucleophile present at any moment.
  8. A small base with time to equilibrate · Equilibration lets the steadier, more substituted enolate win.
Worksheet · LightMySky