Pericyclic Reactions: The Diels-Alder and Orbital Symmetry · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

One step that builds a ring

Science · Chemistry · ages 20-22
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  1. Which diene conformation can react in a Diels Alder step?

    • The s-trans shape, since it is usually stabler
    • Any twisted shape, since shape never matters
    • The s-cis shape, with both double bonds on the same side

  2. What are the two partners and the product of a Diels Alder reaction?

    • A diene plus a dienophile giving a cyclohexene
    • Two dienes giving an open chain with three double bonds
    • A diene plus water giving an alcohol

  3. A Diels Alder reaction runs through a carbocation intermediate before closing the ring.

    Circle one:   True   False

  4. A cis dienophile enters a Diels Alder reaction. What geometry comes out?

    • Trans, since the step always flips the geometry
    • Cis, since both bonds form together with nothing to scramble it
    • A mix, since the intermediate spins freely

  5. Which substituent pair speeds a normal demand Diels Alder?

    • Withdrawing groups on the diene plus donating groups on the dienophile
    • Donating groups on the diene plus withdrawing groups on the dienophile
    • Withdrawing groups on both partners at once

  6. What does the endo rule say about the product?

    • Dienophile substituents tuck under the diene pi system
    • Substituents always point away from the diene system
    • The diene must stay s-trans through the whole step

  7. Why is the six pi Diels Alder allowed with heat while some other cycloadditions need light?

    • Heat always allows every cycloaddition whatever the count
    • Six pi electrons moving suprafacially in a ring match orbital symmetry for heat
    • Light is needed whenever a diene is present

  8. A diene locked s-trans is heated with a dienophile and nothing happens. What is the diagnosis?

    • The dienophile has the wrong substituents for any reaction
    • Heat destroys all Diels Alder pairings on principle
    • The locked shape cannot reach s-cis, so both ends can never meet the partner
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Answer key

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

One step that builds a ring W1-mt_I4i9d6aDgD-s1

  1. The s-cis shape, with both double bonds on the same side · Only s-cis can reach both ends of the dienophile at the same time.
  2. A diene plus a dienophile giving a cyclohexene · The four pi component plus the two pi component close into a six membered ring with one double bond.
  3. False · Both sigma bonds form in a single concerted transition state with no intermediate.
  4. Cis, since both bonds form together with nothing to scramble it · With no intermediate to twist, the partner geometry is preserved exactly in the adduct.
  5. Donating groups on the diene plus withdrawing groups on the dienophile · That push pull pair narrows the HOMO LUMO gap and quickens the concerted bond formation.
  6. Dienophile substituents tuck under the diene pi system · Extra orbital overlap steadies that tucked arrangement in the transition state.
  7. Six pi electrons moving suprafacially in a ring match orbital symmetry for heat · The thermal rule follows the electron count and the symmetry of the meeting orbitals.
  8. The locked shape cannot reach s-cis, so both ends can never meet the partner · Conformation feeds into rate absolutely here: no s-cis reach means no concerted step.
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