Olefin Metathesis and Ring-Closing Strategy · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Trading ends to close a ring

Science · Chemistry · ages 23-24
Name ______________________   Date ____________

  1. What drives a ring-closing metathesis forward?

    • Extra catalyst added at the end
    • The small alkene fragment escaping as a gas
    • Heating until the solvent boils away

  2. What is a metallacyclobutane?

    • A four-membered ring of metal and carbon that swaps alkene ends
    • A cage of solvent around the catalyst
    • The small fragment that escapes as a gas

  3. Metathesis trades the ends of two alkenes, so each product mixes halves.

    Circle one:   True   False

  4. The same diene is run very concentrated. What happens?

    • Only small rings form, and faster
    • Nothing about the outcome changes
    • Molecules join neighbours into chains

  5. A diene chain of 8 atoms closes and a 2-carbon fragment leaves. How many atoms are in the ring?

    Answer: ______________

  6. Why does the escaping fragment move the mixture forward?

    • Escaped product cannot react back, so the ring accumulates
    • The gas stirs the mixture for free
    • The gas feeds fresh catalyst into the flask

  7. A student draws the products keeping each alkene's original partner. What is wrong?

    • Nothing is wrong: partners never change
    • The drawing is fine but drawn too slowly
    • The ends trade partners, so products must mix halves

  8. You must close a large ring whose ends can both be alkenes. Which cut fits metathesis?

    • A cut that leaves matching polar handles for a carbonyl route
    • A cut that leaves an alkene on each side to stitch shut
    • A cut through the longest plain single-bond chain
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Answer key

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

Trading ends to close a ring W1-mt_J9UAxKsgSk-s1

  1. The small alkene fragment escaping as a gas · A product that leaves cannot come back. Escape pulls the balance forward.
  2. A four-membered ring of metal and carbon that swaps alkene ends · Metal plus carbons in a loop of four. The loop is where partners trade.
  3. True · Partners swap. No product keeps only its original half.
  4. Molecules join neighbours into chains · Crowding favours meetings between molecules. Neighbours link into chains.
  5. 6 · Keep the chain atoms minus the leaving atoms. The rest form the ring.
  6. Escaped product cannot react back, so the ring accumulates · The back reaction needs the fragment. Gone means no way back.
  7. The ends trade partners, so products must mix halves · Original pairs break in the metal ring. Keeping them contradicts the mechanism.
  8. A cut that leaves an alkene on each side to stitch shut · Metathesis joins alkene ends. Cut where those ends can form.
Worksheet · LightMySky