Planning a Multi-Step Synthesis: Order, Protection and Yield · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Ordering steps, hiding groups, counting yield

Science · Chemistry · ages 21-22
Name ______________________   Date ____________

  1. When do you mask a group with protection?

    • To make the route longer
    • To shield a group from a later step
    • Only after the synthesis is finished

  2. What makes the order of steps decide success or failure?

    • Reagents must fit all groups present
    • Distillation always fails on scale
    • Short routes always need more protection

  3. Ethers make good protecting groups because they are famously unreactive.

    Circle one:   True   False

  4. Three stages each give 80 percent yield. What is the overall yield in percent, rounded to a whole number?

    Answer: ______________

  5. You oxidise an alcohol to a carbonyl, then add a Grignard reagent. What goes wrong?

    • It attacks the carbonyl just made
    • The Grignard deprotects silyl ethers safely
    • Nothing, the order is ideal

  6. A ten step route and a six step route both give 80 percent per step. Which do you run?

    • The ten step route for higher overall yield
    • The six step route loses less
    • Either, since overall yields match

  7. Your target holds a fragile ring and several steps. When do you build the ring?

    • Early so it guides later steps
    • Never, since rings cannot be made
    • Late, to survive fewer steps

  8. Your route needs extreme cold and endless chromatography at one stage. What do you do?

    • Force it through on larger scale
    • Skip purification to save time
    • Redesign around the risky step
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Answer key

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

Ordering steps, hiding groups, counting yield W1-mt_3PYRELbNfu-s1

  1. To shield a group from a later step · You hide it first, run the step, then reveal it.
  2. Reagents must fit all groups present · A wrong order lets reagents attack the wrong site or destroy earlier work.
  3. True · Their low reactivity lets the rest of the route proceed untouched.
  4. 51 · Multiply 0.8 times 0.8 times 0.8 to get about 51 percent.
  5. It attacks the carbonyl just made · You oxidised too early, so the later reagent destroys earlier work.
  6. The six step route loses less · Ten steps give about 11 percent, while six steps give 26 percent.
  7. Late, to survive fewer steps · Late construction exposes the fragile ring to the fewest steps.
  8. Redesign around the risky step · Extreme conditions and hard purification mean the step will likely fail on scale.
Worksheet · LightMySky