Radical Selectivity, Stability and Allylic Positions · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Which hydrogen gets taken first

Science · Chemistry · ages 19-21
Name ______________________   Date ____________

  1. A C-H bond is unusually weak. What does that tell you?

    • It breaks faster, leaving a stabler radical
    • It breaks slower and leaves a weaker radical
    • It cannot break at all

  2. Which radical is the most stable?

    • Methyl
    • Secondary
    • Tertiary

  3. Propene reacts with bromine under light. What is the main product?

    • 1-bromopropene with Br on the double bond
    • 2-bromopropane with Br in the middle
    • 3-bromopropene, allylic Br

  4. The allyl radical spreads over two carbons. How many resonance forms can you draw for it? Type the number.

    Answer: ______________

  5. Why does bromination distinguish C-H types better than chlorination?

    • Bromine atoms are smaller and faster
    • Bromine uphill, late transition state
    • Chlorine runs out before the reaction finishes

  6. A substrate has 6 primary H (reactivity 1 each) and 2 secondary H (reactivity 82 each). Type the weighted score of the secondary product.

    Answer: ______________

  7. Chlorination of an alkane gives one clean product because chlorine is unselective.

    Circle one:   True   False

  8. A substrate has 9 primary H and 1 allylic H, and bromination gives almost only allylic product. Why does the headcount of 9 lose?

    • Primary hydrogens repel bromine
    • Allylic C-H weaker, gap beats nine
    • Light destroys primary radicals first
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Answer key

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

Which hydrogen gets taken first W1-mt_tgIlRLvc9x-s1

  1. It breaks faster, leaving a stabler radical · Weak bonds break easily, and the radical left behind is stable.
  2. Tertiary · Stability rises with substitution: tertiary beats secondary beats primary beats methyl.
  3. 3-bromopropene, allylic Br · The allylic radical is delocalised, so bromine lands on the allylic carbon.
  4. 2 · The radical sits on either end carbon, giving two resonance forms.
  5. Bromine uphill, late transition state · A late transition state feels the stability gaps between radicals.
  6. 164 · Two hydrogens times reactivity 82 gives 164.
  7. False · Unselective means many products, since chlorine attacks nearly every C-H type.
  8. Allylic C-H weaker, gap beats nine · Product ratios follow count times reactivity, and the allylic reactivity is enormous.
Worksheet · LightMySky