Benzene: Delocalisation and Electrophilic Substitution · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Benzene keeps its ring

Science · Chemistry · ages 17-18
Name ______________________   Date ____________

  1. What does the hydrogenation energy gap tell you?

    • Benzene reacts faster than any alkene
    • Benzene contains no pi electrons at all
    • Benzene holds extra stability beyond three double bonds

  2. What does delocalisation mean in benzene?

    • Six pi electrons spread over the whole ring
    • Three fixed double bonds that never move
    • Six electrons locked onto one carbon

  3. Benzene decolourises bromine water rapidly, just like cyclohexene.

    Circle one:   True   False

  4. What happens after the benzene ring attacks NO2+?

    • The ring opens into a straight chain
    • A second NO2+ adds to the same carbon
    • A proton leaves and the shared loop is restored

  5. How is the NO2+ electrophile made for nitration?

    • Concentrated nitric acid with concentrated sulfuric acid
    • Dilute nitric acid with water alone
    • Nitrogen gas bubbled through benzene

  6. Why does benzene substitute instead of adding bromine?

    • Benzene cannot meet bromine molecules
    • Substitution keeps the stable ring intact
    • Addition is always slower than substitution

  7. A student ends the nitration mechanism right after the ring attacks, with no proton loss. What is missing?

    • The proton must leave so the delocalised ring is restored
    • The nitro group must leave again at once
    • The sulfuric acid must be filtered out first

  8. A student draws benzene with alternating double bonds and predicts fast bromine water bleaching. What is wrong?

    • Nothing, since the drawing proves three alkenes
    • Equal bond lengths and orange bromine water deny fixed double bonds
    • Bromine water only reacts with single bonds
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Answer key

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

Benzene keeps its ring W1-mt_jFqF7cZ0Ij-s1

  1. Benzene holds extra stability beyond three double bonds · Less energy out means a lower, calmer starting point than three cyclohexenes.
  2. Six pi electrons spread over the whole ring · The electrons share the whole loop instead of sitting in fixed bonds.
  3. False · Benzene leaves bromine water orange, which argues against fixed double bonds. The sentence is false.
  4. A proton leaves and the shared loop is restored · Losing a proton closes the delocalised loop again as nitrobenzene.
  5. Concentrated nitric acid with concentrated sulfuric acid · Sulfuric acid protonates nitric acid so water leaves and NO2+ remains.
  6. Substitution keeps the stable ring intact · Addition would destroy the delocalised ring and its bonus stability.
  7. The proton must leave so the delocalised ring is restored · Without losing the proton the loop never closes, and nitrobenzene never forms.
  8. Equal bond lengths and orange bromine water deny fixed double bonds · One shared bond length and no bleaching both contradict the alternating picture.
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