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3.3.10

Flashcards

Interactive study deck for benzene structure and reactions.

Aromatic Chemistry: read all 18 questions and answers
  1. What was the structure of benzene proposed by Kekule?
    Cyclohexa-1,3,5-triene, featuring alternating carbon-carbon single and double bonds in a ring.
  2. State the physical evidence from bond lengths that disproves Kekule's structure of benzene.
    X-ray diffraction shows all six C-C bonds are identical in length (0.140 nm), which is intermediate between a C-C single bond (0.154 nm) and a C=C double bond (0.134 nm).
  3. How does the chemical reactivity of benzene disprove Kekule's cyclohexa-1,3,5-triene?
    Benzene resists addition reactions (e.g. it does not decolorise bromine water) and instead undergoes electrophilic substitution reactions to keep the stable ring intact.
  4. Compare the enthalpy of hydrogenation of benzene with the predicted value for cyclohexa-1,3,5-triene.
    Benzene is -208 kJ/mol, whereas cyclohexa-1,3,5-triene is predicted to be -360 kJ/mol (three times cyclohexene's value of -120 kJ/mol).
  5. Explain the significance of the 152 kJ/mol difference in hydrogenation enthalpies for benzene.
    It represents the delocalisation stability energy (resonance energy) of benzene, showing it is much more stable than predicted due to delocalised pi-electrons.
  6. Describe the bonding and structure of benzene in the modern delocalised model.
    Benzene is a planar, hexagonal ring of 6 sp2 hybridised carbon atoms with bond angles of 120 degrees. The remaining six p-orbitals overlap sideways to form a delocalised pi-electron system above and below the ring plane.
  7. Why does benzene undergo substitution rather than addition reactions?
    Addition reactions would break up the highly stable delocalised pi-electron system, whereas substitution preserves this delocalisation and stability.
  8. Write the chemical equation for the generation of the nitronium ion (NO₂⁺) electrophile in nitration.
    HNO₃ + 2H₂SO₄ → NO₂⁺ + H₃O⁺ + 2HSO₄⁻
  9. State the role of sulfuric acid in the nitration of benzene.
    It acts as a catalyst and a strong acid, protonating nitric acid (HNO₃) to allow the formation of the nitronium ion.
  10. What temperature must be maintained during the mono-nitration of benzene, and why?
    Below 55 degrees Celsius, to prevent further nitration reactions (dinitration or trinitration) on the ring.
  11. Name a major industrial use of nitrobenzene.
    It is reduced to make phenylamine (aniline), which is an important precursor for the manufacture of dyes and pharmaceuticals.
  12. Write the equation for the formation of the acylium electrophile in Friedel-Crafts acylation using ethanoyl chloride and aluminium chloride.
    CH₃COCl + AlCl₃ → CH₃CO⁺ + AlCl₄⁻
  13. Write the equation showing the regeneration of the AlCl₃ catalyst in Friedel-Crafts acylation.
    AlCl₄⁻ + H⁺ → AlCl₃ + HCl
  14. What is the IUPAC name of the organic product when benzene reacts with propanoyl chloride in the presence of AlCl₃?
    1-phenylpropan-1-one
  15. What types of substituents activate the benzene ring towards further electrophilic substitution?
    Electron-donating groups (e.g. −OH, −NH₂, −CH₃), which release electron density into the delocalised pi-system.
  16. Where do activating groups direct incoming electrophiles on the benzene ring?
    To the 2- (ortho) and 4- (para) positions.
  17. Where do deactivating groups direct incoming electrophiles on the benzene ring?
    Most direct to the 3- (meta) position (e.g. −NO₂, −COOH). However, halogens are a key exception: they are deactivating but 2- and 4- (ortho/para) directing.
  18. Give two examples of electron-withdrawing deactivating groups.
    −NO₂ (nitro) and −COOH (carboxyl) or −CHO (aldehyde) groups.
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