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Question

Why is the heat of hydrogenation of benzene less than expected?


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Solution

Resonance

  • Resonance is a way to represent delocalized electrons within molecules or polyatomic ions where the bonding cannot be represented using a single Lewis formula.
  • Several resonance structures represent a molecule or ion with such delocalized electrons.
  • For example resonance structure of benzene:

Heat of hydrogenation of benzene

  • Heat of hydrogenation is the amount of energy released in any unsaturated system when one mole of hydrogen is added.
  • Benzene has three double bonds, the heat of hydrogenation should be -360kJmol-1.
  • Its observed hydrogenation heat is low -208kJmol-1.
  • Heat of hydrogenation is reduced by -152kJmol-1, benzene is more stable than predicted.
  • Benzene is stabilized by resonance.
  • Thus, the heat of hydrogenation of benzene is lower than predicted.

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