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Aromatic Hydrocarbons

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Estimated time: 8 minutes

Key Points: Aromatic Hydrocarbons

Cyclic, planar hydrocarbons with delocalised π electrons. 

Benzenoids contain benzene ring; Non-benzenoids are aromatic without benzene ring.

Structure of Benzene:

  • All 6 C atoms are sp² hybridised
  • Unhybridised p-orbitals overlap laterally → delocalised π bonds
  • Bond length = 139 pm (uniform, due to resonance)

Huckel's Rule

  1. Cyclic and planar
  2. Each ring atom has a p-orbital
  3. Contains (4n + 2) π electrons (n = 0, 1, 2…)

Examples: Benzene, Naphthalene, Pyridine.

Physical Properties:

  • Colourless liquid, sweet smell
  • BP = 353 K, MP = 278.5 K
  • Immiscible with water; burns with sooty flame

Preparation:

 

Electrophilic Substitution:

 
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