Chapter 9: Aromatic Hydrocarbons

Complete notes for Chapter 9. Covers Benzene structure, Kekule's formula, Resonance, Stability, Preparation, Electrophilic Substitution Reactions, and Orientation (Ortho/Para/Meta directors).

Introduction & Classification

Aromatic Hydrocarbons: Compounds containing benzene ring. Parent member is Benzene (C6H6).

Monocyclic: One benzene ring (e.g., Toluene, Phenol, Aniline).
Polycyclic: Multiple rings. (Isolated: Biphenyl / Fused: Naphthalene, Anthracene).

Structure of Benzene

Kekule's Structure

Proposed cyclic hexagonal structure with alternating double and single bonds.

Modern View & Stability

Benzene is unusually stable. Does not decolorize KMnO4 readily. X-ray analysis shows regular hexagon, bond length 1.397Å (intermediate between single and double).

Resonance

Structure is a hybrid of 5 resonance structures (2 Kekule + 3 Dewar). Resonance Energy = 150.5 kJ/mol. Explains extra stability.

Atomic Orbital Treatment

sp2 hybridization of Carbon. Delocalized electron cloud above and below the ring.

Preparation of Benzene

Dehydrogenation of Cyclohexane: Pt catalyst at high temp.
From Acetylene: Polymerization (Cu tube, 300°C).
From Alkanes: Aromatization of n-Hexane.
Laboratory Methods:
- Decarboxylation of Sodium Benzoate (Soda lime).
- Reduction of Phenol (Zn dust).
- Hydrolysis of Benzenesulphonic Acid.

Reactions of Benzene

Benzene prefers Electrophilic Substitution Reactions over addition due to stability of ring.

Halogenation: In presence of FeX3. NBS mechanism involved.
Nitration: Conc. HNO3 + Conc. H2SO4 (50-60°C). Electrophile NO2+.
Sulphonation: Conc. H2SO4 or Fuming H2SO4 (SO3). Electrophile SO3.
Friedel-Crafts Alkylation: Alkyl halide + AlCl3. Electrophile R+.
Friedel-Crafts Acylation: Acyl halide + AlCl3. Electrophile RCO+.

Other Reactions

Addition: Hydrogenation (Cyclohexane), Chlorination (Hexachlorocyclohexane in sunlight).
Oxidation: Combustion (Smoky flame), Catalytic Oxidation (V2O5 -> Maleic Anhydride), Ozonolysis (Glyoxal).

Orientation in Benzene

Directive effect of substituents on incoming group.

Ortho-Para Directing Groups: Electron donating. Increase reactivity. e.g., -OH, -NH2, -CH3, -Cl, -Br. (Halogens are deactivating but O/P directing).
Meta Directing Groups: Electron withdrawing. Decrease reactivity. e.g., -NO2, -COOH, -CHO, -CN, -SO3H.

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Chapter 9

chemistry • intermediate 12th