Chapter 9 / 9.5 Aromatic Hydrocarbons

Aromatic Hydrocarbons (Arenes)

These compounds are called “aromatic” because many have pleasant smells 🌸. Most contain a benzene ring – a special ring with unique stability despite being unsaturated.

Key Features of Benzene

• Molecular formula: \( \ce{C6H6} \)
• All 6 carbon and 6 hydrogen atoms are identical ⚛️
• Forms only one monosubstituted product
• Disubstituted products have three isomers: ortho (1,2), meta (1,3), para (1,4)

Structure of Benzene

Kekulé proposed a cyclic structure with alternating single/double bonds, but experiments showed:

• All C-C bonds are equal length (139 pm) – between single (154 pm) and double bonds (133 pm)
• Explained by resonance and electron delocalization:
  \[ \text{Resonance hybrid} = \text{Structure A} \leftrightarrow \text{Structure B} \]
• Each carbon is \( sp^2 \)-hybridized with delocalized π-electrons above/below the ring 🍩

Aromaticity Rules (Hückel’s Rule)

A compound is aromatic if it has:

1. Planar ring structure
2. Complete π-electron delocalization
3. \((4n + 2)\) π-electrons (\( n = 0,1,2,…\)) e.g.,
   • Benzene (\( n=1 \), 6π-electrons)
   • Naphthalene (\( n=2 \), 10π-electrons)

Preparing Benzene

1. Cyclic polymerization of ethyne: \[ 3\ce{CH#CH} \xrightarrow[\text{873 K}]{\text{Red hot Fe tube}} \ce{C6H6} \]
2. Decarboxylation: \[ \ce{C6H5COONa} \xrightarrow{\ce{NaOH/CaO}} \ce{C6H6} + \ce{Na2CO3} \]
3. Reduction of phenol: \[ \ce{C6H5OH} \xrightarrow{\text{Zn dust}} \ce{C6H6} + \ce{ZnO} \]

Properties of Aromatic Hydrocarbons

Physical: Colorless liquids/solids 💧, characteristic odor, immiscible with water, burn with sooty flame 🔥.

Chemical:

1. Electrophilic Substitution (main reactions):

   Reaction	Reagents/Conditions	Product
   Nitration	Conc. HNO₃ + H₂SO₄, 323-333 K	Nitrobenzene
   Halogenation	X₂ + Anhyd. AlCl₃/FeCl₃	Haloarene
   Sulfonation	Fuming H₂SO₄, Δ	Benzenesulfonic acid
   Friedel-Crafts Alkylation	RCl + Anhyd. AlCl₃	Alkylbenzene
   Friedel-Crafts Acylation	RCOCl + Anhyd. AlCl₃	Acylbenzene

2. Addition Reactions (under forcing conditions):
   • Hydrogenation: \[ \ce{C6H6} + 3\ce{H2} \xrightarrow[\Delta]{\text{Ni}} \ce{C6H12} \]
   • With Cl₂ (UV light): \[ \ce{C6H6} + 3\ce{Cl2} \xrightarrow{\text{UV}} \ce{C6H6Cl6} \] (BHC/Gammaxane)
3. Combustion: \[ \ce{C6H6} + \frac{15}{2}\ce{O2} -> 6\ce{CO2} + 3\ce{H2O} \] (sooty flame)

Directive Influence of Substituents

Existing groups guide where new substitutions occur:

• Ortho/Para Directors (activate ring): -OH, -NH₂, -OCH₃, -CH₃, halogens (-Cl, -Br) ✨
  Reason: Increase electron density at o/p positions via resonance.
• Meta Directors (deactivate ring): -NO₂, -CN, -COOH, -CHO, -SO₃H ⚠️
  Reason: Withdraw electrons, making meta position less electron-poor.

Carcinogenicity Warning ⚠️

Benzene and fused-ring polycyclic aromatics (e.g., benzpyrene) are toxic and carcinogenic. They form during incomplete combustion of coal/tobacco and can damage DNA.

High-Yield NEET Concepts 🔥

1. Benzene Structure & Resonance: Equal bond lengths, stability, delocalization.
2. Electrophilic Substitution: Mechanism (carbocation intermediate), key reactions (nitration, Friedel-Crafts).
3. Hückel’s Rule: (4n+2) π-electrons for aromaticity.
4. Directive Influence: Ortho/para vs meta directing groups with examples.
5. Benzene Preparation: Decarboxylation & ethyne polymerization.