Unit 15: Aromatic Hydrocarbons
Class 11 Chemistry - Nepal Curriculum
Teaching Hours: 6
1. Introduction and Characteristics of Aromatic Compounds
Aromatic compounds are cyclic organic molecules that contain delocalized π-electrons in a conjugated ring system. They exhibit a special type of stability known as aromaticity.
- Planar, cyclic structures with conjugated double bonds
- Follow Hückel’s rule: (4n + 2) π-electrons
- Show resonance, resulting in high stability
- Prefer substitution reactions over addition reactions
2. Hückel’s Rule of Aromaticity
According to Hückel’s rule, a molecule is aromatic if it:
- Is cyclic and planar
- Has a fully conjugated π-system
- Contains (4n + 2) π-electrons, where n is a whole number
Example: Benzene has 6 π-electrons (n = 1), making it aromatic.
Fig: Benzene follows Hückel’s Rule (4n + 2 π-electrons)
3. Kekulé Structure of Benzene
Benzene (C₆H₆) was first described by Kekulé as a six-membered ring with alternating single and double bonds. However, all bond lengths in benzene are equal due to electron delocalization.
Fig: Resonating Kekulé Structures of Benzene
4. Resonance and Isomerism
Benzene exhibits resonance where electrons are delocalized across the ring. Disubstituted benzene compounds show:
- Ortho- (1,2- positions)
- Meta- (1,3- positions)
- Para- (1,4- positions)
Fig: Ortho, Meta, and Para Isomers of Disubstituted Benzene
5. Preparation of Benzene
- From sodium benzoate:
C₆H₅COONa + NaOH → C₆H₆ + Na₂CO₃ - From phenol:
C₆H₅OH + Zn → C₆H₆ + ZnO - From ethyne (acetylene):
3CH≡CH → C₆H₆
Fig: Trimerization of Ethyne to Benzene
6. Physical Properties of Benzene
- Colorless, aromatic liquid with pleasant smell
- Immiscible in water but soluble in organic solvents
- Less dense than water
- Highly flammable with a sooty flame
7. Chemical Properties of Benzene
A. Addition Reactions
- Hydrogenation:
C₆H₆ + 3H₂ → C₆H₁₂(Ni catalyst) - Halogenation:
C₆H₆ + Cl₂ → C₆H₆Cl₆(sunlight)
B. Electrophilic Substitution Reactions
- Nitration:
C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O - Sulphonation:
C₆H₆ + H₂SO₄ → C₆H₅SO₃H + H₂O - Halogenation:
C₆H₆ + Cl₂ → C₆H₅Cl + HCl - Friedel-Crafts Alkylation:
C₆H₆ + RCl → C₆H₅R + HCl - Friedel-Crafts Acylation:
C₆H₆ + RCOCl → C₆H₅COR + HCl
Fig: Electrophilic Substitution Mechanism in Benzene
C. Combustion
2C₆H₆ + 15O₂ → 12CO₂ + 6H₂O
Fig: Free Combustion of Benzene Producing Sooty Flame
8. Uses of Benzene
- Used as a solvent in industries
- Used in the manufacture of dyes, detergents, plastics, and drugs
- Raw material for styrene and other aromatics