1. Aromatic Chemistry
Benzene refuses to behave like an alkene: instead of adding across its double bonds, it substitutes — because breaking its aromatic ring costs about 150 kJ/mol of resonance stabilization. That stability is Hückel's rule: a flat, cyclic, conjugated system with $4n + 2$ π electrons.
Electrophilic Aromatic Substitution (EAS)
An electrophile attacks the ring, forming a resonance-stabilized arenium ion; a base then removes the ring hydrogen, restoring aromaticity. Nitration (HNO3/H2SO4), halogenation (Br2/FeBr3), sulfonation (SO3/H2SO4), and Friedel–Crafts alkylation/acylation all follow this pattern.
Example: Friedel–Crafts acylation: $$\text{Benzene} + \text{CH}_3\text{COCl} \xrightarrow{\text{AlCl}_3} \text{acetophenone} + \text{HCl}$$
Directing Effects
- Activators (–OH, –NH2, –CH3, halogens) direct ortho/para.
- Deactivators (–NO2, –COOH, –SO3H) direct meta.
Example: Nitration of toluene gives mainly ortho- and para-nitrotoluene (the methyl group is an activator); nitration of nitrobenzene gives meta-dinitrobenzene.
Practice Problems
- What are the major products of nitrating phenol?
- Does the –NO2 group direct ortho/para or meta? Is it activating or deactivating?
- Why doesn't benzene undergo addition reactions like an alkene?
- What reagents are used for sulfonation of benzene?
Answers: 1. ortho- and para-nitrophenol 2. Meta; strongly deactivating 3. Addition would destroy aromaticity, costing ~150 kJ/mol of resonance energy 4. Concentrated H2SO4 (SO3/H2SO4)