Aromatic chemistry (A-level only) revision notes

Aromatic Chemistry Revision Note

Benzene Structure and Stability

Benzene (C₆H₆) is a planar, cyclic molecule with a ring of six carbon atoms. Each carbon atom is bonded to one hydrogen atom and has a p-orbital that contributes to a delocalised π-electron system. This delocalisation results in a lower energy state compared to localized double bonds, making benzene more stable than theoretical cyclohexatriene, which would have alternating single and double bonds.

Electrophilic Substitution Mechanisms

Benzene undergoes electrophilic substitution reactions rather than addition reactions, preserving its aromaticity. Key mechanisms include:

1. Nitration: The introduction of a nitro group (NO₂) via the reaction with nitric acid (HNO₃) in the presence of sulfuric acid (H₂SO₄).
2. Friedel-Crafts Reactions: These involve the introduction of alkyl or acyl groups using alkyl halides or acyl chlorides in the presence of a Lewis acid catalyst (e.g., AlCl₃).

Understanding these mechanisms is crucial for predicting the outcomes of reactions involving aromatic compounds.