Electrophilic Substitution Reactions

Ever wondered why benzene doesn't just add groups like alkenes do? Electrophilic substitution happens because adding groups would destroy benzene's stable π electron system, so substitution (swapping hydrogen for something else) is much more favourable.

The basic mechanism is straightforward: an electrophile (E⁺) attacks the benzene ring, forming an unstable intermediate where a hydrogen temporarily disrupts the aromatic system. Then H⁺ is lost, restoring the stable benzene structure.

Nitration uses concentrated HNO₃ and concentrated H₂SO₄ as a catalyst. The acids react together to generate NO₂⁺ (the electrophile), which attacks benzene to form nitrobenzene. Keep the reaction cool to prevent multiple substitutions happening at once.

Halogenation needs a halogen carrier like FeCl₃ or AlBr₃ because halogens alone aren't electrophilic enough. The carrier polarises Br₂ to create Br⁺, which can then attack the benzene ring.

💡 Remember: These reactions substitute a hydrogen atom rather than adding across a double bond - that's what keeps the aromatic ring stable!

Friedel-Crafts reactions form new C-C bonds to aromatic rings and come in two types. Alkylation adds alkyl chains using haloalkanes and halogen carriers under reflux. Acylation adds acyl groups (C=O chains) using acid chlorides, halogen carriers, and reflux conditions.