Hydrocarbons (A Level)

• Reactions of benzene and methylbenzene: halogenation, nitration, Friedel–Crafts alkylation and acylation, side-chain oxidation, hydrogenation.
• The electrophilic substitution mechanism, shown for nitration and bromination.
• Substitution wins over addition because it keeps the aromatic stabilisation of the delocalised ring.
• Halogen carrier and no light: ring substitution. UV light: side-chain substitution.
• –NH₂, –OH and –R direct to the 2- and 4-positions; –NO₂, –COOH and –COR direct to the 3-position.

Reactions of benzene and methylbenzene

• The products of halogenation are halogenoarenes (aryl halides).
• Methylbenzene reacts in the same way but faster, and gives a mixture of 2- and 4-substituted products, e.g. 2-nitromethylbenzene and 4-nitromethylbenzene.
• Side-chain oxidation turns any alkyl side-chain into –COOH: ethylbenzene also gives benzoic acid. The dilute acid converts the benzoate ion into benzoic acid. The purple KMnO₄ becomes a brown precipitate of MnO₂ in alkaline solution.
• C₆H₅COCH₃ is phenylethanone, a ketone. Friedel–Crafts reactions are the way to add a carbon chain to a benzene ring.

Mechanism: electrophilic substitution

Nitration (forming nitrobenzene)

1. Making the electrophile: HNO₃ + H₂SO₄ → NO₂⁺ + HSO₄⁻ + H₂O. The nitronium ion, NO₂⁺, is the electrophile.
2. Attack: a pair of electrons from the delocalised π system forms a bond to the N of NO₂⁺ (curly arrow from inside the ring to N). This gives a positively charged intermediate in which the delocalised system is partly broken: the ring carbon bonded to NO₂ also carries an H and is sp³.
3. Loss of H⁺: the C–H bond breaks, its electrons return to the ring (curly arrow from the C–H bond into the ring) and the delocalised π system is restored.
4. Catalyst regenerated: H⁺ + HSO₄⁻ → H₂SO₄.

Bromination (forming bromobenzene)

1. Br₂ + AlBr₃ → Br⁺ + AlBr₄⁻. The halogen carrier polarises Br₂ and produces the electrophile Br⁺.
2. Electrons from the π system attack Br⁺, forming the positive intermediate.
3. The C–H bond breaks and the ring is restored: H⁺ + AlBr₄⁻ → HBr + AlBr₃ (catalyst regenerated).

₃ ⁺ _{3 3 3} ⁺ ₄ ⁻ ₃ ⁺

Why substitution and not addition