Synthetic Routes - Aromatic Compounds

Aromatic compounds contain a benzene ring. Benzene's delocalised ring is very stable, so it reacts by electrophilic substitution, which keeps the ring intact. For AQA, two reactions of benzene start almost every aromatic route.1. Nitration: concentrated HNO₃ and concentrated H₂SO₄, at about 50 °C. The acids form the nitronium ion, NO₂⁺, which substitutes for an H on the ring to give nitrobenzene, C₆H₅NO₂. Keeping the temperature low limits further nitration.2. Friedel-Crafts acylation: an acyl chloride with an AlCl₃ catalyst, heated under reflux. The electrophile RCO⁺ substitutes onto the ring to give a phenyl ketone, C₆H₅COR. This is how a new C-C bond is made to the ring.The new groups can then be converted further:
• nitrobenzene + tin and concentrated HCl, then NaOH → phenylamine, C₆H₅NH₂ (a reduction)
• phenylamine + acyl chloride or acid anhydride → N-substituted amide
• phenyl ketone + NaBH₄ → secondary alcohol, C₆H₅CH(OH)R
• phenyl ketone + KCN, then dilute acid → hydroxynitrileWorked example. Plan a two-step synthesis of 1-phenylethanol, C₆H₅CH(OH)CH₃, from benzene. The CH(OH)CH₃ group comes from reducing the ketone C₆H₅COCH₃. Step 1: benzene + ethanoyl chloride with AlCl₃ catalyst → phenylethanone. Step 2: NaBH₄ → 1-phenylethanol.At A level you never build a benzene ring, so an aromatic target needs an aromatic starting material.