Everything from the 2 lessons of this topic on a few pages: the key idea of each lesson, the diagram to remember, the points to learn and the traps examiners set. Read it, cover it, test yourself.
In benzene, C6H6, each carbon is sp2 hybridised: the ring is planar with 120° bond angles, and the six unhybridised p orbitals overlap sideways to form a delocalised π system above and below the ring. All six C–C bonds are the same length (0.139 nm), between C–C and C=C. Delocalisation makes benzene about 152 kJ mol−1 more stable than a ring of three separate double bonds, so it reacts by electrophilic substitution, keeping the ring intact. With a halogen carrier (AlCl3, AlBr3 or FeBr3) it is halogenated; with conc. HNO3 + conc. H2SO4 at 25–60 °C it is nitrated; with RCl or RCOCl and AlCl3 it undergoes Friedel-Crafts alkylation or acylation. An alkyl side chain is oxidised to –COOH by hot alkaline KMnO4 then acid, and is chlorinated by free radicals in UV light; H2 with a Ni or Pt catalyst and heat adds to give cyclohexane.
Benzene does not decolourise bromine water: it needs pure Br2 and a halogen carrier, and the reaction is substitution (HBr is the by-product), not addition. Nitration uses concentrated HNO3 and concentrated H2SO4, kept at or below about 60 °C to avoid further nitration. Chlorine plus UV light attacks the side chain; chlorine plus AlCl3 attacks the ring. Side-chain oxidation turns the whole alkyl group into –COOH, whatever its length, and it needs acid at the end.
In electrophilic substitution a strong electrophile is made first: for nitration, HNO3 + 2H2SO4 → NO2+ + H3O+ + 2HSO4−. A curly arrow from the delocalised π system to the electrophile forms a new C–E bond, giving a positive intermediate (an arenium ion) in which the delocalisation is partly broken: four electrons over five carbons. A curly arrow from the C–H bond back into the ring then removes H+ and restores the aromatic ring. Benzene substitutes rather than adds because addition would destroy the stable delocalised system. Substituents already on the ring direct the next group: –OH, –NH2, –CH3 and –Cl send it to the 2- and 4-positions, while –NO2 and –COOH send it to the 3-position. Electron-donating groups activate the ring; electron-withdrawing groups (and halogens) deactivate it.
The first curly arrow starts from the delocalised ring (the circle), not from a C=C or a C–H bond, and ends at the electrophile's atom. The intermediate's horseshoe must not include the sp3 carbon, and the + goes inside it. The second arrow starts from the C–H bond, and the product is H+, not H2 or H−. In the nitronium equation it is 2H2SO4. Halogens direct to 2 and 4 even though they deactivate the ring; –NO2 and –COOH direct to 3.