Benzene, Electrophilic Substitution and Phenol

Benzene is planar with six equivalent carbon–carbon bonds and a delocalised pi system above and below the ring. Equal bond lengths and a less exothermic hydrogenation enthalpy than predicted for three isolated double bonds support this model. Alternating double-bond drawings are useful representations but do not imply three permanently short and three permanently long bonds. The delocalised system stabilises benzene, so a typical reaction substitutes a hydrogen and restores the pi system rather than permanently adding across it.

Nitration uses concentrated nitric and sulfuric acids to generate NO2+. A ring electron pair attacks the electrophile, forming a positively charged intermediate whose pi delocalisation is temporarily interrupted. Loss of H+ restores the aromatic system and regenerates the acid catalyst through the complete reaction sequence. Friedel–Crafts acylation uses an acyl chloride and a suitable halide catalyst to form an acyl electrophile, then follows the same substitution logic. Label the actual electrophile rather than an entire neutral reagent as positively charged.

In phenol, an oxygen lone pair interacts with the ring, increasing electron density and making electrophilic substitution easier than for benzene. Bromine water can therefore form a white precipitate of 2,4,6-tribromophenol without the catalyst required for bromination of benzene. Phenol is a weak acid because the phenoxide ion is stabilised by delocalisation; it reacts with hydroxide but is too weak to react readily with carbonate in the ordinary test. Do not substitute phenol’s conditions into a benzene reaction.

Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.