Unit 22: Hydroxy Compounds — Long Questions

In phenol, the oxygen of the –OH group donates one of its lone pairs into the aromatic π system by delocalisation, which increases the electron density of the ring. This activates the ring strongly towards electrophilic attack, so phenol reacts with much weaker electrophilic reagents and under much milder conditions than benzene, which lacks this activating group.

Nitration phenol reacts with dilute HNO₃ at room temperature to give a mixture of 2-nitrophenol and 4-nitrophenol (concentrated HNO₃ at room temperature gives 2,4,6-trinitrophenol); benzene, by contrast, needs a mixture of concentrated HNO₃ and concentrated H₂SO₄ refluxed at 25–60°C.

Bromination phenol decolourises bromine water at room temperature without any catalyst, giving a white precipitate of 2,4,6-tribromophenol; benzene only brominates when treated with pure (anhydrous) liquid bromine in the presence of an AlBr₃ catalyst at room temperature.

The conversion is carried out in three steps.
Step 1: Nitrous acid (HNO₂) is generated in situ by reacting sodium nitrite (NaNO₂) with dilute hydrochloric acid (HCl), keeping the temperature below 10°C using an ice bath (HNO₂ is too unstable to be stored/isolated).
Step 2: Phenylamine is added to this cold solution and reacts with the HNO₂ to form an unstable diazonium salt, benzenediazonium chloride: C₆H₅NH₂ + HNO₂ + HCl → C₆H₅N₂⁺Cl⁻ + 2H₂O (below 10°C).
Step 3: On further warming, the diazonium salt is hydrolysed (decomposes), releasing nitrogen gas and giving phenol: C₆H₅N₂⁺Cl⁻ + H₂O --warm--> C₆H₅OH + N₂ + HCl.

Ethanol vs water the ethyl group in ethanol is electron-donating (positive inductive effect), pushing electron density onto the oxygen of the ethoxide ion formed when ethanol loses a proton. This extra electron density destabilises the ethoxide ion (makes it more reactive/basic), so it is more likely to re-accept H⁺ and re-form ethanol; the dissociation equilibrium lies further to the left than for water, so ethanol is a weaker acid than water.

Water vs phenol when phenol loses a proton, the resulting phenoxide ion is stabilised because one of the oxygen's lone pairs delocalises into the aromatic ring, spreading the negative charge over the ring rather than leaving it concentrated on one oxygen atom (as in hydroxide ion from water). This delocalisation makes the phenoxide ion more stable (less likely to reform phenol) than the hydroxide ion, so phenol loses its proton more readily than water does, making phenol more acidic than water.

Overall order of acidity phenol > water > ethanol.