Phenol is made from phenylamine through a diazonium salt. Its O lone pair overlaps with the ring, which makes phenol a weak acid and makes the ring so reactive that bromine water reacts with it at room temperature.
Phenol, C6H5OH, is made by diazotising phenylamine with NaNO2 and HCl below 10 °C, then warming the diazonium salt with water, which releases N2. A lone pair on the O overlaps with the delocalised ring. This spreads the charge of the phenoxide ion into the ring, so phenol (pKa 10.0) is a weak acid: it reacts with Na and NaOH but is too weak to release CO2 from carbonates. The same overlap raises the electron density of the ring, activating it at the 2-, 4- and 6-positions: phenol decolourises bromine water at room temperature to give a white precipitate of 2,4,6-tribromophenol, is nitrated by dilute HNO3, and couples with diazonium salts to give azo dyes. Phenol forms esters with acyl chlorides, not with carboxylic acids.
Compare the acids in the simulator, then use the table to explain where phenol sits and why it fizzes with sodium but not with carbonate.
| Step | Reagents and conditions | Equation |
|---|---|---|
| 1. diazotisation | NaNO2 + HCl(aq), below 10 °C | C6H5NH2 + HNO2 + HCl → C6H5N2+Cl− + 2H2O |
| 2. hydrolysis | warm with water (above 10 °C) | C6H5N2+ + H2O → C6H5OH + N2 + H+ |
| Reagent | Phenol | Ethanol | Ethanoic acid |
|---|---|---|---|
| Na | H2 given off | H2 given off | H2 given off |
| NaOH(aq) | dissolves: C6H5OH + NaOH → C6H5ONa + H2O | no reaction | neutralised |
| Na2CO3(aq) | no CO2 | no CO2 | fizzes, CO2 |
Four quick checks. Each correct answer earns XP and records this skill on your profile.
What is observed when bromine water is added to aqueous phenol at room temperature?
Which reagent reacts with phenol but NOT with ethanol?
What conditions convert phenylamine into a diazonium salt?
Why does phenol react with bromine more readily than benzene does?
Compare each reaction of phenol with the matching reaction of benzene. Every difference comes from the O lone pair feeding electron density into the ring.
| Reaction | Phenol | Benzene |
|---|---|---|
| bromination | Br2(aq), room temperature, no catalyst; orange to colourless, white precipitate: C6H5OH + 3Br2 → C6H2Br3OH + 3HBr | Br2 with FeBr3 halogen carrier; one Br substituted |
| nitration | dilute HNO3, room temperature: 2-nitrophenol + 4-nitrophenol | conc. HNO3 + conc. H2SO4, 55 °C |
| azo coupling | C6H5N2+ in alkaline solution, below 10 °C: coloured azo dye, joined at the 4-position | no reaction |
| ester | CH3COCl, room temperature: phenyl ethanoate + HCl (no reaction with CH3COOH) | not applicable |
Phenol does not react with carbonates: it is too weak an acid to release CO2, which is how it is told apart from a carboxylic acid. Bromine water gives 2,4,6-tribromophenol (three Br, 3HBr) with no halogen carrier. Nitration of phenol uses dilute HNO3 and gives a mixture of 2- and 4-nitrophenol. Esters of phenol are made with an acyl chloride, never by heating with a carboxylic acid.
Unlocks once the four checks above are done. Worth more XP, written to A Level Paper 4 standard.
0.940 g of phenol (Mr 94.0) reacts completely with excess bromine water. What mass of 2,4,6-tribromophenol (Mr 330.7) forms?
Which test distinguishes phenol from ethanoic acid?
Which compound is the ester formed when phenol reacts with ethanoyl chloride?
Why is phenol a stronger acid than ethanol but weaker than ethanoic acid?
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