Phenol

· Phenol = aromatic compound with –OH directly bonded to a benzene ring.
· Functional group: phenolic –OH; formula of phenol: C₆H₅OH.
· Phenol behaves differently from alcohols because the oxygen lone pair interacts with the delocalised π system of the benzene ring.
· The –OH group activates the benzene ring and directs substitution to the 2-, 4- and 6-positions.

Phenol contains a benzene ring bonded directly to an –OH group. This direct attachment makes phenol more acidic and more reactive in electrophilic substitution than benzene. Source

Production of phenol from phenylamine

· Phenylamine → diazonium salt → phenol.
· Step 1: React phenylamine with HNO₂ or NaNO₂ + dilute acid at below 10°C.
· Product of step 1: benzenediazonium salt.
· Step 2: Warm the diazonium salt with H₂O.
· Product: phenol.
· Exam condition to remember: diazonium salt formation must be cold, below 10°C.

Acidic behaviour of phenol

· Phenol is a weak acid because it can lose H⁺ from the –OH group.
· Equation: C₆H₅OH ⇌ C₆H₅O⁻ + H⁺.
· The conjugate base is the phenoxide ion, C₆H₅O⁻.
· Phenol is acidic because the negative charge on the phenoxide ion is delocalised into the benzene ring.
· This delocalisation stabilises the phenoxide ion, making loss of H⁺ more favourable than in ethanol.

The phenoxide ion is formed when phenol loses a proton from the –OH group. The ion is stabilised by delocalisation of negative charge into the aromatic ring, explaining phenol’s weak acidity. Source

Relative acidities: water, phenol and ethanol

· Acid strength order: phenol > water > ethanol.
· Phenol is more acidic than water and ethanol because phenoxide ion is resonance-stabilised.
· Ethanol is the least acidic because the ethoxide ion has no delocalisation; its negative charge remains localised on oxygen.
· Water is less acidic than phenol because OH⁻ is not resonance-stabilised.
· Key exam phrase: phenoxide ion is stabilised by delocalisation of the negative charge over the benzene ring.

Reactions showing phenol as an acid

· With NaOH(aq): phenol forms sodium phenoxide.
· Equation: C₆H₅OH + NaOH → C₆H₅ONa + H₂O.
· This shows phenol is acidic enough to react with a strong base.
· With Na(s): phenol forms sodium phenoxide and hydrogen gas.
· Equation: 2C₆H₅OH + 2Na → 2C₆H₅ONa + H₂.
· Observation with Na: effervescence due to H₂(g).

Azo coupling with diazonium salts

· Phenol reacts with diazonium salts in NaOH(aq) to form azo compounds.
· Phenol is first converted to phenoxide ion, which is more reactive towards electrophilic substitution.
· Product contains the azo group, –N=N–.
· Azo compounds are often coloured dyes because of their extended delocalised electron systems.
· Exam condition: coupling occurs in alkaline solution, usually NaOH(aq).

Azo coupling links two aromatic rings through the –N=N– azo group. Phenol reacts as the activated aromatic component under alkaline conditions, forming a coloured azo compound. Source

Nitration of phenol

· Reagent: dilute HNO₃(aq).
· Conditions: room temperature.
· Products: mixture of 2-nitrophenol and 4-nitrophenol.
· The –OH group activates the ring, so phenol reacts under milder conditions than benzene.
· The –OH group directs substitution to 2- and 4-positions.
· Compare with benzene: benzene nitration requires concentrated HNO₃ + concentrated H₂SO₄, usually with heating.

Bromination of phenol

· Reagent: Br₂(aq) / bromine water.
· Conditions: room temperature.
· Product: 2,4,6-tribromophenol.
· Observation: orange/brown bromine water decolourises and a white precipitate forms.
· Phenol undergoes multiple substitution because the –OH group strongly activates the aromatic ring.
· Compare with benzene: benzene bromination requires Br₂ + AlBr₃ catalyst.

Phenol is much more reactive than benzene in electrophilic substitution. The –OH group activates the ring and directs incoming groups mainly to the 2-, 4- and 6-positions. Source

Why phenol reacts more easily than benzene

· The oxygen lone pair on the –OH group overlaps with the benzene π system.
· This increases electron density in the aromatic ring, especially at the 2-, 4- and 6-positions.
· Phenol therefore attracts electrophiles more strongly than benzene.
· Nitration uses dilute HNO₃ at room temperature, not concentrated acid mixture.
· Bromination uses Br₂(aq) at room temperature, not Br₂ + halogen carrier catalyst.
· Key exam explanation: phenol is more reactive because the –OH group is electron-donating and activating.

Directing effect of the –OH group

· The –OH group directs electrophilic substitution to the 2-, 4- and 6-positions.
· For nitration: mainly 2-nitrophenol and 4-nitrophenol.
· For bromination: substitution occurs at 2, 4 and 6, giving 2,4,6-tribromophenol.
· This directing effect applies to other phenolic compounds, such as naphthol.

Applying phenol reactions to other phenolic compounds

· Other phenolic compounds contain an –OH group directly attached to an aromatic ring.
· They show similar weak acidity due to stabilisation of the conjugate base.
· They undergo electrophilic substitution more easily than benzene.
· The –OH group directs substitution to 2-, 4- and 6-positions where available.
· Example named in syllabus: naphthol.