32.2 Phenol

Phenylamine is converted to a diazonium salt with nitrous acid (or sodium nitrite plus dilute acid) below 10 °C. Warming the diazonium salt with water replaces the diazonium group by –OH to give phenol.

The low temperature stabilises the diazonium salt during preparation; warming is used for the subsequent hydrolysis step. Treat the two stages as separate conditions.

C₆H₅NH₂ → diazonium salt below 10 °C, then aqueous warming → C₆H₅OH. Nitrogen gas is released when the diazonium group leaves.

Do not warm the phenylamine/nitrite mixture during diazotisation or skip the diazonium intermediate in the syllabus route.

Phenol combines the chemistry of an aromatic ring with an –OH group. The hydroxyl group can be deprotonated, while its lone pair also activates the ring towards electrophilic substitution.

Use the reagent to identify which part reacts: bases remove the phenolic proton; bromine or nitric acid substitutes on the ring under phenol-specific conditions.

Phenol reacts with aqueous sodium hydroxide to form sodium phenoxide, and with bromine water to give a 2,4,6-tribrominated product.

Phenol is not simply an alcohol attached to benzene: its acidity and ring reactivity are both altered by conjugation.

Phenol can donate H⁺ to form a phenoxide ion. The negative charge in phenoxide is delocalised into the aromatic ring, making the conjugate base more stable than an alkoxide.

Greater conjugate-base stability makes proton loss more favourable, but phenol remains a weak acid: its aqueous equilibrium is far from complete.

Phenol reacts with NaOH to form sodium phenoxide, but it does not release CO₂ from aqueous sodium carbonate as a carboxylic acid does.

The oxygen–hydrogen bond is not acidic simply because oxygen is electronegative; the key comparison is the stability and delocalisation of the conjugate base.

The relative acidity is phenol > water > ethanol. Phenoxide is stabilised by delocalisation, whereas hydroxide and ethoxide keep the negative charge mainly on oxygen; the ethyl group also pushes electron density towards oxygen.

Use conjugate-base stability rather than the number of hydrogens to compare acidity. All three are weak acids in water, but their equilibria differ.

Phenol reacts with aqueous hydroxide, while ethanol does not react appreciably with aqueous hydroxide because hydroxide is not strong enough to remove its proton under those conditions.

Phenol being “weak” does not make it less acidic than ethanol; weak acids can still have a clear relative order.

The –OH group donates electron density into the ring, activating phenol towards electrophilic substitution. Therefore phenol brominates readily with bromine water and nitrates under milder conditions than benzene.

Activation changes both rate and product distribution. It does not mean the ring has lost aromaticity; the substitution mechanism still restores it.

Phenol reacts with bromine water at room temperature to give 2,4,6-tribromophenol, whereas benzene normally requires a Lewis-acid catalyst for bromination.

Do not apply benzene’s catalyst and temperature requirements automatically to phenol; the hydroxyl substituent changes the ring’s electron density.

In phenol, the –OH group is an activating, 2/4-directing substituent. Electrophiles therefore enter mainly at the two ortho positions (2 and 6) and the para position (4).

The two ortho positions are equivalent in an unsubstituted ring, so products are often described as 2-, 4- or 2,4,6-substituted phenols.

Bromination of phenol gives 2,4,6-tribromophenol because all three favoured positions are available and the ring is strongly activated.

The rule predicts preferred positions, not that every phenol reaction must give all three products; reagent amount and steric effects still matter.

A phenolic compound has an –OH group directly attached to an aromatic ring. Its reactions can be predicted by combining phenol’s two features: an acidic O–H proton and an activated, 2/4-directing ring.

Other substituents on the ring can alter the rate, acidity and available positions, so transfer the pattern only after checking their electronic effects and steric crowding.

Naphthol can form a phenoxide with base and undergo electrophilic substitution at positions made electron-rich by the hydroxyl group, but the fused ring geometry changes which sites are available.

Do not treat every aromatic –OH compound as identical to phenol; ring fusion and additional substituents can change regioselectivity.