Phenol: Reactions and Acidity

This guide covers subtopics 32.1, Alcohols, and 32.2, Phenol, from Topic 32, Hydroxy compounds, of Cambridge International AS & A Level Chemistry 9701, 2025–2027 series. This is A Level content. 32.1 is a single, narrow outcome; the substance of this resource is 32.2, Phenol.

Before studying this

This resource assumes alcohol reactions from Alcohols: Reactions and Oxidation, the electrophilic substitution mechanism and directing effects from Arenes and Halogenoarenes, and the addition-elimination mechanism from Carboxylic Acids and Acyl Chlorides for the 32.1 reaction below.

Syllabus coverage

CAMBRIDGE INTERNATIONAL AS & A LEVEL CHEMISTRY 9701 — A Level, Topic 32

32.1 Alcohols — describing the reaction of alcohols with acyl chlorides to form esters, exemplified by ethyl ethanoate.

32.2 Phenol — recalling the production of phenol from phenylamine via a diazonium salt; recalling phenol’s reactions with bases, with sodium, and with diazonium salts in NaOH(aq) to give azo compounds; recalling nitration and bromination of the aromatic ring; explaining phenol’s acidity; explaining the relative acidities of water, phenol and ethanol; explaining why phenol’s nitration/bromination conditions differ from benzene’s; recalling that the hydroxyl group directs to the 2-, 4- and 6-positions; applying this to other phenolic compounds, e.g. naphthol.

Alcohols and acyl chlorides

Extending the alcohol reactions already met at AS Level, alcohols react with acyl chlorides at room temperature to form an ester (plus HCl) — this is, in fact, the more common laboratory method for making an ester in practice, since it’s faster and goes essentially to completion, unlike the reversible, slower reaction between an alcohol and a carboxylic acid.

CH₃CH₂OH + CH₃COCl → CH₃COOCH₂CH₃ (ethyl ethanoate) + HCl

Carboxylic Acids and Acyl Chlorides covers the addition-elimination mechanism behind this reaction, and acyl chlorides’ full range of reactions, in detail.

Producing phenol

Phenol is produced from phenylamine via a two-step diazotisation route: phenylamine reacts with HNO₂ (generated in situ from NaNO₂ and dilute acid) below 10 °C to form a diazonium salt, benzenediazonium chloride; further warming this diazonium salt with water then displaces the diazonium group, giving phenol.

C₆H₅NH₂ → [C₆H₅N₂]⁺Cl⁻ (below 10 °C) → C₆H₅OH (on warming with H₂O)

Amines: Aliphatic and Aromatic, Basicity and Azo Dyes covers phenylamine’s production and diazonium chemistry in full.

Reactions of phenol

With bases. Phenol reacts with NaOH(aq) to form sodium phenoxide:

C₆H₅OH + NaOH → C₆H₅ONa + H₂O

With sodium metal. Phenol reacts with Na(s), releasing hydrogen gas — the same reaction pattern as an alcohol with sodium, but faster, consistent with phenol being the more acidic of the two (see below):

2C₆H₅OH + 2Na → 2C₆H₅ONa + H₂

With diazonium salts, in NaOH(aq). Phenol couples with a diazonium salt (benzenediazonium chloride) under alkaline conditions to form an azo compound — see Amines: Aliphatic and Aromatic, Basicity and Azo Dyes for the coupling reaction and the azo group itself.

Nitration. Phenol nitrates far more readily than benzene: dilute HNO₃(aq) at room temperature is enough, giving a mixture of 2-nitrophenol and 4-nitrophenol (matching the –OH group’s 2,4-directing effect).

Bromination. Phenol brominates far more readily than benzene too: Br₂(aq) at room temperature substitutes at all three activated positions at once, giving 2,4,6-tribromophenol directly, without needing a catalyst.

Why phenol reacts faster than benzene

Phenol’s oxygen has a lone pair that can delocalise into the ring’s π system (similar in principle to the –NH₂ directing effect discussed in Arenes and Halogenoarenes), substantially increasing the ring’s electron density. This makes the ring far more attractive to an electrophile, which is why phenol’s nitration and bromination need only mild reagents and conditions (dilute HNO₃, room temperature; aqueous Br₂, room temperature) compared with benzene’s requirement for concentrated HNO₃/H₂SO₄ or an AlBr₃ catalyst — the electrophile is generated far more easily, or needed in a far less reactive form, because the ring itself is so much more nucleophilic.

The hydroxyl group directs to the 2-, 4- and 6-positions, exactly as established for other lone-pair-donating substituents — the delocalised lone pair increases electron density most at those ring positions.

Applying this to other phenolic compounds. Any molecule containing a phenolic –OH directly attached to an aromatic ring (for example naphthol, a phenol fused to a second ring) undergoes the same activated, 2,4,6-type substitution chemistry, for the same underlying reason — the principle transfers directly, even though the specific ring system is bigger.

The acidity of phenol

Phenol is a weak acid — considerably weaker than a carboxylic acid, but significantly stronger than a simple alcohol like ethanol, and stronger than water itself.

Explaining phenol’s acidity. When phenol loses H⁺ from its –OH group, the resulting phenoxide ion, C₆H₅O⁻, has its negative charge partially delocalised into the aromatic ring (the oxygen’s lone pair overlaps with the ring’s π system, spreading the charge over several ring carbons rather than leaving it concentrated entirely on oxygen). This delocalisation stabilises the phenoxide ion relative to an alkoxide ion (like CH₃CH₂O⁻, from ethanol), where no such delocalisation is possible — making phenol’s H⁺ loss more energetically favourable, and phenol correspondingly more acidic.

Relative acidities: water, phenol, ethanol.

Species	Relative acidity	Why
Phenol	most acidic of the three	phenoxide ion stabilised by delocalisation into the ring
Water	intermediate	no delocalisation available, but no electron-donating alkyl group either
Ethanol	least acidic	the ethyl group’s electron-donating (positive inductive) effect increases electron density on oxygen, making the alkoxide ion less stable and H⁺ loss less favourable — and there’s no delocalisation to compensate

This ordering is why phenol reacts with NaOH(aq) (strong enough to deprotonate it) while ethanol does not, and why phenol reacts detectably faster with sodium metal than ethanol does.

Common mistakes

Treating phenol like an alcohol for nitration/bromination conditions. Benzene-like conditions (concentrated acid mixture, or a Friedel-Crafts catalyst) are unnecessary and, for bromination, would simply be the wrong answer — phenol’s activated ring needs only mild, dilute conditions.

Explaining phenol’s acidity by comparing it only to benzene, rather than to water and ethanol specifically. Syllabus outcome 32.2.4 asks you to “describe and explain the relative acidities of water, phenol and ethanol”, so be ready to explain all three positions in the order: why phenol is more acidic than water (the phenoxide ion’s charge is delocalised into the ring, whereas water’s conjugate base, OH⁻, has no comparable stabilisation), and why water is more acidic than ethanol (the ethyl group’s electron-donating effect makes the ethoxide ion less stable than OH⁻).

Forgetting that phenol reacts with NaOH but not with the weaker base NaHCO₃. Phenol is a weaker acid than carbonic acid/carboxylic acids, so unlike a carboxylic acid, it does not release CO₂ from sodium hydrogencarbonate — a common exam distinguishing test between a phenol and a carboxylic acid group.

Quick revision checklist

• Alcohol + acyl chloride → ester + HCl (faster, more complete than alcohol + carboxylic acid)
• Phenol production: phenylamine → diazonium salt (below 10 °C) → phenol (warm with H₂O)
• Phenol + NaOH → sodium phenoxide; + Na → sodium phenoxide + H₂
• Phenol + diazonium salt (in NaOH) → azo compound
• Nitration/bromination of phenol: mild conditions (dilute HNO₃ or aqueous Br₂, room temperature) — far milder than benzene needs
• Acidity order: phenol > water > ethanol; delocalisation into the ring stabilises the phenoxide ion
• Hydroxyl group directs 2-, 4-, 6-; applies to any phenolic compound

Related resources

• Alcohols: Reactions and Oxidation — the AS alcohol chemistry this topic extends
• Arenes and Halogenoarenes: Electrophilic Substitution and Reactivity — the directing-effect reasoning applied here to phenol specifically
• Amines: Aliphatic and Aromatic, Basicity and Azo Dyes — phenylamine and diazonium chemistry, phenol’s production route
• Carboxylic Acids and Acyl Chlorides — the acyl-chloride reaction and mechanism from 32.1
• Cambridge AS & A Level Chemistry hub

Written against Cambridge International AS & A Level Chemistry 9701, 2025–2027 series. Always check the current syllabus for your examination year.