Hydroxy Compounds (A Level)

Organic Chemistry Weightage: A2 · Paper 4 Topic 32

“At AS you studied alcohols: how they burn, oxidise, dehydrate and form esters. At A Level two new ideas arrive. First, alcohols form esters far more easily with acyl chlorides than with carboxylic acids. Second, phenol, an –OH group on a benzene ring, is not an alcohol at all. It is a stronger acid, and its ring is so electron-rich that it reacts with bromine water in seconds. This topic shows you how phenol is made, why it is acidic, why it reacts so easily, and how to apply the same chemistry to naphthol.”

— SCORECHEM ACADEMIC TEAM

0. What Examiners Want

This guide covers Topic 32 of Cambridge International AS & A Level Chemistry 9701 (2025–27) and follows the learning outcomes below, word for word. The right-hand column shows where each one is taught. It builds on Topic 16 (alcohols), Topic 29 (electrophilic substitution) and Topic 30 (arenes). The acidity of carboxylic acids compared with phenol, and the reactions of acyl chlorides in general, are the subject of Topic 33; the formation of azo dyes from phenylamine is part of Topic 34.

32 Hydroxy compounds. Candidates should be able to:	Section
32.1 Alcohols
1 describe the reaction with acyl chlorides to form esters using ethyl ethanoate	1, 2
32.2 Phenol
1 recall the reactions (reagents and conditions) by which phenol can be produced: (a) reaction of phenylamine with HNO₂ or NaNO₂ and dilute acid below 10 °C to produce the diazonium salt; further warming of the diazonium salt with H₂O to give phenol	3
2 recall the chemistry of phenol, as exemplified by the following reactions: (a) with bases, for example NaOH(aq) to produce sodium phenoxide (b) with Na(s) to produce sodium phenoxide and H₂(g) (c) in NaOH(aq) with diazonium salts, to give azo compounds (d) nitration of the aromatic ring with dilute HNO₃(aq) at room temperature to give a mixture of 2-nitrophenol and 4-nitrophenol (e) bromination of the aromatic ring with Br₂(aq) to form 2,4,6-tribromophenol	5
3 explain the acidity of phenol	4
4 describe and explain the relative acidities of water, phenol and ethanol	4
5 explain why the reagents and conditions for the nitration and bromination of phenol are different from those for benzene	5, 6
6 recall that the hydroxyl group of a phenol directs to the 2-, 4- and 6-positions	6
7 apply knowledge of the reactions of phenol to those of other phenolic compounds, e.g. naphthol	7

How marks are earned in Hydroxy Compounds (A Level)

Reagents and conditions: NaNO₂ + HCl below 10 °C, then warm with water; Br₂(aq); dilute HNO₃; NaOH(aq) and below 10 °C for coupling. The temperature is usually a mark.
Products and observations: 2,4,6-tribromophenol (white precipitate), 2- and 4-nitrophenol, sodium phenoxide, an azo compound, ethyl ethanoate with HCl.
Explanations of acidity: the delocalisation of the phenoxide charge into the ring, and the electron-donating alkyl group in ethoxide.
Explanations of reactivity: the oxygen lone pair delocalised into the ring, which activates it and directs to the 2-, 4- and 6-positions.
Applying the pattern: predicting the reactions of an unfamiliar phenol, such as naphthol, from the phenol reactions.

⚠️ Examiner Trap: Where this topic is tested. Phenol chemistry appears in a Paper 4 organic question, often as a short sequence. In 2024 (9701/41, question 9) phenol was reacted with sodium and with dilute nitric acid, and you had to write the equation for sodium, draw the two isomeric products of nitration, and complete the electrophilic substitution mechanism of the phenoxide ion with a weak electrophile. Know the reagents, the equations and the mechanism steps (curly arrow from the ring, the intermediate, loss of H⁺).

1. Recap: Reactions of Alcohols

An alcohol has an –OH group on a saturated carbon atom. It is primary, secondary or tertiary according to the number of alkyl groups on the carbon that carries the –OH. At AS you met the reactions in the figure below. Ethanol is the example.

Reactions of ethanol: sodium, oxidation, dehydration, substitution, esterification and the new acyl chloride reaction

Alcohol reactions from AS (green) and the new reaction with an acyl chloride (amber).

With sodium, the O–H bond breaks: 2CH₃CH₂OH + 2Na → 2CH₃CH₂ONa + H₂.
Primary alcohols oxidise (acidified K₂Cr₂O₇, heat) to aldehydes (distil off) and then carboxylic acids (reflux); secondary alcohols give ketones; tertiary alcohols do not oxidise.
Heating with concentrated H₂SO₄ at about 170 °C dehydrates an alcohol to an alkene.
Alcohols form halogenoalkanes with HBr, PCl₅ or SOCl₂.
With a carboxylic acid and a strong acid catalyst under reflux, an alcohol forms an ester in a reversible reaction: CH₃COOH + CH₃CH₂OH ⇌ CH₃COOCH₂CH₃ + H₂O. This reaction is slow, and the mixture at equilibrium still contains reactants, so the yield is not high.

Esters are named with the alcohol part first: the alkyl group comes from the alcohol, and the “-oate” part from the acid. Ethanol and ethanoic acid give ethyl ethanoate; ethanol and propanoic acid give ethyl propanoate.

2. Alcohols and Acyl Chlorides

An acyl chloride has the group –COCl. Ethanoyl chloride is CH₃COCl. The carbonyl carbon is bonded to two highly electronegative atoms (O and Cl), so it carries a large δ+ charge and is attacked very readily by nucleophiles, including the lone pair on the oxygen of an alcohol. The reaction forms an ester and hydrogen chloride.

Equation: CH₃COCl + CH₃CH₂OH → CH₃COOCH₂CH₃ + HCl
Conditions: room temperature, no catalyst. The reaction is vigorous and gives steamy fumes of HCl. It goes to completion (one direction only).

Ethanoyl chloride reacts with ethanol to give ethyl ethanoate and hydrogen chloride

The ester contains the CH₃CO group from the acyl chloride and the OCH₂CH₃ group from the alcohol.

Why use an acyl chloride? Compare the two ways of making ethyl ethanoate.

	Carboxylic acid + alcohol	Acyl chloride + alcohol
Reagents	CH₃COOH + CH₃CH₂OH	CH₃COCl + CH₃CH₂OH
Catalyst	concentrated H₂SO₄	none
Conditions	heat under reflux	room temperature
Rate	slow	fast, vigorous
Extent	reversible; equilibrium mixture	irreversible; goes to completion
Other product	H₂O	HCl (a gas)

The mechanism: addition–elimination. The reaction is a nucleophilic substitution of Cl by OR, which happens in two stages (the pattern is introduced in Topic 29).

Mechanism of the reaction of ethanoyl chloride with ethanol: addition then elimination

Stage 1: the oxygen lone pair attacks the δ+ carbon; the C=O π electrons move onto O. Stage 2: the C=O re-forms, Cl⁻ leaves, and H⁺ is lost, giving HCl.

Addition: a lone pair on the alcohol oxygen attacks the δ+ carbonyl carbon. The C=O π bond breaks and its electrons go onto the oxygen. This gives a tetrahedral intermediate with O⁻ and O⁺(H).
Elimination: the lone pair on O⁻ re-forms the C=O bond, and the C–Cl bond breaks to release Cl⁻. The O⁺–H loses H⁺, which combines with Cl⁻ to give HCl.

Worked example. Ethanoyl chloride reacts with ethanol. Name the ester formed, write the equation and state two advantages over using ethanoic acid. [4]

Ethyl ethanoate [1]. CH₃COCl + CH₃CH₂OH → CH₃COOCH₂CH₃ + HCl [1]. The reaction is fast at room temperature with no catalyst [1], and it is not reversible, so the yield is higher [1].

⚠️ Examiner Trap: Keep the acyl chloride dry. Acyl chlorides react with water to form the carboxylic acid and HCl, so the alcohol and the apparatus must be dry. Do not write H₂SO₄ as a catalyst for this reaction, and do not write water as a product: the second product is HCl.

3. Making Phenol

Phenol, C₆H₅OH, has the –OH group bonded directly to a benzene ring. It is made from phenylamine (the amine from Topic 34) in two stages.

Diazotisation. Phenylamine is treated with nitrous acid, HNO₂, below 10 °C. Nitrous acid is unstable, so it is made in the mixture from NaNO₂ and dilute HCl. The product is the benzenediazonium salt, C₆H₅N₂⁺Cl⁻.
Hydrolysis. The diazonium salt solution is warmed with water. Nitrogen gas is given off and phenol forms.

C₆H₅NH₂ + HNO₂ + HCl → C₆H₅N₂⁺Cl⁻ + 2H₂O (below 10 °C)
C₆H₅N₂⁺ + H₂O → C₆H₅OH + N₂ + H⁺ (warm)

Phenylamine to benzenediazonium chloride to phenol

Diazotisation below 10 °C, then warming with water releases nitrogen and gives phenol.

Benzenediazonium salts are unstable and decompose when warm, so the first step must be kept cold. In the second step the same instability is used on purpose: warming replaces the N₂⁺ group by OH.

Worked example. Give the reagents and conditions for the conversion of phenylamine into phenol in two steps, and name the intermediate. [4]

Step 1: NaNO₂ and dilute HCl (or HNO₂) [1] below 10 °C [1]; intermediate: benzenediazonium chloride [1]. Step 2: warm with water [1].

⚠️ Examiner Trap: Both temperatures are marks. Do not write “heat” for step 1. The diazotisation is kept below 10 °C (ice bath), and the hydrolysis is a separate step in which the solution is warmed. If you heat the first step, the diazonium salt decomposes before you have made it into a useful reagent.

4. The Acidity of Phenol

Phenol is a weak acid. In water it dissociates to a small extent:

C₆H₅OH ⇌ C₆H₅O⁻ + H⁺

It is a stronger acid than water and ethanol but a weaker acid than carboxylic acids. The order is:

ethanoic acid > phenol > water > ethanol (approximate pK_a: 4.8, 10, 15.7, 16)

Why is phenol more acidic than ethanol? The strength of an acid depends on how stable its conjugate base is. The more stable the anion, the more readily the acid releases H⁺.

In the phenoxide ion, a lone pair on the oxygen overlaps with the π system of the ring. The negative charge is delocalised over the oxygen and the ring carbons (positions 2, 4 and 6). Spreading the charge makes the ion more stable.
In the ethoxide ion, the ethyl group is electron-donating (positive inductive effect). It pushes electron density towards the oxygen, which already carries the negative charge. The charge is localised on O, and the ion is less stable.
Water is in between: in OH⁻ there is no alkyl group to destabilise the ion, so water is a stronger acid than ethanol. But the charge is not delocalised, so water is a weaker acid than phenol.

Delocalisation of the negative charge in the phenoxide ion compared with the localised charge in the ethoxide ion

Phenoxide: charge shared with the ring. Ethoxide: the electron-donating ethyl group concentrates the charge on oxygen.

The difference shows in simple reactions.

Table of reactions of ethanoic acid, phenol, water and ethanol with sodium, sodium hydroxide and sodium carbonate

All four react with sodium. Only phenol and ethanoic acid react with NaOH(aq). Only ethanoic acid reacts with sodium carbonate.

With sodium: all four give hydrogen. 2C₆H₅OH + 2Na → 2C₆H₅ONa + H₂. (The reaction of ethanol is slower than that of water; phenol reacts less violently than water.)
With NaOH(aq): phenol reacts and ethanol does not. C₆H₅OH + NaOH → C₆H₅ONa + H₂O. Phenol is only slightly soluble in water, but the ionic salt sodium phenoxide dissolves, so a cloudy mixture of phenol and water turns clear when NaOH(aq) is added.
With Na₂CO₃(aq): phenol does not react; it is too weak an acid to release CO₂. A carboxylic acid does.

Worked example. Explain why phenol is a stronger acid than ethanol. [3]

In the phenoxide ion, a lone pair on O is delocalised into the π system of the ring [1], so the negative charge is spread over the ion and it is stabilised [1]. In ethoxide the alkyl group is electron-donating, so the charge stays on O and the ion is less stable [1].

⚠️ Examiner Trap: Say ‘the anion is stabilised’. The explanation is about the stability of the anion, not about the O–H bond in phenol. Always mention delocalisation for phenoxide and the electron-donating alkyl group for ethoxide. Do not say “phenol has a benzene ring so it is acidic”.

5. Reactions of Phenol

The figure shows the five reactions you must recall. The first two use the O–H bond (acid properties); the last three use the ring (electrophilic substitution).

Reactions of phenol with NaOH, Na, sodium carbonate, bromine water, diazonium salts and dilute nitric acid

The acid reactions (green), no reaction with sodium carbonate (grey), and the three ring substitutions.

(a) With bases and (b) with sodium. Phenol forms sodium phenoxide, C₆H₅ONa, with NaOH(aq) (water is the other product) or with sodium metal (hydrogen is the other product).

(c) With diazonium salts: azo coupling. When a benzenediazonium salt is added to a solution of phenol in NaOH(aq) kept below 10 °C, an azo compound forms. The –N=N– group joins the two rings. The product, 4-hydroxyazobenzene, is yellow-orange.

Benzenediazonium chloride couples with phenoxide to give 4-hydroxyazobenzene

The phenol is dissolved in alkali first, because phenoxide is much more reactive than phenol. The diazonium ion attacks the 4-position.

The diazonium ion is a weak electrophile. It reacts only with very electron-rich rings, such as phenoxide (and phenylamine, Topic 34). The –N=N– link extends the delocalised system, so azo compounds absorb visible light and are used as dyes.

(d) Nitration. Phenol reacts with dilute HNO₃ at room temperature to give a mixture of 2-nitrophenol and 4-nitrophenol. C₆H₅OH + HNO₃ → HOC₆H₄NO₂ + H₂O. No concentrated sulfuric acid is needed.

(e) Bromination. Phenol reacts with Br₂(aq) (bromine water) at room temperature with no catalyst. The orange colour of the bromine disappears and a white precipitate of 2,4,6-tribromophenol forms. C₆H₅OH + 3Br₂ → C₆H₂Br₃OH + 3HBr. This is a good test: ethanol gives no reaction with bromine water.

Reaction	Reagents and conditions	Product(s)	Observation
with base	NaOH(aq), room temp.	sodium phenoxide + H₂O	phenol dissolves
with sodium	Na(s)	sodium phenoxide + H₂	bubbles of gas
coupling	diazonium salt, NaOH(aq), below 10 °C	4-hydroxyazobenzene	yellow-orange colour or precipitate
nitration	dilute HNO₃, room temp.	2-nitrophenol + 4-nitrophenol	yellow-orange oil or solid
bromination	Br₂(aq), room temp.	2,4,6-tribromophenol	orange colour lost; white precipitate

⚠️ Examiner Trap: Nitration gives two isomers, not three. The 2- and 6-positions are equivalent, so the products are 2-nitrophenol and 4-nitrophenol. Bromination goes further because the ring stays very reactive: all three activated positions (2, 4 and 6) are substituted, so the single product is 2,4,6-tribromophenol.

6. Why Phenol Is So Reactive

Benzene needs a halogen carrier for bromination, and concentrated nitric and sulfuric acids for nitration. Phenol reacts with bromine water and with dilute nitric acid. The difference lies in the –OH group.

Bromination and nitration of benzene compared with phenol: reagents, conditions and products

Benzene needs harsh conditions and gives one substitution; phenol reacts under mild conditions, and bromination replaces three hydrogens.

A lone pair on the oxygen is delocalised into the π system of the ring. This increases the electron density of the ring, especially at the 2-, 4- and 6-positions. A more electron-rich ring:

attracts and reacts with weaker electrophiles, so no halogen carrier is needed (the ring itself polarises Br₂ so that one Br becomes δ+), and dilute HNO₃ gives enough NO₂⁺;
reacts at room temperature, because the activation energy is lower;
is substituted at all three activated positions with bromine water, giving 2,4,6-tribromophenol.

Delocalisation of the oxygen lone pair into the ring increases electron density at positions 2, 4 and 6

The –OH group is 2,4,6-directing and activating. Benzene has no such group, so it needs harsher reagents.

The hydroxyl group of a phenol directs to the 2-, 4- and 6-positions. The intermediate formed by attack at these positions is stabilised by the O lone pair, so those routes have the lowest activation energy.

⚠️ Examiner Trap: The phenoxide ion is even more reactive. In sodium phenoxide the oxygen carries a full negative charge, so the ring is even more electron-rich than in phenol. It reacts with electrophiles so weak that phenol does not, such as the diazonium ion (azo coupling) or the δ+ carbon of carbon dioxide. When a question gives a weak electrophile, check whether the reaction starts from the phenoxide ion, and draw the curly arrow from the ring π system to the electrophile.

Worked example. Explain why phenol reacts with bromine water without a catalyst but benzene does not. [3]

A lone pair on the oxygen is delocalised into the ring [1], so the electron density of the ring is higher than in benzene [1], and the ring can polarise Br₂ and attack it without a halogen carrier [1].

7. Other Phenolic Compounds: Naphthol

A phenolic compound has an –OH group bonded directly to an aromatic ring. 2-Naphthol has the –OH group on a naphthalene ring (two fused benzene rings). The syllabus asks you to apply your phenol knowledge to such compounds, so predict their reactions by analogy.

2-naphthol coupled with a benzenediazonium salt to give 1-(phenylazo)-2-naphthol

2-Naphthol reacts like phenol. Coupling in NaOH(aq) below 10 °C gives an orange-red azo dye, with the azo group on the carbon next to the –OH.

With NaOH(aq): sodium 2-naphthoxide and water (the –OH is acidic, like phenol).
With Na(s): sodium 2-naphthoxide and hydrogen.
With a diazonium salt in NaOH(aq), below 10 °C: an azo compound, an orange-red dye. The azo group adds at carbon 1, next to the –OH.
With Br₂(aq), or dilute HNO₃: ring substitution under mild conditions, with no catalyst, because the –OH group activates the ring.

Worked example. 2-Naphthol is added to NaOH(aq) and the mixture is cooled below 10 °C; benzenediazonium chloride is then added. State what you would observe and name the type of compound formed. [3]

2-Naphthol dissolves in NaOH(aq) to give the 2-naphthoxide ion [1]. On adding the diazonium salt an orange-red colour (or precipitate) forms [1]. The product is an azo compound (an azo dye) [1].

8. Quick Sheet and Checklist

Topic	What to remember
Alcohol + acyl chloride	CH₃COCl + ROH → ester + HCl; room temp.; no catalyst; goes to completion
Phenol from phenylamine	NaNO₂ + HCl, below 10 °C; then warm with H₂O (N₂ given off)
Acidity	ethanoic acid > phenol > water > ethanol
Phenol + NaOH / Na	sodium phenoxide (+ H₂O / H₂)
Phenol + Br₂(aq)	2,4,6-tribromophenol, white precipitate, room temp., no catalyst
Phenol + dilute HNO₃	2-nitrophenol + 4-nitrophenol, room temp.
Phenol + diazonium	azo compound, NaOH(aq), below 10 °C
Directing effect	–OH: 2-, 4-, 6-directing and activating
Naphthol	reacts like phenol; couples at carbon 1

[ ] I can write the equation and conditions for ethanoyl chloride reacting with an alcohol, and say why it is better than using the acid.
[ ] I can draw the addition–elimination mechanism of an acyl chloride with an alcohol.
[ ] I can give the reagents and temperatures for making phenol from phenylamine.
[ ] I can explain the order of acidity of ethanoic acid, phenol, water and ethanol.
[ ] I can write equations for phenol with NaOH, Na, Br₂(aq) and dilute HNO₃.
[ ] I can describe azo coupling and state the conditions.
[ ] I can explain why phenol reacts under milder conditions than benzene and why it directs to 2, 4 and 6.
[ ] I can predict the reactions of 2-naphthol.