CIE AS/A Level Chemistry 32.2 Phenol Study Notes- 2025-2027 Syllabus
CIE AS/A Level Chemistry 32.2 Phenol Study Notes – New Syllabus
CIE AS/A Level Chemistry 32.2 Phenol Study Notes at IITian Academy focus on specific topic and type of questions asked in actual exam. Study Notes focus on AS/A Level Chemistry latest syllabus with Candidates should be able to:
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 phenolrecall 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-tribromophenolexplain the acidity of phenol
describe and explain the relative acidities of water, phenol and ethanol
explain why the reagents and conditions for the nitration and bromination of phenol are different
from those for benzenerecall that the hydroxyl group of a phenol directs to the 2-, 4- and 6-positions
apply knowledge of the reactions of phenol to those of other phenolic compounds, e.g. naphthol
Preparation of Phenol from Phenylamine
Phenol can be prepared from phenylamine via a two-step process involving diazotisation followed by hydrolysis. You must be able to recall the reagents and conditions for each step.
Step 1: Formation of the Diazonium Salt (Diazotisation)
Phenylamine reacts with nitrous acid at low temperature to form a diazonium salt.
Nitrous acid is generated in situ from sodium nitrite and a dilute acid (usually dilute HCl).
Conditions:
- \( \mathrm{NaNO_2} \) and dilute \( \mathrm{HCl} \)
- temperature below 10°C
Equation:
\( \mathrm{C_6H_5NH_2 + HNO_2 + HCl \rightarrow C_6H_5N_2^+Cl^- + 2H_2O} \)
The low temperature is essential because the diazonium salt is unstable above 10°C.
Step 2: Formation of Phenol
On warming, the diazonium salt reacts with water to form phenol.
Conditions:
- warm with \( \mathrm{H_2O} \)
Equation:
\( \mathrm{C_6H_5N_2^+Cl^- + H_2O \rightarrow C_6H_5OH + N_2 + HCl} \)
Nitrogen gas is released, providing a strong driving force for the reaction.
Key Exam Points
- diazotisation must be carried out below 10°C
- diazonium salts are intermediates
- warming with water converts the diazonium salt into phenol
Example
State the reagents and conditions needed to convert phenylamine into a diazonium salt.
▶️ Answer / Explanation
Sodium nitrite and dilute hydrochloric acid at a temperature below 10°C.
Example
Describe how phenylamine can be converted into phenol, including reagents and conditions.
▶️ Answer / Explanation
Phenylamine is first reacted with sodium nitrite and dilute hydrochloric acid below 10°C to form a diazonium salt.
The diazonium salt is then warmed with water, causing it to decompose to form phenol, nitrogen gas and hydrochloric acid.
Chemistry of Phenol
Phenol shows distinctive chemistry due to the presence of the –OH group directly attached to a benzene ring. The lone pair on oxygen interacts with the aromatic π system, making the ring more reactive towards electrophilic substitution and giving phenol weakly acidic properties.
(a) Reaction with Bases (e.g. NaOH(aq))
Phenol is a weak acid and reacts with aqueous sodium hydroxide.
\( \mathrm{C_6H_5OH + NaOH \rightarrow C_6H_5ONa + H_2O} \)
The product is sodium phenoxide. Phenol is acidic enough to react with NaOH, but not strong enough to react with sodium carbonate.
(b) Reaction with Sodium Metal
Phenol reacts with sodium metal in an acid–metal reaction.
\( \mathrm{2C_6H_5OH + 2Na \rightarrow 2C_6H_5ONa + H_2} \)
Hydrogen gas is released and sodium phenoxide is formed. This reaction is slower than with aliphatic alcohols.
(c) Coupling with Diazonium Salts
Phenol reacts with diazonium salts in alkaline conditions to form azo compounds.
Conditions:
- cold diazonium salt solution
- aqueous NaOH
This is an electrophilic substitution reaction, producing a brightly coloured azo dye, often used as a pH indicator or dye.
(d) Nitration with Dilute HNO₃(aq)
Phenol undergoes nitration with dilute nitric acid at room temperature, without the need for a catalyst.
Products:
- 2-nitrophenol (ortho)
- 4-nitrophenol (para)
The –OH group is an activating, ortho/para-directing substituent, increasing electron density in the ring.
(e) Bromination with Br₂(aq)
Phenol reacts rapidly with bromine water at room temperature.
\( \mathrm{C_6H_5OH + 3Br_2 \rightarrow C_6H_2Br_3OH + 3HBr} \)
The product is 2,4,6-tribromophenol, which forms as a white precipitate. The reaction does not require a halogen carrier catalyst.
Why Phenol Is So Reactive
The lone pair on the oxygen atom overlaps with the delocalised π system of the benzene ring.
This increases electron density at the ortho and para positions, making electrophilic substitution faster and easier than in benzene.
Example
Explain why phenol reacts with aqueous sodium hydroxide but ethanol does not.
▶️ Answer / Explanation
Phenol is a weak acid because the phenoxide ion is stabilised by delocalisation into the benzene ring.
Ethanol does not form a stabilised alkoxide ion, so it is not acidic enough to react with NaOH.
Example
Describe the reaction of phenol with bromine water and explain why multiple substitution occurs.
▶️ Answer / Explanation
Phenol reacts rapidly with bromine water to form 2,4,6-tribromophenol as a white precipitate.
The –OH group donates electron density into the ring, strongly activating it.
As a result, repeated electrophilic substitution occurs at the ortho and para positions.
Acidity of Phenol
Phenol is a weak acid. It is more acidic than aliphatic alcohols, but much less acidic than carboxylic acids. You must be able to explain the acidity of phenol in terms of stability of the conjugate base.
Acid–Base Behaviour of Phenol
Phenol can donate a proton from the O–H group:
\( \mathrm{C_6H_5OH \rightleftharpoons C_6H_5O^- + H^+} \)
The equilibrium lies only slightly to the right, showing that phenol is a weak acid.
Stability of the Phenoxide Ion
When phenol loses a proton, it forms the phenoxide ion.
The negative charge on the oxygen atom is delocalised into the benzene ring. This occurs because the oxygen lone pair overlaps with the delocalised π system of the ring.
As a result:
- the negative charge is spread over the oxygen and the ring
- the phenoxide ion is stabilised
- loss of a proton is favoured compared with alcohols
Comparison with Alcohols
In aliphatic alcohols, such as ethanol, the alkoxide ion formed has a localised negative charge on oxygen.
There is no delocalisation into a π system, so the conjugate base is less stable.
Therefore:
- phenol is acidic enough to react with \( \mathrm{NaOH(aq)} \)
- aliphatic alcohols are not acidic enough to react with \( \mathrm{NaOH(aq)} \)
Why Phenol Is Still a Weak Acid
Although the phenoxide ion is stabilised by delocalisation, phenol is still much weaker than a carboxylic acid.
In carboxylic acids, the negative charge in the carboxylate ion is delocalised between two highly electronegative oxygen atoms, giving much greater stability.
Key Exam Statement
Phenol is acidic because the phenoxide ion formed is stabilised by delocalisation of the negative charge into the benzene ring.
Example
Explain why phenol reacts with aqueous sodium hydroxide but ethanol does not.
▶️ Answer / Explanation
Phenol forms a phenoxide ion that is stabilised by delocalisation into the benzene ring.
Ethanol forms an alkoxide ion with a localised negative charge, which is much less stable.
Therefore phenol is acidic enough to react with NaOH, whereas ethanol is not.
Example
Explain why phenol is more acidic than an aliphatic alcohol but less acidic than a carboxylic acid.
▶️ Answer / Explanation
Phenol is more acidic than an aliphatic alcohol because the phenoxide ion is stabilised by delocalisation of the negative charge into the benzene ring.
Aliphatic alcohols do not have this delocalisation, so their conjugate bases are less stable.
Phenol is less acidic than a carboxylic acid because the carboxylate ion has the negative charge delocalised over two electronegative oxygen atoms, giving greater stabilisation.
Relative Acidities of Water, Phenol and Ethanol
Water, phenol and ethanol can all act as weak acids. You must be able to describe and explain their relative acidities by comparing the stability of their conjugate bases.
Order of Acidity
The relative acidities are:
phenol > water > ethanol
General Principle
The strength of an acid depends on the stability of the conjugate base formed when the acid loses a proton.
The more stable the conjugate base, the stronger the acid.
Phenol
Phenol loses a proton to form the phenoxide ion:
\( \mathrm{C_6H_5OH \rightleftharpoons C_6H_5O^- + H^+} \)
The negative charge on the oxygen atom is delocalised into the benzene ring via overlap with the delocalised π system.
This delocalisation stabilises the phenoxide ion, making phenol the most acidic of the three.
Water
Water loses a proton to form the hydroxide ion:
\( \mathrm{H_2O \rightleftharpoons OH^- + H^+} \)
The negative charge on the hydroxide ion is localised on the oxygen atom, but oxygen is highly electronegative, giving some stabilisation.
Water is therefore a weak acid, but more acidic than ethanol.
Ethanol
Ethanol loses a proton to form the ethoxide ion:
\( \mathrm{CH_3CH_2OH \rightleftharpoons CH_3CH_2O^- + H^+} \)
The ethyl group has a +I (electron-donating) effect, which pushes electron density towards the oxygen atom.
This destabilises the negative charge on the ethoxide ion, making ethanol the least acidic of the three.
Summary Explanation
- Phenol is most acidic because its conjugate base is stabilised by delocalisation.
- Water is less acidic because the negative charge is localised, but on a highly electronegative oxygen atom.
- Ethanol is least acidic because the alkyl group destabilises the negative charge by electron donation.
Example
Arrange water, phenol and ethanol in order of increasing acidity and give one reason.
▶️ Answer / Explanation
Ethanol < water < phenol.
Phenol is most acidic because the phenoxide ion is stabilised by delocalisation of the negative charge into the benzene ring.
Example
Explain why ethanol is less acidic than water, but water is less acidic than phenol.
▶️ Answer / Explanation
Ethanol forms an ethoxide ion in which the alkyl group donates electron density, destabilising the negative charge on oxygen.
Water forms a hydroxide ion with a localised negative charge on oxygen, which is more stable than the ethoxide ion.
Phenol forms a phenoxide ion in which the negative charge is delocalised into the benzene ring, giving the greatest stabilisation.
Why the Reagents and Conditions for Nitration and Bromination of Phenol Differ from Benzene
Phenol reacts much more readily than benzene in electrophilic substitution reactions. You must be able to explain why different reagents and milder conditions are used for the nitration and bromination of phenol compared with benzene.
Key Difference Between Phenol and Benzene
The key difference is the presence of the –OH group directly attached to the benzene ring in phenol.
The lone pair of electrons on the oxygen atom overlaps with the delocalised π system of the ring.
This donates electron density into the ring, especially at the ortho and para positions.
Effect on Ring Reactivity
As a result of electron donation:
- the benzene ring in phenol is activated
- phenol is much more reactive towards electrophiles than benzene
- a strong electrophile is not required
Nitration: Phenol vs Benzene
Benzene requires a mixture of concentrated nitric acid and concentrated sulfuric acid at elevated temperature.
This is necessary to generate the powerful electrophile, the nitronium ion, \( \mathrm{NO_2^+} \).
Phenol reacts with dilute nitric acid at room temperature.
The activated ring means that nitration occurs easily without sulfuric acid.
Bromination: Phenol vs Benzene
Benzene requires bromine in the presence of a halogen carrier catalyst such as AlBr₃.
The catalyst is needed to generate a sufficiently strong electrophile.
Phenol reacts rapidly with bromine water at room temperature with no catalyst.
The ring is so activated that multiple substitution occurs, forming 2,4,6-tribromophenol.
Why Strong Conditions Are Avoided for Phenol
Using the same harsh conditions as benzene would cause uncontrolled multiple substitution or oxidation of phenol.
Therefore, milder reagents and conditions are used to control the reaction.
Phenol requires milder conditions than benzene because the –OH group donates electron density into the ring, activating it towards electrophilic substitution.
Example
Explain why phenol can be nitrated using dilute nitric acid, whereas benzene requires concentrated nitric and sulfuric acids.
▶️ Answer / Explanation
The –OH group in phenol donates electron density into the benzene ring.
This activates the ring, so a strong electrophile is not required.
Benzene is less reactive and therefore needs concentrated acids to generate the nitronium ion.
Example
Explain why bromination of phenol occurs readily with bromine water, but bromination of benzene requires a halogen carrier catalyst.
▶️ Answer / Explanation
In phenol, the –OH group donates electron density into the aromatic ring, increasing its reactivity towards electrophiles.
As a result, bromine is sufficiently electrophilic on its own and no catalyst is required.
Benzene is stabilised by a delocalised π system and is less reactive, so a halogen carrier catalyst is needed to generate a stronger electrophile.
Directing Effect of the Hydroxyl Group in Phenol
In electrophilic substitution reactions, the hydroxyl group attached to a benzene ring has a strong directing effect. You must be able to recall that the –OH group in phenol directs substitution to the 2-, 4- and 6-positions on the ring.
2-, 4- and 6-Positions
When the –OH group is assigned position 1:
- 2-position → ortho
- 4-position → para
- 6-position → ortho
These positions are activated towards electrophilic substitution.
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Reason for This Directing Effect
The lone pair of electrons on the oxygen atom of the –OH group overlaps with the delocalised π system of the benzene ring.
This electron donation increases electron density particularly at the 2-, 4- and 6-positions, making electrophilic attack more likely at these sites.
Evidence from Phenol Reactions
This directing effect is seen in reactions such as:
- nitration → 2-nitrophenol and 4-nitrophenol
- bromination → 2,4,6-tribromophenol
Key Exam Statement
The hydroxyl group in phenol is an activating, ortho/para-directing group and therefore directs electrophilic substitution to the 2-, 4- and 6-positions of the benzene ring.
Example
State the positions on the benzene ring that are activated by the –OH group in phenol.
▶️ Answer / Explanation
The 2-, 4- and 6-positions.
Example
Phenol reacts with bromine water to form 2,4,6-tribromophenol. Explain why substitution occurs specifically at these positions.
▶️ Answer / Explanation
The –OH group donates electron density into the benzene ring by overlap of a lone pair with the delocalised π system.
This increases electron density at the 2-, 4- and 6-positions, so electrophilic substitution occurs at these positions.
Applying the Chemistry of Phenol to Other Phenolic Compounds
The reactions of phenol can be used to predict the reactions of other phenolic compounds, such as naphthol. You must be able to apply your knowledge of phenol chemistry to these compounds.
What Is a Phenolic Compound?
A phenolic compound contains a hydroxyl group (–OH) directly bonded to an aromatic ring.
Examples include:
- phenol (one benzene ring)
- naphthol (two fused benzene rings)
Key Similarity: The –OH Group
In both phenol and naphthol, the –OH group donates electron density into the aromatic system by overlap of a lone pair with the delocalised π system.
Therefore, other phenolic compounds show similar behaviour to phenol.
Acid–Base Reactions
Like phenol, naphthol is a weak acid.
- reacts with \( \mathrm{NaOH(aq)} \) to form a phenoxide-type salt
- reacts with sodium metal to produce hydrogen gas
This occurs because the conjugate base is stabilised by delocalisation into the aromatic rings.
Electrophilic Substitution Reactions
Phenolic compounds undergo electrophilic substitution more readily than benzene.
For naphthol, this means:
- nitration occurs under milder conditions than for naphthalene
- bromination can occur without a halogen carrier catalyst
The –OH group activates the aromatic system, just as in phenol.
Azo Coupling Reactions
Like phenol, naphthol reacts with diazonium salts in alkaline solution.
This produces azo compounds, many of which are intensely coloured dyes.
Predicting Products
When applying phenol chemistry to naphthol:
- expect substitution to occur at positions activated by the –OH group
- expect faster reactions than for non-phenolic aromatic compounds
- use similar reagents and milder conditions
Any compound containing an –OH group attached directly to an aromatic ring will show phenol-like acidity and enhanced electrophilic substitution.
Example
Predict how naphthol would react with aqueous sodium hydroxide.
▶️ Answer / Explanation
Naphthol is a phenolic compound and is a weak acid.
It reacts with aqueous sodium hydroxide to form a naphthoxide salt and water.
Example
Naphthol reacts rapidly with bromine water, whereas naphthalene does not. Explain this difference.
▶️ Answer / Explanation
Naphthol contains a hydroxyl group that donates electron density into the aromatic system.
This activates the ring towards electrophilic substitution.
Naphthalene does not contain an –OH group and is therefore much less reactive towards bromine water.