Ch 2 · Phenol · AS & A Level Chemistry 9701

Topic 2 of 2 in Hydroxy compounds — AS & A Level Chemistry 9701 — 68 lessons.

Introduction

Phenol, C_6H_5OH, has an -OH group joined directly to a benzene ring. This single structural feature makes phenol behave quite differently from both an aliphatic alcohol and from benzene itself: its ring is far more reactive than benzene's, yet its -OH group is far more acidic than an alcohol's. Both differences trace back to the same cause, the interaction between the oxygen lone pair and the ring.

Key ideas — Preparation from the diazonium salt of phenylamine

Phenol is prepared by warming a solution of benzenediazonium chloride (formed from phenylamine, sodium nitrite and dilute hydrochloric acid below 10 °C) with water. The diazonium salt decomposes, releasing nitrogen gas and forming phenol: C_6H_5N_2^+Cl^- + H_2O xrightarrowwarm C_6H_5OH + N_2 + HCl

🧑‍🏫 Teacher's explanation

Think of phenol as a benzene ring with a very 'greedy' oxygen group attached. This oxygen pulls electron density away from the ring, but more importantly, it can let go of a hydrogen ion (H^+) much more easily than an alcohol like ethanol can. This makes phenol much more acidic than alcohols.

📖 Definitions

🔍 Worked example — Easy

Write the balanced chemical equation for the reaction of benzenediazonium chloride with water to produce phenol, including state symbols.

🔍 Worked example — Standard

Explain, in terms of bond enthalpy and stability, why phenol is more acidic than ethanol.

🔍 Worked example — Exam-style

A sample of phenol is reacted with bromine water (Br_2(aq)). Predict the observation and the organic product formed.

⚠️ Common errors

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🎯 Exam question

Key ideas — Reactions with bases and with sodium

Phenol is weakly acidic and reacts with aqueous sodium hydroxide in a neutralisation reaction, forming sodium phenoxide and water: C_6H_5OH + NaOH arrow C_6H_5O^-Na^+ + H_2O Phenol also reacts with sodium metal, releasing hydrogen gas and forming the same salt: 2C_6H_5OH + 2Na arrow 2C_6H_5O^-Na^+ + H_2 Unlike carboxylic acids, phenol is not acidic enough to react with sodium carbonate or sodium hydrogencarbonate; no effervescence is observed.

🧑‍🏫 Teacher's explanation

Think of phenol as a 'reluctant' acid. In chemistry, acidity is all about how easily a molecule can let go of a hydrogen ion (H^+). Ethanol is like a person holding onto their keys very tightly; it won't give them up even to a strong base. Phenol, however, is a bit more generous. Because the oxygen is attached to a benzene ring, the negative charge left behind after losing H^+ can be spread out (delocalized) around the ring, making the whole molecule more stable. This stability is the 'secret sauce' that makes phenol more acidic than alcohols.

📖 Definitions

🔍 Worked example — Easy

Write the balanced equation for the reaction between phenol and sodium hydroxide, including state symbols.

🔍 Worked example — Standard

Calculate the moles of hydrogen gas produced when 0.10 moles of phenol reacts completely with excess sodium metal.

🔍 Worked example — Exam-style

Explain, in terms of bond enthalpy and stability, why phenol is more acidic than ethanol.

⚠️ Common errors

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🎯 Exam question

(a) Write a balanced equation for this reaction, including state symbols. [2] (b) State the observation that would be made during this reaction. [1] (c) Explain why phenol reacts with sodium metal but ethanol does not. [2] [5 marks]

Key ideas — Coupling with diazonium salts to form azo compounds

Below 10 °C, an alkaline solution of phenol reacts with a diazonium salt in an electrophilic substitution called a coupling reaction, forming an intensely coloured azo compound joined by an -mathrmN=N- (azo) linkage: C_6H_5N_2^+ + C_6H_5O^- arrow mathrmC_6H_5-N=N-C_6H_4-OH Alkali converts phenol into the more strongly activating phenoxide ion, which is essential for the coupling reaction to proceed.

🧑‍🏫 Teacher's explanation

Imagine you are trying to snap two LEGO bricks together. Some bricks are 'sticky' and want to bond easily, while others are 'slippery' and resist. In this reaction, the diazonium salt is a highly reactive, 'sticky' electrophile looking for electrons. However, phenol itself is actually quite stubborn because its electrons are tucked away in the benzene ring.

📖 Definitions

🔍 Worked example — Easy

Identify the active species of phenol in an alkaline solution that undergoes coupling with a diazonium salt.

🔍 Worked example — Standard

Write the balanced chemical equation for the coupling of benzenediazonium chloride with phenol in an alkaline solution to form 4-hydroxyazobenzene.

🔍 Worked example — Exam-style

A sample of azo dye is prepared. If the reaction requires the phenol to be in its phenoxide form, why is it necessary to use an alkaline solution rather than pure water?

⚠️ Common errors

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🎯 Exam question

(a) State the type of reaction occurring. [1] (b) Draw the structure of the organic product formed, assuming substitution occurs at the para position. [2] (c) Explain why sodium hydroxide is added to the reaction mixture. [1] [4 marks]

Key ideas — Nitration and bromination of the ring

Phenol reacts with dilute nitric acid at room temperature to give a mixture of 2-nitrophenol and 4-nitrophenol, with no need for concentrated sulfuric acid. Phenol also decolourises bromine water instantly at room temperature, with no halogen carrier, giving a white precipitate of 2,4,6-tribromophenol: C_6H_5OH + 3Br_2 arrow C_6H_2Br_3OH + 3HBr

🧑‍🏫 Teacher's explanation

Think of the benzene ring as a crowd of people. In benzene, the electrons are shared equally, making it relatively stable and 'stubborn' to react. However, the -OH group on phenol is like a loud, energetic person in that crowd; it pushes electron density into the ring through the resonance effect. This makes the ring much more 'nucleophilic' (electron-rich) and eager to attack other molecules.

📖 Definitions

🔍 Worked example — Easy

Calculate the molar ratio of phenol to 2,4,6-tribromophenol in the reaction: ceC_6H_5OH + 3Br_2 arrow C_6H_2Br_3OH + 3HBr

🔍 Worked example — Standard

A sample of phenol reacts with 0.50 mol of bromine water to produce 0.15 mol of 2,4,6-tribromophenol. How many moles of phenol were initially present?

🔍 Worked example — Exam-style

Explain, in terms of electron density, why phenol reacts with bromine water without a catalyst, whereas benzene requires a Lewis acid catalyst.

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🎯 Exam question

Key ideas — Why phenol's conditions differ from benzene's

One lone pair on phenol's oxygen atom overlaps with the ring's π system and becomes delocalised into it, substantially increasing electron density around the ring, especially at the 2, 4 and 6 positions. This makes phenol's ring far more attractive to electrophiles than benzene's, so nitration needs only dilute nitric acid (not the concentrated acid/concentrated sulfuric acid mixture benzene requires) and bromination needs no halogen carrier catalyst at all, occurring instantly at room temperature where benzene would not react without one.

🧑‍🏫 Teacher's explanation

Think of benzene as a very stable, calm lake. It is content in its state and requires a massive 'push' (concentrated acids and heat) to react. Phenol, however, is like a lake with a constant, energetic current running through it. This 'current' comes from the oxygen atom. The oxygen has lone pairs of electrons that don't just sit on the oxygen; they spill over into the benzene ring through a process called delocalisation.

📖 Definitions

🔍 Worked example — Easy

Compare the conditions required for the nitration of benzene and phenol.

🔍 Worked example — Standard

Predict the major products when phenol reacts with bromine water (Br_2(aq)) and explain why no catalyst is needed.

🔍 Worked example — Exam-style

Explain why phenol is more acidic than ethanol.

⚠️ Common errors

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🎯 Exam question

(a) State the observation made during this reaction. [1] (b) Explain why phenol reacts more rapidly than benzene when treated with bromine. [2] [3 marks]

Key ideas — Acidity of phenol, water and ethanol

The order of acidity is phenol > water > ethanol. When phenol loses a proton, the resulting phenoxide ion's negative charge is delocalised into the aromatic ring, spreading the charge over several atoms and stabilising the ion; this delocalisation is not available to an alkoxide ion, so ethanol is a weaker acid than water, since the electron-donating ethyl group instead increases electron density on oxygen and destabilises its (undelocalised) conjugate base. Phenol is nonetheless a much weaker acid than a carboxylic acid, whose carboxylate ion is stabilised even more effectively by charge delocalisation over two equivalent oxygen atoms. This same reasoning applies to other phenolic compounds, such as substituted phenols and naphthols: an electron-withdrawing ring substituent further stabilises the delocalised anion and increases acidity, while an electron-donating substituent reduces it, and any phenolic -OH group undergoes the same ring-activated nitration, bromination and coupling reactions described above.

🧑‍🏫 Teacher's explanation

Think of acidity as how 'willing' a molecule is to give up a hydrogen ion (H^+). Imagine a person holding a heavy ball (the proton). If they can pass that ball to a group of friends who can share the weight, they are much more likely to let go of it. In chemistry, this 'group of friends' is the aromatic ring.

📖 Definitions

🔍 Worked example — Easy

Arrange the following in order of increasing acidity: CH_3CH_2OH, H_2O, C_6H_5OH.

🔍 Worked example — Standard

Explain why 4-nitrophenol is a stronger acid than phenol.

🔍 Worked example — Exam-style

Calculate the K_a of phenol if the pK_a is 10.0.

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🎯 Exam question

(b) Predict whether 2-methylphenol is more or less acidic than phenol and justify your answer. [2] [4 marks]

Worked example

Question. Phenol and cyclohexanol (a saturated, non-aromatic alcohol) are both treated separately with bromine water. (a) State what would be observed with phenol. [1] (b) State what would be observed with cyclohexanol. [1] (c) Explain, in terms of structure and bonding, why phenol reacts so much more readily with bromine than cyclohexanol does. [3]

Ch 2 · Phenol · AS & A Level Chemistry 9701

All chapters in Hydroxy compounds