9701 · 32.2
Phenol flashcards
Revision flashcards for Cambridge 9701 Phenol (syllabus 32.2). Flip, recall, then mark a real past-paper question.
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What is the structure of phenol?
A hydroxyl (-OH) group directly bonded to a carbon atom within a benzene ring. Its formula is C₆H₅OH.
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Why is phenol more acidic than ethanol?
The negative charge on the phenoxide ion (C₆H₅O⁻) is delocalised into the π-system of the benzene ring, stabilising the conjugate base. The charge on the ethoxide ion (C₂H₅O⁻) is localised on the oxygen atom.
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Why is phenol less acidic than ethanoic acid?
The carboxylate ion (CH₃COO⁻) delocalises the negative charge over two highly electronegative oxygen atoms, which is more stable than the phenoxide ion where the charge is delocalised over one oxygen and the carbon ring.
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What is the approximate pKa of phenol?
Approximately 10. For comparison, ethanol's pKa is ~16 and ethanoic acid's is ~4.8.
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What reagent is used for the bromination of phenol, and what is observed?
Aqueous bromine (Br₂(aq)). The orange-brown bromine water is decolourised, and a white precipitate of 2,4,6-tribromophenol is formed.
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Why does phenol not require a halogen carrier for bromination?
The -OH group is strongly activating, making the benzene ring electron-rich enough to polarise the Br-Br bond and react without a Lewis acid catalyst.
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What reagents are used for the nitration of phenol?
Dilute nitric acid (HNO₃) at room temperature. This is a much milder condition than the concentrated HNO₃/H₂SO₄ needed for benzene.
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What are the products of the nitration of phenol?
A mixture of 2-nitrophenol and 4-nitrophenol.
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How can you chemically distinguish between phenol and a carboxylic acid like ethanoic acid?
Add sodium hydrogencarbonate (NaHCO₃). The carboxylic acid will produce effervescence (CO₂ gas), while phenol is not acidic enough to react.
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How can you chemically distinguish between phenol and an alcohol like cyclohexanol?
Add aqueous sodium hydroxide (NaOH). Phenol will dissolve to form a colourless solution of sodium phenoxide. The alcohol will not react as it is not acidic enough.
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What is the directing effect of the -OH group in phenol?
It is a 2,4,6-directing (ortho- and para-directing) and activating group for electrophilic aromatic substitution.
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Name a polymer made from phenol.
Phenol-methanal resin, also known as Bakelite. It is formed by a condensation polymerisation reaction between phenol and methanal.