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CraftExam
medium difficulty7 min

Acidity of Alcohols and Phenols

Phenols are more acidic than alcohols because phenoxide ion is stabilised by resonance, whereas alkoxide ions do not get similar resonance stabilisation.

In one line: Acid strength follows the comfort of the ion left behind.

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Understand it

  • Acidity depends on the stability of the conjugate base formed after loss of H+.
  • Phenol loses H+ to form phenoxide ion, in which the negative charge is delocalised over oxygen and the aromatic ring.
  • Alcohols form alkoxide ions, where the negative charge remains localised mainly on oxygen and is often intensified by the electron-releasing effect of alkyl groups.
  • Electron-withdrawing groups on phenol increase acidity, while electron-donating groups generally decrease it.

Example

Phenol is more acidic than ethanol because phenoxide ion is resonance-stabilised, while ethoxide ion is not.

Rule to remember

Acidity rule: greater conjugate-base stability means stronger acid.

Phenol gives C6H5O-, and resonance delocalisation stabilises this phenoxide ion.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

Compare ethanol, phenol and p-nitrophenol in acidity.

  1. 1Ethoxide lacks resonance stabilisation, phenoxide is resonance-stabilised, and p-nitrophenoxide is further stabilised by the electron-withdrawing nitro group.
  2. 2Therefore acidity order is p-nitrophenol > phenol > ethanol.

Method you can reuse

Remove H+ mentally, draw or identify the conjugate base, check resonance and substituent effects, then arrange acidity by conjugate-base stability.

Visual tip

A resonance diagram of phenoxide ion is important because it shows delocalisation of negative charge into the aromatic ring.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

Why is p-nitrophenol more acidic than phenol?

Think first, then show answer
The nitro group is electron-withdrawing and stabilises the phenoxide ion, so p-nitrophenol is more acidic.

Common wrong answer

Placing ethanol above phenol in acidity.

Why it loses marks: Alkoxide ion is less stable than resonance-stabilised phenoxide ion.

Don't mix this up

A common mistake is saying phenol is acidic only because benzene is present; the key reason is resonance stabilisation of phenoxide ion.

Exam tip

For acidity comparisons, compare conjugate-base stability, then mention resonance or substituent effect clearly.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

It appears as acidity order, assertion-reason, explanation of phenol's acidic nature, and substituent-effect questions.
  1. 1

    Enough for 1 mark

    Phenols are more acidic than alcohols because phenoxide ion is stabilised by resonance, whereas alkoxide ions do not get similar resonance stabilisation.

  2. 2

    Add for 2 marks

    Example: Phenol is more acidic than ethanol because phenoxide ion is resonance-stabilised, while ethoxide ion is not.

  3. 3

    Add for 3 marks

    Acidity depends on the stability of the conjugate base formed after loss of H+.

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