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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.

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Student-friendly explanation

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.

How to write this in exams

  1. 1

    Start with the exact idea

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

  2. 2

    Then show how to use it

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

  3. 3

    Add one concrete example

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

  4. 4

    Avoid this incomplete answer

    Placing ethanol above phenol in acidity is wrong because alkoxide ion is less stable than resonance-stabilised phenoxide ion.

Definition

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

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.

Memory hook

Acid strength follows the comfort of the ion left behind.

Examples and method

Worked example

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

Method to apply

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

Diagram support

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

How CBSE asks it

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

Avoid common mistakes

Common confusion

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

Common wrong answer

Placing ethanol above phenol in acidity is wrong because alkoxide ion is less stable than resonance-stabilised phenoxide ion.

Exam tip

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

Quick check

Why is p-nitrophenol more acidic than phenol?

The nitro group is electron-withdrawing and stabilises the phenoxide ion, so p-nitrophenol is more acidic.

Answer writing and exam use

1-mark answer

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

2-mark answer

Phenols are more acidic than alcohols because phenoxide ion is stabilised by resonance, whereas alkoxide ions do not get similar resonance stabilisation. Acidity rule: greater conjugate-base stability means stronger acid. Phenol gives C6H5O-, and resonance delocalisation stabilises this phenoxide ion. Phenol is more acidic than ethanol because phenoxide ion is resonance-stabilised, while ethoxide ion is not.

3-mark answer

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. Acidity rule: greater conjugate-base stability means stronger acid. Phenol gives C6H5O-, and resonance delocalisation stabilises this phenoxide ion. Compare ethanol, phenol and p-nitrophenol in acidity. Ethoxide lacks resonance stabilisation, phenoxide is resonance-stabilised, and p-nitrophenoxide is further stabilised by the electron-withdrawing nitro group. Therefore acidity order is p-nitrophenol > phenol > ethanol. It appears as acidity order, assertion-reason, explanation of phenol's acidic nature, and substituent-effect questions. Placing ethanol above phenol in acidity is wrong because alkoxide ion is less stable than resonance-stabilised phenoxide ion.
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