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Acidic Nature of Phenol

Why Phenols are acidic, Resonance Stabilization, and Substituent Effects.

By chemca Team • Updated Jan 2026

Phenols turn blue litmus red and react with aqueous NaOH to form salts, indicating their acidic nature. However, they are weaker acids than carboxylic acids and do not react with sodium bicarbonate ($NaHCO_3$).

$$ C_6H_5OH + H_2O \rightleftharpoons C_6H_5O^- \text{ (Phenoxide)} + H_3O^+ $$

1. Why is Phenol Acidic?

Resonance Stabilization

The acidity of phenol is due to the stability of the Phenoxide ion formed after the loss of a proton.

• In the phenoxide ion, the negative charge on the oxygen atom is delocalized over the benzene ring via resonance (at ortho and para positions).
• This delocalization disperses the charge, making the ion highly stable.
• Hybridization: The $sp^2$ hybridized carbon of the benzene ring is more electronegative than the $sp^3$ carbon in alcohols, further pulling electron density and facilitating proton release.

Comparison with Alcohol: In alcohols ($RO^-$), the negative charge is localized on the oxygen and destabilized by the +I effect of the alkyl group. Hence, Phenols are much stronger acids ($pK_a \approx 10$) than Alcohols ($pK_a \approx 16-18$).

2. Effect of Substituents

The acidity of phenol is significantly affected by the nature and position of substituent groups on the benzene ring.

Electron Withdrawing Groups (EWG)

Groups like $-NO_2, -CN, -CHO, -X$ withdraw electrons via -I and/or -R effects.

• They disperse the negative charge on the phenoxide ion.
• They stabilize the phenoxide ion.
• Acidity Increases.

Example: Nitrophenols

$$ \text{p-Nitrophenol} > \text{o-Nitrophenol} > \text{m-Nitrophenol} > \text{Phenol} $$

Note: o-Nitrophenol is slightly less acidic than p-Nitrophenol due to Intramolecular Hydrogen Bonding, which makes H release difficult.

Picric Acid (2,4,6-Trinitrophenol): With three $-NO_2$ groups, it is a very strong acid ($pK_a \approx 0.7$), stronger than even acetic acid.

Electron Releasing Groups (ERG)

Groups like $-CH_3, -OCH_3, -NH_2$ release electrons via +I or +R effects.

• They intensify the negative charge on the phenoxide ion.
• They destabilize the phenoxide ion.
• Acidity Decreases.

Example: Cresols (Methyl Phenols)

$$ \text{Phenol} > \text{m-Cresol} > \text{p-Cresol} > \text{o-Cresol} $$

3. Comparison of Acidity

General Order

$$ RCOOH > H_2CO_3 > C_6H_5OH > H_2O > ROH > HC \equiv CH $$

1. Phenol vs Carboxylic Acid: Carboxylic acids are stronger. The carboxylate ion ($RCOO^-$) has resonance where negative charge is delocalized over two equivalent electronegative oxygen atoms. In phenoxide, charge is on Carbon (less electronegative), making resonance less effective.

2. Phenol vs Water: Phenol is more acidic than water due to resonance stabilization.

3. Test for Distinction: Carboxylic acids react with $NaHCO_3$ to evolve $CO_2$ (effervescence). Phenols do not react with $NaHCO_3$.

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