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ChemistryMediumClass 12

Phenols — Acidity, electrophilic substitution

Alcohols, Phenols & Ethers

8

JEE Qs

8%

Hard

75

min

Master the electronic effects (inductive, resonance) of the -OH group and common substituents to accurately predict the acidity and regioselectivity of electrophilic substitution reactions in phenols.

🧮 Key Formulas

C6H5OH + H2O <=> C6H5O- + H3O+
Ka = [C6H5O-][H3O+]/[C6H5OH]
pKa = -log(Ka)

✅ Key Points for JEE

  • 1Phenols are significantly more acidic than alcohols (but less than carboxylic acids) due to the resonance stabilization of the phenoxide ion, which delocalizes the negative charge over the benzene ring.
  • 2Electron-withdrawing groups (EWGs) at ortho and para positions increase phenol acidity by stabilizing the phenoxide ion; electron-donating groups (EDGs) decrease acidity. The inductive effect (-I) is present at all positions, but the mesomeric effect (+M/-M) is dominant at ortho/para.
  • 3The -OH group is a powerful activating group and an ortho-para director for Electrophilic Aromatic Substitution (EAS) reactions due to its strong +M effect, which increases electron density at these positions.
  • 4Due to the strong activation by the -OH group, phenols undergo EAS very readily, often requiring controlled conditions (e.g., non-polar solvents like CS2 for monohalogenation) to prevent poly-substitution.
  • 5Important named reactions involving electrophilic substitution of phenols include Kolbe's reaction (carboxylation for salicylic acid) and Reimer-Tiemann reaction (formylation for salicylaldehyde).

⚠️ Common Mistakes

  • ✕Incorrectly comparing the relative acidity of phenols with alcohols, water, or carboxylic acids (order: Carboxylic acids > Carbonic acid > Phenols > Water > Alcohols).
  • ✕Misidentifying the directing effect of the -OH group in phenols during EAS, or underestimating its activating strength, leading to incorrect product predictions.
  • ✕Confusing the reaction conditions for monohalogenation (e.g., Br2/CS2) versus polyhalogenation (e.g., Br2/H2O, which gives tribromophenol immediately).
  • ✕Failing to recall the specific reagents, conditions, or products for important named reactions like Kolbe's and Reimer-Tiemann reactions.

NCERT Chapters

  • Class 12 Chemistry Ch 11: Alcohols, Phenols and Ethers