Why are carboxylic acids significantly stronger acids than phenols, despite both compounds possessing acidic hydroxyl (-OH) groups attached to unsaturated carbon atoms?

Answer and explanation

Correct answer: B. The carboxylate base is stabilized by two equivalent resonance structures on oxygen atoms

  1. Step 1: Acid strength is determined by the relative thermodynamic stability of the conjugate base formed upon deprotonation.
  2. Step 2: Deprotonation of a carboxylic acid yields a carboxylate anion (), which is stabilized by two completely equivalent resonance structures where the negative charge is equally shared between two identical, highly electronegative oxygen atoms.
  3. Step 3: In contrast, deprotonation of phenol yields a phenoxide anion (). Although phenoxide has five resonance structures, the negative charge is delocalized over three carbon atoms of the aromatic ring, which are far less electronegative than oxygen, making resonance stabilization of phenoxide much less effective than that of carboxylate.
Answer
The carboxylate conjugate base is stabilized by two equivalent resonance structures where negative charge is delocalized over two highly electronegative oxygen atoms, whereas in phenoxide the charge is delocalized onto less electronegative carbon atoms
Why not the others: 'Phenols cannot lose a proton...' is false; phenols are weakly acidic (); 'Carboxylic acids dissociate into metallic crystals...' is nonsensical; 'Carbon atom in carboxylic acids is sp-hybridized...' is false as the carbonyl carbon is -hybridized.

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