Why is ortho-substituted benzoic acid (such as o-nitrobenzoic acid or o-toluic acid) a stronger acid than benzoic acid and its corresponding meta- and para-isomers (the ortho-effect)?

Answer and explanation

Correct answer: A. Steric crowding twists carboxyl out of plane to inhibit resonance and aid stabilization

  1. Step 1: The 'ortho-effect' in substituted benzoic acids refers to the experimental observation that nearly all ortho-substituted benzoic acids are stronger acids than benzoic acid, regardless of whether the substituent is electron-donating or electron-withdrawing.
  2. Step 2: The mechanism is steric inhibition of resonance (SIR): a bulky group at the ortho position sterically repels the group, forcing it out of the plane of the benzene ring.
  3. Step 3: Being out of plane, the carbonyl group can no longer participate in resonance with the benzene -electrons.
  4. Step 4: This loss of benzene-carbonyl resonance allows the two oxygen atoms of the carboxylate anion to share the negative charge without competition from the aromatic ring, making the carboxylate anion significantly more stable and driving higher acidity.
Answer
Steric hindrance by the ortho-substituent twists the carboxyl group out of the aromatic plane, inhibiting resonance with the benzene ring and increasing carboxylate stabilization
Why not the others: 'The ortho-substituent forms an ionic bond with hydronium...' is false; 'The ortho-position has lower atomic mass...' is physically impossible; 'Ortho-substituents eliminate all protons...' is chemically absurd.

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