Which of the following acids has the lowest $p K_{a}$ (strongest acid)? ( $p K_{a}$ : benzoic acid 4.20; 2-methylbenzoic acid 3.91; 3-methylbenzoic acid 4.27; 4-methylbenzoic acid 4.37)

A2-Methylbenzoic acid
BBenzoic acid
C3-Methylbenzoic acid
D4-Methylbenzoic acid

Answer: A

Any ortho-substituted benzoic acid is stronger than benzoic acid (ortho effect), even for an electron-releasing group like methyl, because the group twists the

COOH

out of the ring plane. The

p K_{a}

values agree: 3.91 is the lowest.

Practice General Organic Chemistry II: Acids and Bases More on Ortho effect and solvent effects

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Which of the following acids has the lowest pKa​ (strongest acid)? (pKa​: benzoic acid 4.20; 2-methylbenzoic acid 3.91; 3-methylbenzoic acid 4.27; 4-methylbenzoic acid 4.37)

Answer: A

More questions on Ortho effect and solvent effects

Which of the following acids has the lowest $p K_{a}$ (strongest acid)? ( $p K_{a}$ : benzoic acid 4.20; 2-methylbenzoic acid 3.91; 3-methylbenzoic acid 4.27; 4-methylbenzoic acid 4.37)