Which of the following will have the highest pK_a value;

The correct answer is Option (3) → acetic acid (\( \ce{CH3COOH} \))

\( \mathrm{p}K_a = -\log K_a \), so the highest pK_a belongs to the weakest acid. A carboxylic acid is stronger when the group attached to –COOH withdraws electrons and stabilises the carboxylate ion, and weaker when the group donates electrons.

• Acetic acid: the methyl group donates electrons (+I effect) and destabilises the carboxylate ion, so it is the weakest acid of the four (pK_a about 4.76).

• Phenylacetic acid: the phenyl group (sp² carbons) withdraws electrons slightly through the \( \ce{CH2} \) group (pK_a about 4.31).

• Benzoic acid: the phenyl group is bonded directly to –COOH (pK_a about 4.19).

• 3-Chloropropanoic acid: chlorine withdraws electrons (−I effect) (pK_a about 4.0).

So acetic acid has the highest pK_a.

Why the other options are incorrect

• Option (1) benzoic acid — Incorrect. The electron-withdrawing phenyl ring on the carboxyl carbon makes it a stronger acid than acetic acid, so its pK_a is lower.

• Option (2) phenylacetic acid — Incorrect. The phenyl group still withdraws electrons through the \( \ce{CH2} \) group, so it is slightly stronger than acetic acid.

• Option (4) 3-chloropropanoic acid — Incorrect. The −I effect of chlorine stabilises the carboxylate ion, making it the strongest acid of the four; the effect is smaller than in 2-chloro acids because chlorine is one carbon further away.