Carboxylic Acids - Substituent Acidity

Acid strength of a carboxylic acid increases when electron-withdrawing groups stabilise the carboxylate anion by the inductive effect, and a lower pKa indicates a stronger acid. The inductive effect weakens rapidly with distance through the carbon chain, so a chlorine closer to the carboxyl group stabilises the anion more strongly. In 2-chlorobutanoic acid the chlorine sits on the carbon adjacent to the carboxyl, giving the strongest electron withdrawal, the most stabilised carboxylate, and therefore the lowest pKa, so it is the correct answer. 3-chlorobutanoic acid has the chlorine one carbon further away, so its effect is weaker and pKa is higher. 4-chlorobutanoic acid has the most distant chlorine and the smallest acid-strengthening effect among the chlorinated acids. Plain butanoic acid lacks any electron-withdrawing substituent and is the weakest acid with the highest pKa. This distance dependence of the inductive effect is core NCERT content and a frequent JEE acidity-ranking question. The inductive effect is transmitted through the sigma framework of the chain and weakens by roughly a factor for each intervening carbon, which is why even a single position closer to the carboxyl produces a measurable drop in pKa. Adding a second electron-withdrawing group, as in dichloro or trichloro acids, lowers pKa further still. As a check, acidity falls in the order alpha > beta > gamma chloro > unsubstituted, confirming 2-chlorobutanoic acid as strongest.