See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

Concept: Acid strength (Ka) increases as the stability of the conjugate base (carboxylate anion) increases. Electron-withdrawing groups (EWGs) stabilize the conjugate base by dispersing negative charge, thereby increasing Ka (lowering pKa). The closer and more numerous the EWGs, the stronger the inductive effect. Step 1 – Compare the structures: (a) Acetic acid (CH3COOH): The methyl group is weakly electron-donating. pKa ≈ 4.76. (b) Dichloroacetic acid (Cl2CHCOOH): Two chlorine atoms are directly on the alpha carbon (adjacent to COOH). Chlorine is strongly electron-withdrawing by induction. Two Cl atoms provide a powerful combined inductive effect very close to the carboxyl group. pKa ≈ 1.48. (c) 4-Iodocyclohexane-1-carboxylic acid: Iodine is at the 4-position (para) on the cyclohexane ring, far from the COOH group. Iodine is a weak EWG by induction (large, polarizable, but poor inductive transmitter over distance). pKa ≈ 4.6–4.8. (d) 4-Chlorocyclohexane-1-carboxylic acid: Chlorine at C4, also remote from COOH. Chlorine is a better inductive EWG than iodine, but the distance (four bonds away) greatly attenuates the effect. pKa ≈ 4.4–4.5. Step 2 – Rank acid strengths: Dichloroacetic acid (b) has two strongly electron-withdrawing Cl atoms directly on the alpha carbon, providing the greatest stabilization of the conjugate base and the highest Ka among these options. Options (c) and (d) have a single halogen far from the acid group, so their inductive effects are minimal. Option (a) has no EWG at all. Step 3 – Why other options fail: (a) Acetic acid: methyl is electron-donating; weakest acid here. (c) 4-Iodocyclohexane-1-COOH: one EWG, remote; weak effect. (d) 4-Chlorocyclohexane-1-COOH: one EWG, remote; slightly stronger than (c) or (a), but far weaker than (b). (b) Dichloroacetic acid: two Cl atoms alpha to COOH gives the lowest pKa (highest Ka). Therefore, the correct answer is B.