Which of the following four compounds is the most acidic?

Question

Quizplus · Accepted Answer

The acidity of a compound depends on the stability of the corresponding conjugate base. The compound with the most stable conjugate base will be the most acidic. In this case, compound III has a phenol group (-OH) which can lose a proton to form a stable phenoxide ion. The negative charge on the oxygen can be delocalized over the entire benzene ring, making the conjugate base more stable. On the other hand, the conjugate bases of compounds I, II, and IV do not have any resonance stabilization, and are therefore less stable. Therefore, compound III is the most acidic.

Which of the Following Four Compounds Is the Most Acidic