Which one is the most acidic compound?

Solution & Explanation

\textbf{Answer:} (D) 2,4,6-Trinitrophenol (picric acid) \textbf{Solution:} Acidity of phenols increases with electron-withdrawing groups, which stabilise the phenoxide ion. The three $-\mathrm{NO}{}_2$ groups in picric acid exert strong $-I$ and $-R$ (electron-withdrawing) effects, the ortho/para nitro groups delocalising the negative charge of the phenoxide. Thus 2,4,6-trinitrophenol is far more acidic than o-nitrophenol (one $-\mathrm{NO}{}_2$), phenol, or o-cresol (the $-\mathrm{CH}{}_3$ group is electron-donating, making o-cresol the least acidic). Order of acidity: picric acid $>$ o-nitrophenol $>$ phenol $>$ o-cresol.