Which of the following will not be soluble in sodium hydrogen carbonate?

o-Nitrophenol will not be soluble in sodium hydrogen carbonate. Sodium hydrogen carbonate (NaHCO3), also known as sodium bicarbonate or baking soda, is a weak base. It can react with and dissolve acidic compounds to form soluble salts. Benzoic acid, 2,4,6-trinitrophenol (picric acid), and benzene sulphonic acid are all acidic enough to react with NaHCO3, forming soluble sodium salts. However, o-Nitrophenol is a weaker acid than the others. Its pKa is higher, meaning it is less likely to donate a proton to the bicarbonate ion. Therefore, it will not be soluble in NaHCO3. The acidity of the compounds depends on the electron-withdrawing or electron-donating groups attached to the phenol. The nitro groups in 2,4,6-trinitrophenol significantly increase the acidity, making it soluble in NaHCO3. Similarly, the sulfonic acid group in benzene sulphonic acid is strongly acidic. The carboxylic acid group in benzoic acid also makes it sufficiently acidic for reaction. The single nitro group in o-nitrophenol is not sufficient to increase its acidity to the point where it readily reacts with NaHCO3. Thus, only o-Nitrophenol will remain insoluble.