Which of the following compounds will not be soluble in sodium bicarbonate?

To determine which compound will not be soluble in sodium bicarbonate (NaHCO₃), we must recall that sodium bicarbonate is a weak base and reacts only with acids stronger than carbonic acid (H₂CO₃). Carbonic acid has a pKa value of approximately 6.3. Therefore, any acid with a pKa less than or equal to 6.3 will react with NaHCO₃, producing carbon dioxide gas and dissolving in the solution. Acids with pKa greater than 6.3 are weaker than carbonic acid and will not react with NaHCO₃, hence they are insoluble.

Now, let's evaluate each option:

Option A: 2,4,6-Trinitrophenol (Picric acid)
This compound has three nitro groups attached to the phenol ring, which are strong electron-withdrawing groups. These groups significantly increase the acidity. The pKa of picric acid is about 0.4, which is much less than 6.3. Therefore, it is a strong acid and will react with NaHCO₃, making it soluble.

Option B: Benzene sulphonic acid
Sulphonic acids are very strong acids. The pKa of benzene sulphonic acid is approximately -6.5, which is far less than 6.3. Hence, it will readily react with NaHCO₃ and dissolve.

Option C: o-Nitrophenol
This is a phenol derivative with a nitro group at the ortho position. While the nitro group is electron-withdrawing and increases acidity compared to phenol (pKa ≈ 10), the pKa of o-nitrophenol is about 7.23. Since 7.23 is greater than 6.3, o-nitrophenol is a weaker acid than carbonic acid. Thus, it will not react with NaHCO₃ and will not dissolve.

Option D: Benzoic acid
Benzoic acid is a carboxylic acid. Its pKa is approximately 4.2, which is less than 6.3. Therefore, it is stronger than carbonic acid and will react with NaHCO₃, leading to solubility.

Based on the pKa values, o-Nitrophenol (Option C) is the only compound that will not dissolve in sodium bicarbonate because it is too weak an acid to react with it.

Hence, the correct answer is Option C.