Which of the following diazonium salt is insoluble in water?

C₆H₅N₂⁺BF₄^–

The correct answer is option 1. C₆H₅N₂⁺BF₄^–.

To determine which diazonium salt is insoluble in water, we need to consider the solubility properties of the counterions attached to the diazonium cation. Let's analyze each of the given diazonium salts:

1. \(C_6H_5N_2^+BF_4^−\) (Phenyl diazonium tetrafluoroborate): Tetrafluoroborate is a bulky anion with a weakly coordinating nature. It tends to form salts that are less soluble in water because the large, non-polarizable \(BF_4^−\) anion does not interact strongly with water molecules. The solubility of such salts is generally lower compared to salts with smaller, more hydrophilic anions.

2. \(C_6H_5N_2^+HSO_4^−\) (Phenyl diazonium hydrogen sulfate): Hydrogen sulfate is highly soluble in water due to its ionic nature and ability to form hydrogen bonds with water molecules. This makes the phenyl diazonium hydrogen sulfate salt water-soluble.

3. \(C_6H_5N_2^+Br^−\) (Phenyl diazonium bromide):

Bromide ions are small and highly soluble in water due to their ionic nature and strong interaction with water molecules. Phenyl diazonium bromide is therefore soluble in water.

4. \(C_6H_5N_2^+Cl^−\) (Phenyl diazonium chloride):

Similar to bromide, chloride ions are small and highly soluble in water. Phenyl diazonium chloride is also soluble in water.

Among the given options, **C6H5N2+BF4−** (phenyl diazonium tetrafluoroborate) is the diazonium salt that is insoluble in water. This is due to the bulky and non-coordinating nature of the BF4− anion, which does not interact strongly with water molecules, leading to lower solubility.

Therefore, the correct answer is 1. C₆H₅N₂⁺BF₄^–