In the diazotisation of anline with sodium nitrite and hydrochloride acid, an excess of hydrochloric acid is used primarily to

Options

A. Suppress the concentration of free aniline for coupling
B. Suppress hydrolysis of phenol
C. Ensure a stoichiometric amount of nitrous acid
D. Neutralize the base liberated

Explanation

To answer the question regarding the use of excess hydrochloric acid in the diazotisation of aniline with sodium nitrite, we can break down the process step by step.

### Step-by-Step Solution:

1. **Understanding Diazotisation**:
- Diazotisation is a chemical reaction that involves converting an amine (in this case, aniline) into a diazonium salt. Aniline (C6H5NH2) reacts with sodium nitrite (NaNO2) in the presence of hydrochloric acid (HCl) to form benzene diazonium chloride (C6H5N2Cl).

2. **Reaction Components**:
- The main components of the reaction are:
- Aniline (C6H5NH2)
- Sodium nitrite (NaNO2)
- Hydrochloric acid (HCl)

3. **Role of Hydrochloric Acid**:
- Hydrochloric acid serves multiple purposes in this reaction. It provides the acidic medium necessary for the formation of the diazonium ion from the aniline.

4. **Excess Hydrochloric Acid**:
- An excess of hydrochloric acid is used to suppress the concentration of free aniline in the solution. This is crucial because free aniline can lead to unwanted side reactions, particularly coupling reactions, where the diazonium salt could react with the aniline itself, leading to the formation of azo compounds.

5. **Preventing Coupling Reactions**:
- If the concentration of free aniline is high, the diazonium salt (C6H5N2+) can couple with the aniline, resulting in undesired products. By using excess HCl, we ensure that the aniline is protonated (forming anilinium ions, C6H5NH3+), which reduces the availability of free aniline for coupling reactions.

6. **Conclusion**:
- Therefore, the primary reason for using excess hydrochloric acid in the diazotisation of aniline is to suppress the concentration of free aniline, thus preventing coupling reactions and ensuring the successful formation of the diazonium salt.

### Final Answer:
The excess hydrochloric acid is used primarily to suppress the concentration of free aniline for coupling.