Excess of HCl is used to convert free aniline to aniline hydrochloride otherwise free aniline would undergo coupling reaction with benzenediazonium chloride.

Excess Hydrochloric Acid in the Diazotization of Arylamines

Introduction
The diazotization of arylamines is an important reaction in organic chemistry, often used to synthesize azo compounds. This reaction involves the conversion of an arylamine (such as aniline) into a diazonium salt, which can then be coupled with a suitable coupling partner to form the desired azo compound.

Role of Excess Hydrochloric Acid
In the diazotization reaction, an excess of hydrochloric acid (HCl) is used primarily to suppress the concentration of free aniline available for coupling. This ensures that the diazonium salt is the major species present in the reaction mixture.

Explanation
When an arylamine reacts with sodium nitrite (NaNO2) in the presence of hydrochloric acid, the following steps occur:

1. Protonation of the arylamine: HCl acts as a source of H+ ions, which protonate the arylamine to form the corresponding arylammonium chloride salt. This protonation is necessary for the subsequent reaction with sodium nitrite.

2. Nitrosation: The protonated arylamine reacts with sodium nitrite to form the corresponding diazonium salt. This reaction proceeds through the formation of nitrous acid (HNO2), which is in equilibrium with its protonated form (H2NO2+). The diazonium salt is the desired product for further coupling reactions.

3. Coupling: The diazonium salt can undergo coupling reactions with a variety of coupling partners, such as phenols or aromatic amines, to form azo compounds. The choice of coupling partner determines the final product.

By using an excess of hydrochloric acid, the concentration of free arylamine (such as aniline) is minimized. This is important because the arylamine can also undergo coupling reactions with the diazonium salt, leading to undesired side products. By suppressing the concentration of free arylamine, the selectivity for the formation of the diazonium salt is increased.

Additionally, excess hydrochloric acid helps to maintain an acidic environment, which is necessary for the stability of the diazonium salt. The diazonium salt is prone to decomposition, particularly under basic conditions. The excess hydrochloric acid helps to neutralize any base that may be liberated during the reaction, preventing diazonium salt decomposition.

Therefore, the primary role of excess hydrochloric acid in the diazotization of arylamines is to suppress the concentration of free arylamine available for coupling, ensuring the formation of the desired diazonium salt as the major species in the reaction mixture.