Preparation of Chlorobenzene from Aniline:

Reaction: Aniline + Nitrous Acid -> Benzenediazonium chloride + Water

Step 1: Conversion of Aniline to Benzenediazonium chloride
Aniline reacts with nitrous acid (HNO2) to form a highly reactive intermediate called benzenediazonium chloride. This reaction is known as diazotization.

Step 2: Conversion of Benzenediazonium chloride to Chlorobenzene
The benzenediazonium chloride is then heated with cuprous chloride (CuCl) to give chlorobenzene. This reaction is known as Sandmeyer reaction.

Explanation:
- Aniline (C6H5NH2) is treated with nitrous acid (HNO2) to form benzenediazonium chloride (C6H5N2Cl).
- Nitrous acid is generated in situ by the reaction of sodium nitrite (NaNO2) with hydrochloric acid (HCl).
- The diazotization reaction involves the replacement of the amino group (-NH2) of aniline with a diazonium group (-N2Cl).
- The reaction proceeds via the formation of a diazonium ion intermediate, which is then converted to the diazonium chloride salt in the presence of hydrochloric acid.
- The benzenediazonium chloride obtained is unstable and highly reactive.
- It can undergo various substitution reactions to form different products.
- In this case, it is heated with cuprous chloride (CuCl) to give chlorobenzene (C6H5Cl).
- The Sandmeyer reaction involves the replacement of the diazonium group with a chlorine atom, resulting in the formation of chlorobenzene.
- Cuprous chloride acts as a catalyst in this reaction, facilitating the substitution of the diazonium group.
- The reaction is carried out in an inert solvent such as ether or benzene.
- The overall reaction can be represented as:
C6H5N2Cl + CuCl -> C6H5Cl + N2 + CuCl2

Key Points:
- Aniline is converted to benzenediazonium chloride by diazotization with nitrous acid.
- Benzenediazonium chloride is then heated with cuprous chloride to give chlorobenzene.
- Nitrous acid is generated in situ by the reaction of sodium nitrite with hydrochloric acid.
- Cuprous chloride acts as a catalyst in the Sandmeyer reaction.
- The Sandmeyer reaction involves the substitution of the diazonium group with a chlorine atom.