What is the by-product formed along with the haloarene at the end of Sandmeyer’s reaction?

N₂

The by-product formed along with the haloarene at the end of Sandmeyer’s reaction is nitrogen gas \((N_2)\). So the answer is (4).

The Sandmeyer reaction is a chemical reaction that is used to synthesize aryl halides from diazonium salts. The reaction is initiated by the reaction of a diazonium salt with a copper halide, which produces an aryl radical and nitrogen gas. The aryl radical then reacts with a halide ion to form the aryl halide product.

The other options are not produced in the Sandmeyer reaction. \(NO_2\) is a product of the diazotization reaction, which is the first step in the Sandmeyer reaction. \(HCl\) and \(H_2O\) are not produced in the Sandmeyer reaction.