Read the passage carefully and answer.

Organic compounds derived from $NH_3$ by replacing one or more H-atoms with alkyl or aryl groups will form primary, secondary and tertiary amines. The common methods which include their preparations are reduction of nitro group, amide group and nitrile group, Gabriel phthalimide synthesis and ammonolysis of alkyl halides. Amines are basic in nature due to the presence of a lone pair of electrons on the nitrogen atom. Important chemical properties of amines include acylation, alkylation and their interaction with nitrous acid. There are various tests which can distinguish different types of amines from each other, for e.g. carbylamine test, Hinsberg's test, coupling reaction etc. Another important class of compounds which can be derived from primary amines is azo compounds. These compounds are intensely colored and widely used as dyes in textile, leather and food industries.

The final product of the following reaction is;

Nitrobenzene

The correct answer is Option (2) → Nitrobenzene

Step-by-Step Reaction Mechanism

This sequence describes a specialized method for introducing a nitro group into a benzene ring, often called the Schiemann-like nitration or a variation of the Balz-Schiemann reaction:

Step (i): Diazotization

Aniline ($C_6H_5NH_2$) reacts with nitrous acid ($HNO_2$), typically generated in situ from $NaNO_2$ and $HCl$ at low temperatures ($0\text{–}5^\circ\text{C}$), to form the benzene diazonium cation ($C_6H_5N_2^+$).

Step (ii): Formation of Fluoroborate Salt

The addition of fluoroboric acid ($HBF_4$) precipitates the diazonium salt as benzene diazonium fluoroborate ($C_6H_5N_2^+BF_4^-$). This salt is relatively stable and can be isolated.

Step (iii): Nitration via Nitro-de-diazoniation

Heating the benzene diazonium fluoroborate with aqueous sodium nitrite ($NaNO_2$) in the presence of copper powder catalyst ($Cu, \Delta$) results in the replacement of the diazonium group with a nitro group ($-NO_2$).

Conclusion

While $HBF_4$ is often associated with the synthesis of fluorobenzene (Balz-Schiemann reaction), the addition of $NaNO_2$ and $Cu$ in the final step specifically directs the reaction to form Nitrobenzene.