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;
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Nitrobenzene
Aniline is first converted into a diazonium salt, and the diazonium group is then replaced by another group. The third reagent decides which group comes in.
First, nitrous acid (from \( \ce{NaNO2} \) and HCl at 273–278 K) diazotises aniline:
\( \displaystyle \ce{C6H5NH2 + NaNO2 + 2HCl} \xrightarrow{\text{273--278 K}} \ce{C6H5N2^+Cl^- + NaCl + 2H2O} \)
Then fluoroboric acid precipitates the diazonium fluoroborate:
\( \displaystyle \ce{C6H5N2^+Cl^- + HBF4 -> C6H5N2^+BF4^- v + HCl} \)
Finally, the fluoroborate is heated with aqueous sodium nitrite in the presence of copper, and the diazonium group is replaced by –NO2, with loss of nitrogen gas:
\( \displaystyle \ce{C6H5N2^+BF4^- + NaNO2} \xrightarrow[\Delta]{\ce{Cu}} \ce{C6H5NO2 + N2 ^ + NaBF4} \)
So the final product is nitrobenzene.
Why the other options are incorrect
Option (1) Fluorobenzene — Incorrect. It is formed when the fluoroborate is heated on its own, without \( \ce{NaNO2/Cu} \): \( \ce{C6H5N2^+BF4^-} \xrightarrow{\Delta} \ce{C6H5F + BF3 + N2} \). Here the nitrite step replaces the diazonium group by –\( \ce{NO2} \) instead.
Option (3) 1-Fluoro-4-nitrobenzene — Incorrect. Only one group, the diazonium group, is replaced; no second substituent enters the ring.
Option (4) 4-Fluoro-1-nitrobenzene — Incorrect. It is the same disubstituted product as Option (3), which is not formed, and the name breaks the alphabetical rule for locants (fluoro takes the lower number).
NCERT (2026-27 edition):
Replacement by Fluoride Ion
