What will be the major product of the given reaction sequence?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → p-Dinitrobenzene
The acetylated –NH2 group steers the first nitro group to the para position; the –NH2 group is then freed, turned into a diazonium group and replaced by a second –NO2 group.
(i) Nitration: the –NHCOCH3 group directs to the ortho and para positions, and the p-nitro derivative is the major product (p-nitroacetanilide).
\( \displaystyle \ce{C6H5NHCOCH3 + HNO3} \xrightarrow{\ce{H2SO4}} \ce{O2N-C6H4-NHCOCH3 + H2O} \)
(ii) \( \ce{H3O+} \): hydrolysis of the amide removes the acetyl group and gives p-nitroaniline.
\( \displaystyle \ce{O2N-C6H4-NHCOCH3 + H2O} \xrightarrow{\ce{H3O+}} \ce{O2N-C6H4-NH2 + CH3COOH} \)
(iii) \( \ce{NaNO2} \), \( \ce{HBF4} \): sodium nitrite and the acid give nitrous acid in the mixture; this diazotises the –NH2 group in the cold, and with fluoroboric acid present the diazonium salt is precipitated as p-nitrobenzenediazonium fluoroborate.
\( \displaystyle \ce{NaNO2 + HBF4 -> HNO2 + NaBF4} \)
\( \displaystyle \ce{O2N-C6H4-NH2 + HNO2 + HBF4} \xrightarrow{\text{273-278 K}} \ce{O2N-C6H4-N2^+BF4^- v + 2H2O} \)
(iv) \( \ce{NaNO2} \), Cu: heating the diazonium fluoroborate with aqueous sodium nitrite and copper replaces the diazonium group by –NO2, with loss of nitrogen gas.
\( \displaystyle \ce{O2N-C6H4-N2^+BF4^- + NaNO2} \xrightarrow[\Delta]{\ce{Cu}} \ce{O2N-C6H4-NO2 + N2 ^ + NaBF4} \)
The second –NO2 group takes the place of the old –NH2 group, para to the first: p-dinitrobenzene.
Why the other options are incorrect
Option (1) p-Nitroaniline — Incorrect. It is the product of step (ii) only; steps (iii) and (iv) replace its –NH2 group.
Option (2) p-Nitroacetanilide — Incorrect. It is formed in step (i) and is hydrolysed in step (ii).
Option (3) m-Dinitrobenzene — Incorrect. The first nitro group enters para to the nitrogen, and the second takes the nitrogen’s own place, so the two groups end up para, not meta.
NCERT (2026-27 edition):
9.6 Chemical Reactions – “However, by protecting the –NH2 group by acetylation reaction with acetic anhydride, the nitration reaction can be controlled and the p-nitro derivative can be obtained as the major product.”
“… This can be done by protecting the -NH2 group by acetylation with acetic anhydride, then carrying out the desired substitution followed by hydrolysis of the substituted amide to the substituted amine.”
9.7 Method of Preparation of Diazonium Salts – “Benzenediazonium chloride is prepared by the reaction of aniline with nitrous acid at 273-278K. … The conversion of primary aromatic amines into diazonium salts is known as diazotisation.”
9.9 Chemical Reactions – “3. Replacement by fluoride ion: When arenediazonium chloride is treated with fluoroboric acid, arene diazonium fluoroborate is precipitated which on heating decomposes to yield aryl fluoride.”
“6. Replacement by –NO2 group: When diazonium fluoroborate is heated with aqueous sodium nitrite solution in the presence of copper, the diazonium group is replaced by –NO2 group.”