The correct sequence for converting aniline to p-bromoaniline is
(A) \( Br_2/CH_3COOH \)
(B) Alkaline Hydrolysis
(C) \( (CH_3CO)_2O \) and Pyridine
(D) Separation of ortho and para products
Arrange the above option from the sentence meaning. Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (C), (A), (D), (B)
The –\( \ce{NH2} \) group of aniline activates the benzene ring so strongly that bromine water substitutes all three ortho and para positions at once, giving 2,4,6-tribromoaniline. To get a monobromo product, the activating effect must first be reduced by protecting the –NH2 group as an amide; the protecting group is removed at the end.
(C) \( \ce{(CH3CO)2O} \) and pyridine: aniline is acetylated to acetanilide. The lone pair on nitrogen is now shared with the C=O group by resonance, so the ring is only moderately activated. Pyridine removes the acid formed.
\( \displaystyle \ce{C6H5NH2 + (CH3CO)2O} \xrightarrow{\text{pyridine}} \ce{C6H5NHCOCH3 + CH3COOH} \)
(A) \( \ce{Br2/CH3COOH} \): acetanilide is brominated once. The bulky –\( \ce{NHCOCH3} \) group directs to the ortho and para positions, and the para product is the major one.
\( \displaystyle \ce{C6H5NHCOCH3 + Br2} \xrightarrow{\ce{CH3COOH}} p\text{-}\ce{BrC6H4NHCOCH3}\,(\text{major}) + \ce{HBr} \)
(D) Separation of ortho and para products: the small amount of the ortho isomer is removed, so that the final amine is pure p-bromoaniline.
(B) Alkaline hydrolysis: the amide is hydrolysed and the free –\( \ce{NH2} \) group is restored.
\( \displaystyle p\text{-}\ce{BrC6H4NHCOCH3} \xrightarrow[\text{heat}]{\ce{OH-}/\ce{H2O}} p\text{-}\ce{BrC6H4NH2} + \ce{CH3COO-} \)
Note: It seems that there is a mistake in the original NTA paper, which prints “Arrange the above option from the sentence meaning.” when it should read “Arrange the above steps in the correct sequence.”
Shortcut: The amino group must be protected before bromination, so the sequence must start with (C). Only Option (4) starts with (C).
NCERT (2026-27 edition):
Electrophilic Substitution – “… The main problem encountered during electrophilic substitution reactions of aromatic amines is that of their very high reactivity. Substitution tends to occur at ortho- and para-positions. … 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.”