Arrange the following compounds in the decreasing order of their reactivity towards Nucleophilic substitution reaction:
(A) Bromobenzene
(B) 4-Nitrobromobenzene
(C) 2,4,6-Trinitrobromobenzene
(D) 2,4-Dinitrobromobenzene
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (C), (D), (B), (A)
Aryl halides undergo nucleophilic substitution only with great difficulty, because the C–X bond has partial double bond character and the electron-rich ring repels the nucleophile. An electron-withdrawing \( \ce{-NO2} \) group at the ortho or para position to the halogen makes the reaction easier: it pulls electron density out of the ring and stabilises the negatively charged intermediate (the carbanion) by resonance. The more such nitro groups, the more reactive the halide.
Bromobenzene itself, like chlorobenzene, needs very harsh conditions (NaOH at about 623 K and 300 atm) for the halogen to be replaced by –OH.
For example, the trinitro bromo compound is hydrolysed to picric acid:
\( \displaystyle \ce{(O2N)3C6H2Br + H2O} \xrightarrow{\text{warm}} \ce{(O2N)3C6H2OH + HBr} \)
Shortcut: The trinitro compound is the most reactive and bromobenzene the least, so the order starts with (C) and ends with (A). Only Option (4) does this.
NCERT (2026-27 edition):
Reactions of Haloarenes – …
