What is the decreasing order of reactivity of the following compounds towards nucleophilic substitution?
(I) m-Nitrobromobenzene
(II) 2,4,6-Trinitrobromobenzene
(III) p-Nitrobromobenzene
(IV) 2,4-Dinitrobromobenzene
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → II > IV > III > I
A haloarene reacts with a nucleophile only slowly. An electron withdrawing –NO2 group at the ortho or para position pulls electron density from the ring and stabilises the negatively charged intermediate, so the reaction becomes faster. The more such groups, the faster the reaction. A –NO2 group at the meta position has no such effect.
(II) 2,4,6-Trinitrobromobenzene — first: three –NO2 groups, at both ortho positions and the para position.
(IV) 2,4-Dinitrobromobenzene — second: two –NO2 groups, one ortho and one para.
(III) p-Nitrobromobenzene — third: one –NO2 group, at the para position.
(I) m-Nitrobromobenzene — last: its –NO2 group is at the meta position, where it does not stabilise the intermediate.
NCERT (2026-27 edition):
6.7.2 Reactions of Haloarenes – “The presence of an electron withdrawing group (-NO2) at ortho- and para-positions increases the reactivity of haloarenes.”
“The effect is pronounced when (-NO2) group is introduced at ortho- and para- positions. However, no effect on reactivity of haloarenes is observed by the presence of electron withdrawing group at meta-position.”