Replacement of \(Cl\) of \(C_6H_5Cl\) to give \(C_6H_5OH\) occurs only in drastic conditions but \(Cl\) of 2,4-dinitrochlorobenzene is readily replaced. This is because:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → \(-NO_2\) withdraws electrons from ortho and para position.
Aryl halides undergo nucleophilic substitution with difficulty: the C–Cl bond has partial double-bond character by resonance. Chlorobenzene gives phenol only with NaOH at 623 K and 300 atm. An electron withdrawing NO2 group at the ortho and para positions increases the reactivity: it makes the carbon carrying Cl electron deficient, and it stabilises the carbanion formed when \( \ce{OH^{-}} \) adds, because the negative charge is delocalised onto the \( \ce{NO2} \) group by resonance.
\( \ce{(NO2)2C6H3Cl + 2NaOH} \xrightarrow{368\ \mathrm{K}} \ce{(NO2)2C6H3ONa + NaCl + H2O} \)
\( \ce{(NO2)2C6H3ONa} \xrightarrow{\ce{H+}} \text{2,4-dinitrophenol} \)
With one \( \ce{NO2} \) (4-nitrochlorobenzene) 443 K is needed; with two, 368 K; with three (2,4,6-trinitrochlorobenzene), warm water at 323 K is enough.
Why the other options are incorrect
Option (2) \(-NO_2\) group withdraws electron from meta position — Incorrect. A meta \( \ce{NO2} \) group has no resonance effect on the carbon carrying Cl; the negative charge of the intermediate cannot reach it, so the activation comes from the ortho and para groups.
Option (3) \(-NO_2\) makes the ring electron rich at ortho and para position — Incorrect. The reverse; \( \ce{NO2} \) makes those positions electron poor, and an electron rich ring would repel the nucleophile.
Option (4) \(-NO_2\)group donates electrons at meta position — Incorrect. \( \ce{NO2} \) is an electron withdrawing group (−I, −R); it does not donate electrons to any position.
NCERT (2026-27 edition):
Replacement by Hydroxyl Group – “… 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. … As shown, the presence of nitro group at ortho- and para-positions withdraws the electron density from the benzene ring and thus facilitates the attack of the nucleophile on haloarene. The carbanion thus formed is stabilised through resonance.”