Which of the following is not a correct resonance structure for a mechanism of the following chemical reaction?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → structure 2
1-Chloro-4-nitrobenzene reacts with \( \ce{OH^{-}} \) by nucleophilic aromatic substitution, an addition–elimination mechanism:
Addition: \( \ce{OH^{-}} \) attacks the carbon carrying Cl (C-1). This carbon becomes sp3 and holds both Cl and OH, and the ring gains a negative charge.
Delocalisation: this charge is shared by resonance over the ortho (C-2, C-6) and para (C-4) carbons of the ring, and from C-4 onto the oxygen atoms of the –NO2 group, which is why a para nitro group makes the reaction easy.
Elimination: \( \ce{Cl^{-}} \) leaves and the aromatic ring is restored, giving 4-nitrophenol:
\( \displaystyle \ce{O2N-C6H4-Cl + OH^- -> O2N-C6H4-OH + Cl^-} \)
Every correct resonance structure of the intermediate must keep C-1 as a saturated (sp3) carbon with four bonds and place the negative charge only on C-2, C-4, C-6 or the nitro oxygen.
Option (1) structure 1 — A correct resonance structure. The negative charge is on an ortho carbon next to C-1.
Option (2) structure 2 — Not a correct resonance structure. C-1 is drawn with a double bond while it still carries Cl, OH and another ring bond (five bonds), and the charge is on a meta carbon, which it cannot reach by resonance.
Option (3) structure 3 — A correct resonance structure. The negative charge is on the other ortho carbon.
Option (4) structure 4 — A correct resonance structure. The negative charge is on the para carbon that carries the –\( \ce{NO2} \) group; from here it moves onto the nitro oxygen.
NCERT (2026-27 edition):
Reactions of Haloarenes – …
