______ has the highest rate of reaction towards nucleophilic substitution
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 1-chloro-2-nitrobenzene
Haloarenes are much less reactive than haloalkanes towards nucleophilic substitution: chlorobenzene gives phenol only under drastic conditions. The reactivity increases when an electron withdrawing group such as –\( \ce{NO2} \) is present at the ortho or para position to the halogen. The nucleophile adds to the carbon carrying the halogen and forms a negatively charged intermediate (carbanion). A nitro group at the ortho or para position withdraws this negative charge onto its own oxygen atoms by resonance (−R) and by its −I effect, which stabilises the intermediate. A nitro group at the meta position cannot take the charge by resonance, so its effect is small.
\( \displaystyle \ce{C6H5Cl} \xrightarrow[\text{(ii) }\ce{H+}]{\text{(i) NaOH, 623 K, 300 atm}} \ce{C6H5OH} \)
\( \displaystyle \ce{O2N-C6H4-Cl} \xrightarrow[\text{(ii) }\ce{H+}]{\text{(i) NaOH, 443 K}} \ce{O2N-C6H4-OH} \quad (\text{nitro group ortho or para}) \)
Of the given compounds, only 1-chloro-2-nitrobenzene (Option 2) has a nitro group ortho to chlorine, so it reacts fastest.
Why the other options are incorrect
Option (1) chlorobenzene — Incorrect. It has no activating group; the C–Cl bond has partial double bond character due to resonance, so substitution needs about 623 K and 300 atm.
Option (3) 1-chloro-3-nitrobenzene — Incorrect. The nitro group is meta to chlorine; it cannot delocalise the negative charge of the intermediate by resonance, so the rate rises only a little.
Option (4) 2-chlorotoluene — Incorrect. The methyl group is electron donating; it destabilises the negatively charged intermediate and makes substitution even slower than in chlorobenzene.
NCERT (2026-27 edition):
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.”