Question

The correct order of the following compounds showing increasing tendency towards nucleophilic substitution reaction is : (Note: The compounds are chloro benzene with different nitro positions: (i) Chlorobenzene, (ii) 4-nitrochlorobenzene, (iii) 2,4-dinitrochlorobenzene, (iv) 2,4,6-trinitrochlorobenzene)

• A. $(iv)<(iii)<(ii)<(i)$
• B. $(iv)<(i)<(iii)<(ii)$
• C. $(i)<(ii)<(iii)<(iv)$
• D. $(iv)<(i)<(ii)<(iii)$

Hint:

EWG at ortho/para positions increase $S_NAr$ reactivity by lowering the electron density of the ring and stabilizing the negative charge formed during the reaction.

Answer 1

The Correct Option is C

Step 1: Nucleophilic aromatic substitution ($S_NAr$) is difficult in chlorobenzene because of resonance.
Step 2: Electron-withdrawing groups (EWG) like $-NO_2$ at ortho and para positions stabilize the carbanion intermediate (Meisenheimer complex).
Step 3: More $-NO_2$ groups = greater stabilization = faster reaction.
Step 4: Thus, Trinitrochlorobenzene (Picryl chloride) is the most reactive, and chlorobenzene is the least.