The introduction of which substituent at the ortho and para positions of chlorobenzene dramatically enhances its reactivity toward nucleophilic aromatic substitution ()?
Answer and explanation
Correct answer: D. Nitro group
- Step 1: In nucleophilic aromatic substitution (), the rate-determining step is the attack of the nucleophile to form a negatively charged Meisenheimer carbanion complex.
- Step 2: Strong electron-withdrawing groups () placed at the ortho and para positions stabilize this negative charge via powerful resonance delocalization into the nitro oxygen atoms.
- Step 3: Consequently, 2,4,6-trinitrochlorobenzene (picryl chloride) hydrolyzes to picric acid simply upon warming with water, whereas chlorobenzene requires and .
Answer
Nitro group ().
Why not the others: is electron-donating via hyperconjugation and deactivates towards nucleophilic attack; is a strong donor that severely deactivates the ring toward nucleophiles; is an extremely strong electron-donating group.
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