The correct order of the following compounds showing increasing tendency towards nucleophilic substitution reaction is:

Correct Order: (i) < (ii) < (iii) < (iv)

The Core Concept: Nucleophilic Aromatic Substitution (SNAr) Normally, aryl halides (like chlorobenzene) are highly unreactive towards nucleophilic substitution. This is because the carbon-chlorine bond has partial double-bond character due to resonance, making it hard to break. Furthermore, the electron-rich benzene ring naturally repels incoming nucleophiles.

The Role of Electron-Withdrawing Groups To make an aryl halide reactive, you need strong electron-withdrawing groups (EWGs), such as the nitro group (-NO₂), attached to the ring.

• Location matters: These groups are most effective when located at the ortho and para positions relative to the halogen.
• Mechanism: During the reaction, an intermediate with a negative charge (a carbanion) is formed. The -NO₂ groups at the ortho and para positions help stabilize this negative charge through strong resonance (-R) and inductive (-I) effects. The more stabilized this intermediate is, the faster the reaction happens.

Evaluating the Compounds: The tendency to undergo nucleophilic substitution increases directly with the number of electron-withdrawing -NO₂ groups at the ortho and para positions:

• (i) Chlorobenzene: Has zero -NO₂ groups. The ring is electron-rich, making it the least reactive.
• (ii) 1-Chloro-4-nitrobenzene: Has one -NO₂ group at the para position. It is more reactive than chlorobenzene.
• (iii) 1-Chloro-2,4-dinitrobenzene: Has two -NO₂ groups (one ortho, one para). The combined withdrawing effect makes it significantly more reactive than (ii).
• (iv) 1-Chloro-2,4,6-trinitrobenzene: Has three -NO₂ groups (two ortho, one para). This ring is extremely electron-deficient, making the intermediate highly stable. This is the most reactive compound of the set.

Therefore, the reactivity steadily increases as you add more nitro groups: (i) < (ii) < (iii) < (iv).