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JEE Chemistry 2023 Question with Solution

The compound which will have the lowest rate towards nucleophilic aromatic substitution on treatment with OH−\text{OH}^- is:

Option 1 structure showing chlorobenzene with a nitro group para to chlorine on the benzene ring.Option 2 structure showing chlorobenzene with one nitro group ortho and one nitro group para to chlorine.Option 3 structure showing chlorobenzene with a nitro group ortho to chlorine on the benzene ring.Option 4 structure showing chlorobenzene with a nitro group meta to chlorine on the benzene ring.
  • A
    Option 1 structure showing chlorobenzene with a nitro group para to chlorine on the benzene ring.
  • B
    Option 2 structure showing chlorobenzene with one nitro group ortho and one nitro group para to chlorine.
  • C
    Option 3 structure showing chlorobenzene with a nitro group ortho to chlorine on the benzene ring.
  • D
    Option 4 structure showing chlorobenzene with a nitro group meta to chlorine on the benzene ring.

Answer

Correct answer:D

Step-by-step solution

Standard Method

Given: Four chloronitrobenzene derivatives are compared for nucleophilic aromatic substitution with OH−\text{OH}^-.

Find: Which compound has the lowest rate.

For nucleophilic aromatic substitution, electron-withdrawing groups such as NO2\text{NO}_2 increase the rate strongly when they are present at the ortho or para position to the leaving group, because they stabilize the anionic Meisenheimer intermediate.

A nitro group at the meta position does not provide this resonance stabilization effectively.

Therefore:

  • para-nitrochlorobenzene reacts faster than chlorobenzene,
  • ortho-nitrochlorobenzene also reacts faster,
  • ortho, para-dinitrochlorobenzene reacts the fastest,
  • meta-nitrochlorobenzene has the lowest rate among the given compounds.

Hence the compound shown in option (4) has the lowest rate towards nucleophilic aromatic substitution.

The correct option is D.

Common mistakes

  • Assuming that any NO2\text{NO}_2 group increases the rate equally is incorrect. In nucleophilic aromatic substitution, position matters; only ortho and para nitro groups stabilize the Meisenheimer intermediate well. Always check the relative position with respect to the leaving group.

  • Judging the answer only by the number of NO2\text{NO}_2 groups can be misleading. A single meta nitro group may be less activating than a single ortho or para nitro group because it lacks effective resonance stabilization of the intermediate.

  • Confusing nucleophilic aromatic substitution with electrophilic aromatic substitution leads to wrong trends. Here the key factor is stabilization of the negatively charged intermediate, not the usual directing effects used in electrophilic substitution.

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