Question

Arrange the following set of compounds in order of their decreasing relative reactivity with an electrophile, E⁺

1. Chlorobenzene, 2,4-dinitrochlorobenzene, p-nitrochlorobenzene 
2. Toluene, p-H₃C-C₆H₄-NO₂, p-O₂N-C₆H₄-NO₂.

Answer 1

Electrophiles are reagents that participate in a reaction by accepting an electron pair in order to bond to nucleophiles. 
The higher the electron density on a benzene ring, the more reactive is the compound towards an electrophile, E⁺ (Electrophilic reaction). 
(a) The presence of an electron withdrawing group (i.e., NO₂^- and Cl^-) deactivates the aromatic ring by decreasing the electron density. 
Since NO₂^- group is more electron withdrawing (due to resonance effect) than the Cl^- group (due to inductive effect), the decreasing order of reactivity is as follows:
Chlorobenzene \(>\) p - nitrochlorobenzene \(>\) 2, 4 - dinitrochlorobenzene

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(b) While CH₃^- is an electron donating group, NO₂^- group is electron withdrawing. Hence, toluene will have the maximum electron density and is most easily attacked by E⁺.
NO₂^- is an electron withdrawing group. Hence, when the number of NO₂^- substituents is greater, the order is as follows: 
Toluene \(>\) p-CH₃-C₆H₄-NO₂, p -O₂ N-C₆H₄-NO₂