Question

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

(a) Chlorobenzene, 2,4-dinitrochlorobenzene, p-nitrochlorobenzene

(b) 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

(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