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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, pH3CC6H4NO2,pO2NC6H4NO2.

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Solution

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

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 > pCH3C6H4NO2,pO2NC6H4NO2


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