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Question 22

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

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

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