Arrange the following set of compounds in order of their decreasing relative reactivity with an electrophile, E+
Chlorobenzene, 2,4-dinitrochlorobenzene, p-nitrochlorobenzene
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
The typical reactions of benzene are electrophilic substitution reactions. Higher the electron-density in the benzene ring, more reactive is the compound towards these reactions. Since N02 is a more powerful electron-withdrawing group than Cl, therefore, more the number of nitro groups, less reactive is the compound. Thus, the overall reactivity decreases in the order:
Chlorobenzene > p-nitrochlorobenzene > 2, 4-dinitrochlorobenzene