Arrange the following set of compounds in order of their decreasing relative reactivity with an electrophile, E+
Toluene, p-H3C–C6H4–NO2, p-O2N–C6H4–NO2.
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).
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
Here, CH3 group is electron donating but N02 group is electron-withdrawing. Therefore, the maximum electron-density will be in toluene, followed by p-nitrotoluene followed by p-dinitrobenzene. Thus, the overall reactivity decreases in the order:
Toluene >p—H3C—C6H4—NO2, p—O2N—C6H4—NO2.