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-H_(3)C-C_(6)H_(4)-NO_(2) p-O_(2)N-C_(6)H_(4)-NO_(2)

Arrange the following set of compounds in order of their decreasing re

1.	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-H_(3)C-C_(6)H_(4)-NO_(2) p-O_(2)N-C_(6)H_(4)-NO_(2)
Answer» 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., `NO_(2)^(-)` and `Cl^(-)`) deactivates the aromatic ring by decreasing the electron density. Since `NO_(2)^(-)` group is more electorn withdrawing (due to resonance EFFECT) than the `Cl^(-)` group (due toinductive effect), the decreasing order of reactivity is as follows : Chlorobenzene `gt` p-Nitroclorobenzene `gt` 2, 4-Dinitrochlorobenzene (b) While `CH_(3)^(-)` is an electron donating group `NO_(2)^(-)` group is electron with drawing. HENCE, toluene will have the maximum electron density and is most easily attacked by `E^(+)`. `NO_(2)^(-)` is an electron withdrawing group. Hence, when the number of `NO_(2)^(-)` substitution is GREATER, the order is as follows : Toluene `lt p-H_(3)C-C_(6)H_(4)-NO_(2), p-O_(2)N-C_(6)H_(4)-NO_(2)` (Because the no. of inactive `-NO_(2)` groups decreases.)