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,$p-CH_3-C_6H_4-NO_2$,$p-O_2N-C_6H_4-NO_2$

(N/A) Electrophiles are reagents that participate in a reaction by accepting an electron pair to bond with nucleophiles. The higher the electron density on a benzene ring,the more reactive the compound is towards an electrophile,$E^{+}$ (Electrophilic aromatic substitution).
$(a)$ The presence of electron-withdrawing groups (i.e.,$NO_2$ and $Cl$) deactivates the aromatic ring by decreasing the electron density. Since the $NO_2$ group is more electron-withdrawing (due to resonance and inductive effects) than the $Cl$ group (which is deactivating due to its strong inductive effect despite resonance donation),the decreasing order of reactivity is:
Chlorobenzene $> p-$Nitrochlorobenzene $> 2,4-$Dinitrochlorobenzene
$(b)$ While $CH_3$ is an electron-donating group (activating),the $NO_2$ group is electron-withdrawing (deactivating). Toluene has the highest electron density and is most easily attacked by $E^{+}$. As the number of $NO_2$ substituents increases,the electron density decreases. Thus,the order is:
Toluene $> p-CH_3-C_6H_4-NO_2 > p-O_2N-C_6H_4-NO_2$