Question

Arrange the following set of compounds in the order of their decreasing relative reactivity with an electrophile. Give reason.

Answer 1

Electron donating groups increase the reactivity of arenes towards electrophile as they increase electron density on the benzene ring.

On the other hand, electron withdrawing group decreases the reactivity of arenes towards electrophile as they withdraw the electron density from the benzene ring.

Anisole is most reactive among the given molecules because methoxy group $(-OC{H}_3)$ is an electron releasing/ donating group and thus increases electron density on benzene by resonance effect $(+R)$.


In case of alkyl halides, the electron density increases at ortho and para positions due to $+R$ effect. However, the halogen atom also withdraws electrons from the ring because of its $-I$ effect.


Since the $-I$ effect is stronger than the $+R$ effect, the halogens are moderately deactivating. Thus, overall electron density on benzene ring decreases, which makes electrophilic substitution difficult.

Nitrobenzene is the least reactive because nitro group $(-N{O}_2)$ is a strong electron withdrawing group and withdraws electron density by inductive effect $(-I)$ and resonance effect $(-R)$.


So, the order of their decreasing relative reactivity with an electrophile is:

$Anisole>Chlorobenzene>Nitrobenzene$