Q.Arrange the following set of compounds in the order of their decreasing relative reactivity with an electrophile. Give reason.
C6H5-OCH3 (anisole), C6H5-Cl (chlorobenzene), C6H5-NO2 (nitrobenzene)

The key idea is Reactivity in Electrophilic Aromatic Substitution.

The reactivity of substituted benzene rings towards electrophiles depends on the electronic effects (inductive and mesomeric) of the substituent groups.

1. Anisole ($\text{C}_6\text{H}_5\text{-OCH}_3$): The $-\text{OCH}_3$ group is a strong electron-donating group due to its significant positive mesomeric ($+\text{M}$) effect, which outweighs its negative inductive ($-\text{I}$) effect. This activates the benzene ring towards electrophilic attack.
2. Chlorobenzene ($\text{C}_6\text{H}_5\text{-Cl}$): The $-\text{Cl}$ group is an electron-withdrawing group due to its strong negative inductive ($-\text{I}$) effect, which is more dominant than its positive mesomeric ($+\text{M}$) effect. This deactivates the benzene ring, but to a lesser extent than a strong deactivating group. …

The reactivity of an aromatic compound towards an electrophile in an Electrophilic Aromatic Substitution (EAS) reaction is directly related to the electron density on the benzene ring. An electrophile, being electron-deficient, seeks regions of high electron density. Therefore, any substituent that increases the electron density on the benzene ring will activate it towards electrophilic attack, making it more reactive. Conversely, any substituent that decreases the electron density on the benzene ring will deactivate it, making it less reactive.

Substituents influence electron density primarily through two effects:

1. Inductive effect (I-effect): This is the transmission of charge through a chain of atoms in a molecule. Electron-withdrawing groups (like halogens, $-\text{NO}_2$) exert a -I effect, pulling electron density away. Electron-donating groups (like alkyl groups) exert a +I effect, pushing electron density towards the ring.
2. Mesomeric effect (M-effect) or Resonance effect (R-effect): This involves the delocalisation of $\pi$ electrons or lone pairs through conjugation.
   • +M effect: Groups with lone pairs on the atom directly attached to the ring (e.g., $-\text{OCH}_3$, $-\text{OH}$, $-\text{NH}_2$, $-\text{Cl}$) can donate these electrons to the ring via resonance, increasing electron density. These are generally activating groups.
   • -M effect: Groups with $\pi$ bonds conjugated with the ring and an electronegative atom (e.g., $-\text{NO}_2$, $-\text{CHO}$, $-\text{COOH}$) can withdraw electrons from the ring via resonance, decreasing electron density. These are generally deactivating groups.

We need to analyze the effect of each substituent on the benzene ring:

1. Anisole ($\text{C}_6\text{H}_5\text{-OCH}_3$):

   • The $-\text{OCH}_3$ (methoxy) group has an oxygen atom directly attached to the benzene ring. This oxygen atom possesses two lone pairs of electrons.
   • These lone pairs can be donated to the benzene ring through resonance, exhibiting a strong +M effect. This significantly increases the electron density on the benzene ring, particularly at the ortho and para positions.
   • While oxygen is electronegative and exerts a -I effect, the +M effect is much stronger and predominates.
   • Therefore, the $-\text{OCH}_3$ group is a strong activating group, making anisole highly reactive towards electrophiles.

2. Chlorobenzene ($\text{C}_6\text{H}_5\text{-Cl}$):

   • The $-\text{Cl}$ (chloro) group has a chlorine atom directly attached to the benzene ring. Chlorine is an electronegative atom.
   • It exerts an electron-withdrawing -I effect, pulling electron density away from the ring.
   • However, chlorine also has lone pairs of electrons that can be donated to the benzene ring through resonance, exhibiting a +M effect.
   • For halogens, the -I effect is stronger than the +M effect. This means there is a net withdrawal of electron density from the benzene ring. …