Reactions of Haloarenes - Nucleophilic Substitution Reactions

Description:

Chlorobenzene can be converted into phenol under drastic conditions (Temperature:623 K and Pressure:300 atm).

The presence of an electron-withdrawing group (−NO₂) at ortho − and para−positions increases the reactivity towards NS reactions.

Mechanism of Aromatic Nucleophilic Substitution Reaction

Note: When electron-withdrawing groups are present at meta −postions, no effect is observed on reactivity. Why??

• When electron-withdrawing groups are present at meta −postions, no effect is observed on reactivity.

• During resonance, the negative charge is developed at ortho − and para − positions which are stabilized via resonance by −NO₂ group.

• Electron density is withdrawn by −NO₂ group when it is present at ortho − and para − positions such that it facilitates the attack of incoming nucleophile.

• However, when  −NO₂ is present at meta position, no resonance structure has negative charge on carbon bearing −NO₂ group as electron density is at ortho and para positions.

• Thus, no effect on reactivity towards nucleophilic substitution reaction is observed when −NO₂ is present at meta position.