-nitrobenzene, none of the resonating structures bear the negative charge on carbon atom bearing the –NO group. Therefore, the presence of nitro group at meta - position does not stabilise the negative charge and no effect on reactivity is observed by the presence of –NO group at meta -position. . Electrophilic substitution reactions Haloarenes undergo the usual electrophilic reactions of the benzene ring such as halogenation, nitration, sulphonation and Friedel-Crafts reactions.
Halogen atom besides being slightly deactivating is o, p - directing; therefore, further substitution occurs at ortho- and para - positions with respect to the halogen atom. The o, p -directing influence of halogen atom can be easily understood if we consider the resonating structures of halobenzene as shown: Due to resonance, the electron density increases more at ortho- and para -positions than at meta -positions. Further, the halogen atom because of its –I effect has some tendency to withdraw electrons from the benzene ring. As a result, the ring gets somewhat deactivated as compared to benzene and hence the electrophilic substitution reactions in haloarenes occur slowly and require more drastic conditions as compared to those in benzene.
(i) Halogenation (ii) Nitration (iii) Sulphonation Haloalkanes and Haloarenes (iv) Friedel-Crafts reaction Although chlorine is an electron withdrawing group, yet it is ortho -, para - directing in electrophilic aromatic substitution reactions. Why? Chlorine withdraws electrons through inductive effect and releases electrons through resonance. Through inductive effect, chlorine destabilises the intermediate carbocation formed during the electrophilic substitution.
Through resonance, halogen tends to stabilise the carbocation and the effect is more pronounced at ortho - and para - positions. The inductive effect is stronger than resonance and causes net electron withdrawal and thus causes net deactivation. The resonance effect tends to oppose the inductive effect for the attack at ortho - and para - positions and hence makes the deactivation less for ortho - and para - attack. Reactivity is thus controlled by the stronger inductive effect and orientation is controlled by resonance effect.