carbon atom in C— X bond: In haloalkane, the carbon atom attached to halogen is sp hybridised while in case of haloarene, the carbon atom attached to halogen is sp -hybridised. The sp hybridised carbon with a greater s -character is more electronegative and can hold the electron pair of C— X bond more tightly than sp -hybridised carbon in haloalkane with less s -chararcter. Thus, C— Cl bond length in haloalkane is 177pm while in haloarene is pm. Since it is difficult to break a shorter bond than a longer bond, therefore, haloarenes are less reactive than haloalkanes towards nucleophilic substitution reaction.
(iii) Instability of phenyl cation: In case of haloarenes, the phenyl cation formed as a result of self-ionisation will not be stabilised by resonance and therefore, S N mechanism is ruled out. (iv) Because of the possible repulsion, it is less likely for the electron rich nucleophile to approach electron rich arenes. Replacement by hydroxyl group Chlorobenzene can be converted into phenol by heating in aqueous sodium hydroxide solution at a temperature of 623K and a pressure of atmospheres. The presence of an electron withdrawing group (-NO ) at ortho - and para -positions increases the reactivity of haloarenes.
Haloalkanes and Haloarenes The effect is pronounced when (-NO ) group is introduced at ortho - and para - positions. However, no effect on reactivity of haloarenes is observed by the presence of electron withdrawing group at meta -position. Mechanism of the reaction is as depicted: Can you think why does NO group show its effect only at ortho - and para - positions and not at meta - position? As shown, the presence of nitro group at ortho - and para -positions withdraws the electron density from the benzene ring and thus facilitates the attack of the nucleophile on haloarene.
The carbanion thus formed is stabilised through resonance. The negative charge appeared at ortho - and para - positions with respect to the halogen substituent is stabilised by –NO group while in case of meta