Why does benzene undergo electrophilic substitution reactions easily and nucleophilic substitutions with difficulty?
Benzene is a hybrid of various resonating structures, resulting in six delocalised electrons between the six carbon atoms of the ring. This creates a region of high electron density that easily attracts electrophiles. Hence, benzene undergoes electrophilic substitution reactions easily.
Conversely, nucleophiles are electron-rich species and are repelled by the electron-rich benzene ring, making it difficult for them to attack. Additionally, benzene possesses unusual stability due to resonance. While electrophilic substitution retains this stable resonating system, nucleophilic attack is unfavorable. Therefore, benzene undergoes nucleophilic substitutions with difficulty.
Explanation
The textbook context explains that benzene has six delocalised electrons due to resonance, which gives it unusual stability. The high electron density from these delocalised electrons attracts electrophiles easily, facilitating electrophilic substitution. On the other hand, nucleophiles, being electron-rich, are repelled by the electron-rich benzene ring, making nucleophilic substitution difficult. The preference for substitution over addition is also attributed to the resonance stability of the benzene ring.