Why do alkenes prefer to undergo electrophilec addition reaction while arenes prefer electrophilic substitution reactions ? Explain.

Explanation

Due to the presence of `pi`-electron cloud above and below the plane of alkenes and arenes, these are electron rich molecules and therefore, provide sites for the attack of electrophiles. Hence, they undergo electrophilic reactions, The alkenes undergo electrophilic addition reactions because alkenes are unsaturated molecules. For example,

Arenes, on the other hand, cannot undergo electrophilic addition reactions. This is because benzene has a large resonance energy of `150.4 kJ mol^(-1)`. During electrophilic addition reaction, two new `sigma`-bonds are formed but the aromatic character of benzene gets destroyed and therefore, resonance energy of benzene ring is lost. Hence, electrophilic addition reactions of arenes are not energetically favourable, Arenes, in contrast undergo electrophilic H substitution reactions in which `sigma` C-H bond is broken and new `sigma` C-X bond is formed. The aromatic character of benzene ring is not destroyed and benzene retains its resonance energy. Hence, arenes undergo electrophilic substitution reactions,