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

Explanation

Due to the presence of a `pi`-electron cloud above and below the plane of alkenes and arenes , these are electron rich molecules and hence attract electrophiles and thus undergo electrophilic reactions. Alkenes , undergo electrophilic addition rather than substitution reactions because of the following two reasons :

(i) In electrophilic substitution reactions, one `sigma`-bond is broken and another is formed . Since the bond energies of the `sigma` C-H bond broken and new `sigma` C-X bond formed are not much different , therefore, electrophilic substitution reactions of alkenes are not accompanied by large energy changes and hence are energetically not favourable
(ii)In electrophillic addition reactions, one weak `pi`-bond is broken and two strong `sigma`-bonds (C-X and C-Y) are formed , therefore , electrophilic addition reactions of alkenes are accompanied by large energy changes and hence are energetically favourable . In other words, the typical reactions of alkenes are electrophilic addition reactions and not electrophilic substitution reactions.
In contrast, arenes undergo electrophilic substitution reactions rather than electrophilic addition reactions as explained below :

Benzene has a large resonance energy of `150.6 "kJ mol"^(-1)` . During electrophilic addition reactions , two new `sigma`-bonds are formed but the aromatic character of the benzene ring is destroyed, i.e., the resonance energy of the benzene ring is lost. Since the loss in resonance energy is more than the energy released during formation of two additional `sigma`-bonds, therefore , electrophilic addition reaction is not energetically favourable.
In contrast, during electrophilic substitution reactions, one `sigma`C-H bond is broken and one new `sigma` C-X bond is formed but the aromatic character of the benzene ring remains intact , i.e., resonance energy of the benzene ring remains intact. In other words, small energy changes , if any , during making and breaking of `sigma`-bonds is more than compensated by the large resonance energy of the benzene ring. Thus, electrophilic substitution reactions of arenes are energetically more favourable than electrophilic addition reactions. Thus, from the above discussion, it follows that alkenes undergo electrophilic addition reactions while arenes undergo electrophilic substitution reactions.