There are certain similarities between the electrophilic aromatic substitution reactions of benzene and electrophilic addition reactions of olefins. But before we summarise the various points of similarities, let us rewrite the mechanism of both the reactions.
Now let us summarize the similarities between the two reactions.
(i) Both types of reactions are polar, two-step processes and involve the attack of elctrophilic reagent on the sp2 hybridised carbon.
(ii) In both cases the rate determining step is the attack of an electrophile at carbon (first step) to form carbocation.
So we see that the two reactions are quite similar upto the formation of carbocation but after this the two reactions differ very much as one regenerates the original structure with a substituent, Le a substituted product is formed, while the other (addition on olefins) gives an addition product and thus the original structure (presence of double bond) is destroyed. The difference is due to the fact that in substitution reactions the regenerated benzene nucleus, obtained by the loss of proton from the carbocation, stabilizes itself by resonance while there is no such comparable large resonance stabilisation in case of addition reactions on alkenes which thus tend to add the nucleophile.
