Question

Explain what the difference is between the alkyl substitution reactions and addition/elimination type reactions in EAS (electrophilic aromatic substitution).

Answer 1

In saturated hydrocarbons, substitution takes place , for this C-H bond Breaks and in place of it C-X bond will form where X is an atom which gets substituted.

In benzene or other aromatic compounds double bonds are present in conjugation. If a molecule gets add up across double bond its conjugation gets lost it will not show resonance anymore in short system gets destabilised.

For this reason, substitution occurs in these systems in which double bond remain intact. For e.g

Benzene + Cl₂ ---lewis acid----> chlorobenzene

Here Cl⁺attacks on benzene ring as an electrophille which is generated in presence of lewis acid New C-Cl bond forms and H⁺ gets removed by breaking of old C-H bond. This is electrophillic substitution in which addition followed by elimination takes place.

In alkyl substitution bond breaking and bond forming takes place simultaneously because sp³ C cannot form 5 bonds .

In aromatic compounds each C is sp² which after attachment with electrophille becomes sp³ and then as it looses H⁺ again it becomes sp² .