Why do we get isopropyl benzene on treating benzene with -chloropropane instead of n -propyl benzene? (v) Friedel-Crafts acylation reaction: The reaction of benzene with an acyl halide or acid anhydride in the presence of Lewis acids (AlCl ) yields acyl benzene. ( . ) ( .
) If excess of electrophilic reagent is used, further substitution reaction may take place in which other hydrogen atoms of benzene ring may also be successively replaced by the electrophile. For example, benzene on treatment with excess of chlorine in the presence of anhydrous AlCl can be chlorinated to hexachlorobenzene (C Cl ) ( . ) Mechanism of electrophilic substitution reactions: According to experimental evidences, S E (S = substitution; E = electrophilic) reactions are supposed to proceed via the following three steps: (a) Generation of the eletrophile (b) Formation of carbocation intermediate (c) Removal of proton from the carbocation intermediate (a) Generation of electrophile E ⊕ : During chlorination, alkylation and acylation of benzene, anhydrous AlCl , being a Lewis acid helps in generation of the elctrophile Cl ⊕ , R ⊕ , RC ⊕ O (acylium ion) respectively by combining with the attacking reagent. In the case of nitration, the electrophile, nitronium ion, is produced by transfer of a proton (from sulphuric acid) to nitric acid in the following manner: Step I Step II Protonated Nitronium nitric acid ion It is interesting to note that in the process of generation of nitronium ion, sulphuric acid serves as an acid and nitric acid as a base.
Thus, it is a simple acid-base equilibrium. (b) F o r m a t i o n o f C a r b o c a t i o n (arenium ion): Attack of electrophile results in the formation of σ -complex or arenium ion in which one of the carbon is sp hybridised. sigma complex (arenium ion) The arenium ion gets stabilised by resonance: Sigma complex or arenium ion loses its aromatic character because delocalisation of electrons stops at sp hybridised carbon. (c) Removal of proton: To restore the aromatic character, σ