Question

Electrophilic Aromatic Substitution.

Answer 1

The electrophilic aromatic substitution reactions proceed by a common mechanism. Therefore, we will first deal the mechanism, in general, and then apply it to specific substitution reactions like nitration, halogenation, etc. General mechanism. Electrophilic aromatic substitution reactions are believed to proceed by bimolecular (SE₂) mechanism which involves three steps. 

(i) Generation of an electrophile (electrophilic reagent) First of all the attacking electiophile, say X⁺, is generated from the reagent, say XY.

\(X : Y \longrightarrow \underset {Electrophile}{X^+} + : Y ^-\)

(ii) Formation of the intermediate carbonium ion. On the approach of the electrophile X⁺, benzene undergoes electromeric effect (as shown by the curved arrow) with the result one carbon atom of the benzene ring gets positively charged and the other gets negatively charged. The elctrophile then attacks on the negative centre resulting in the formation of an intermediate known as aronium (carbonium) cation, \(\sigma\)-complex² or the pentadienyl cation³, I.

Electromeric effect in benzene	Resonating structures of the carbonium ion	Resonance hybrid of the carbonium ion
	Where X+ = \(NO _2 \, ^+\) (nitronium ion) in nitration
	= CI+ (chloronium ion) in chlorination
	SO3 (sulphur trioxide) in sulphonation
	\(=CH_3 \,^+\) (methyl cation) in alkylation
	\(=\overset {+}{C} O CH _3\) (acetyl cation) in acetylation

The carbonium ion I is stabilised by resonating structures I, II and III; and the resonance hybrid can be represented by the structure IV. Thus note the positive charge on the intermediate carbonium ion is not confined to one carbon but is disperesed over the entire molecule especially on the o- and p-positions with respect to the carbon carrying the attacking reagent.

Since formation of intermediate carbonium ion is a slow step, it constitutes the rate determining step of the complete reaction.

(iii) Formation of the product. The reaction is completed by the abstraction of a proton from the carbon atom bearing electrophile by an anionic species (: Y^-) present in the reaction mixture to form a substituted product.