Question

Regarding the mechanism of electrophilic substitution, the false statement is:
A).Rate limiting step is formation of arenium ion
B).Arenium ion can stabilize through resonance
C).Arenium ion is aromatic
D).Initial step is generation of electrophile

Answer 1

At first think about the electrophilic substitution reactions. Electrophilic substitution reactions are chemical reactions in which an electrophile displaces a functional group in a compound but not always a hydrogen atom.

Complete step by step answer:
Electrophilic substitution reactions generally proceed via a three step mechanism that involves the following steps,

1. Generation of an electrophile
   2)Formation of a carbocation
   3)The removal of a proton from the intermediate
   Step $1$ :-Anhydrous aluminum chloride is a very useful lewis acid in the generation of electrophile from the chlorination, alkylation and acylation of the aromatic ring. The resulting electrophiles are $C{l^ + },{R^ + },R{C^ + }O$ respectively as shown below.
   $Cl - Cl + AlC{l_3} \to C{l^ + } + {[AlC{l_4}]^ - } \\\ C{H_3} - Cl + AlC{l_3} \to C{H_3}^ + + {[AlC{l_4}]^ - } \\\ C{H_3} - CO - Cl + AlC{l_3} \to C{H_3} - {C^ + }O + {[AlC{l_4}]^ - } \\\$
   Step $2$ :-The electrophile attacks the aromatic ring forming a sigma complex or an arenium ion. One of the carbons in this arenium ion is $s{p^3}$hybridized.
   ${C_6}{H_6} + {E^ + } \to {C_6}{H_4}E - {H^ + }$
   Step $3$ :-In order to restore the aromatic character, the sigma complex releases a proton from the $s{p^3}$hybridized carbon when it is attacked by the ${[AlC{l_4}]^ - }$. The reaction describing the removal of a proton from the sigma complex is given below
   ${C_6}{H_4}E - {H^ + }\xrightarrow{{{{[AlC{l_4}]}^ - }}}{C_6}{H_5}E + HCl + AlC{l_3}$
   In the mechanism of electrophilic substitution reaction, rate limiting step is formation of arenium ions. Arenium ions can stabilize through resonance. Initial step is generation of electrophile. Arenium is not aromatic because it has only $5\pi$ electrons.

The false statement is C.

Note:
Arenium ion finds stability in a resonance structure. Since the delocalization of the electron stops at the $s{p^3}$ hybridized carbon, the sigma complex or the arenium ion loses its aromatic character. The aromatic compound has $(4n + 2)\pi$ electrons.