Question

Which of the following can’t be used in Friedel-Crafts reaction?
A) ${\text{FeC}}{{\text{l}}_3}$
B) ${\text{SnC}}{{\text{l}}_4}$
C) ${\text{AlC}}{{\text{l}}_3}$
D) ${\text{NaCl}}$

Answer 1

Hint : The Friedel Craft reaction takes place in presence of Lewis acid. Lewis acid is electron deficient in nature. ${\text{FeC}}{{\text{l}}_3}$ , ${\text{SnC}}{{\text{l}}_4}$ and ${\text{AlC}}{{\text{l}}_3}$ act as Lewis acid in Friedel Craft reaction.

Complete Step By Step Answer:
Friedel craft reaction takes place in presence of Lewis acid. Lewis acid is an electron-pair acceptor which means it has to have a vacant orbital to accept those electrons.
${\text{FeC}}{{\text{l}}_3}$ , ${\text{SnC}}{{\text{l}}_4}$ and ${\text{AlC}}{{\text{l}}_3}$ act as Lewis acid as explained below-
The ${\text{Cl}}$ atom due to high electronegativity reduces electron density from central atom
The central atom has a vacant orbital so it acts as an electrophile and attacks electron rich species.
This replaces the aromatic proton and the number of carbon atoms in the product formed is increased.
${\text{NaCl}}$ does not act as Lewis acid because it forms cation ${\text{N}}{{\text{a}}^ + }$ which has very less tendency to accept electrons , as it has inert gas configuration.
Hence, the correct answer is ‘D’.

Additional Information:
Friedel craft reaction is of two types-
1. Friedel craft alkylation reaction- In this, aromatic ring reacts with alkyl halide in presence of Lewis acid to form alkyl aromatic compound. The reaction can be written as-
$\Rightarrow$ ${\text{Aromatic ring + Alkyl Halide}}\xrightarrow{{{\text{Lewis acid}}}}{\text{Alkyl aromatic compound}}$
Example-

Here the aromatic ring is benzene.
2. Friedel craft acylation reaction- In this reaction, addition of acyl group takes place in presence of Lewis acid. The reaction can be written as-
$\Rightarrow$ ${\text{Aromatic ring + RCOX}}\xrightarrow{{{\text{Lewis acid}}}}{\text{Acyl aromatic compound}}$
Example-

Here, the aromatic ring is benzene.

Note :
In Friedel craft reaction, the Lewis acid is used as it helps in the formation of carbocation which is an electrophile. This electrophile replaces the aromatic proton and increases the number of carbon atoms in the product during conversions.