Question

In the sulphonation of benzene active electrophile is
A. ${\text{S}}{{\text{O}}_2}$
B. ${\text{S}}{{\text{O}}_3}$
C. ${\text{SO}}_4^{2 - }$
D. ${\text{HSO}}_4^ - $

Answer 1

Hint: Sulphonation of benzene takes place in presence of sulphuric acid. The sulphonation adds an electrophile sulphur trioxide on benzene. The product of sulphonation of benzene is benzene sulphonic acid.

Step by step answer: 

Sulphonation of benzene takes place as follows:
All steps of sulphonation are reversible.
First, the formation of electrophile:
${{\text{H}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}} \rightleftarrows \,{{\text{H}}_{\text{2}}}{\text{O}}\,{\text{ + }}\,{\text{S}}{{\text{O}}_{\text{3}}}$
The sulphuric acid generates an electrophile sulphur trioxide.
Second, the attack of benzene on electrophile:

The attack of benzene on electrophile generates a positive charge on the ring which is delocalized. The delocalized form is known as arenium ion.
Third, removal of hydrogen of benzene by oxygen of sulphur trioxide:

So, the active electrophile in sulphonation of benzene is sulphur trioxide.
The sulphur atom in ${\text{S}}{{\text{O}}_2}$ has a lone pair, so it works as a nucleophile, so option (A) is incorrect.
Sulphur trioxide is an electrophile so, (B) is correct.
${\text{SO}}_4^{2 - }$ has a negative charge, so it will work as a nucleophile, so option (C) is incorrect.
${\text{HSO}}_4^ - $ has a negative charge, so it will work as a nucleophile, so option (D) is incorrect.

Therefore, option (B) ${\text{S}}{{\text{O}}_3}$, is correct.

Note: Sulphonation of benzene is an electrophilic substitution reaction. In an electrophilic substitution reaction, an electrophile substitutes another electrophile. An electrophile can be identified as the species having positive charge or incomplete octet. In sulphur trioxide, the electronegativity of oxygen is more than sulphur. Three oxygens attract the electron density toward them so, sulphur gets slightly positive nature hence, sulphur trioxide work as an electrophile