Question

Question 9 

Identify the electrophile in the nitration of benzene reaction. 

a) NO₂⁺ 

b) NO₃⁺ 

c) NO₂ 

d) HNO₃ 

Question 10 

Which of the marked hydrogens is most acidic? 

Answer 1

Nitration: The process of nitration of an aromatic compound is the replacement of an acidic proton by nitronium ion. It is an example of aromatic electrophilic substitution reaction. An electron donating group increases the electron density in the aromatic compound. This favours and so increases the rate of the nitration. An electron withdrawing group on the other hand decreases the rate of nitration.

Ans 9. A mixture of sulphuric acid (H₂SO₄) and nitric acid (HNO₃) is used as the nitrating agent for the nitration of benzene. The nitrating agent upon reaction gives nitronium ion that acts as an electrophile.

The reaction of H₂SO₄ and HNO₃ is given below.

$HO-NO_{2}+H-SO_{4}H\rightarrow H_{2}O^{+}-NO_{2}+HSO_{4}^{-}$

Sulphuric acid being more acidic than nitric acid gives away its proton. This proton is accepted by nitric acid.

$H_{2}O^{+}-NO_{2}+HSO_{4}^{-}\rightarrow NO_{2}^{+}+HSO_{4}^{-}+H_{2}O$

Protonated nitric acid releases the water molecule. This rearrangement results in the formation of nitronium ion NO₂⁺, HSO₄^- and H₂O.

Therefore, the electrophile in the nitration of benzene is nitronium ion NO₂⁺.