15. In the reaction shown below, in the nitrating mixture HNO3 act as a:
a. acid
b. base
c. catalyst
d. reducing agent

In the nitrating mixture used for nitration of benzene, HNO₃ acts as a base because it accepts a proton from the stronger acid H₂SO₄ and then generates the nitronium ion NO2+NO2+, the actual electrophile.​​

Introduction

The role of HNO₃ in nitrating mixture is a classic concept in electrophilic aromatic substitution and a favorite MCQ in exams like CSIR NET and GATE. In a mixed acid system of concentrated HNO₃ and concentrated H₂SO₄, nitric acid surprisingly behaves as a Brønsted base, accepting a proton from sulfuric acid and ultimately providing the nitronium ion NO2+NO2+ required for nitration of benzene.​

Question and correct option

The question shown on the slide is:

“In the reaction shown below, in the nitrating mixture HNO₃ acts as:
A) acid
B) base
C) catalyst
D) reducing agent”

The correct option is B) base.​

Mechanism: why HNO₃ acts as a base

In the nitrating mixture, concentrated H₂SO₄ is a stronger acid than concentrated HNO₃, so it donates a proton to nitric acid. The key steps are:​

1. Proton transfer (acid–base step)
   HNO3+H2SO4⇌H2NO3++HSO4−HNO3+H2SO4⇌H2NO3++HSO4−
   Here, HNO₃ accepts a proton from H₂SO₄ and therefore behaves as a Brønsted base.​

2. Generation of nitronium ion
   H2NO3+→NO2++H2OH2NO3+→NO2++H2O
   The protonated nitric acid loses water to give the nitronium ion NO2+NO2+, the active electrophile that attacks benzene.​

3. Overall electrophile-forming step
   Combining the above:
   HNO3+H2SO4→NO2++HSO4−+H2OHNO3+H2SO4→NO2++HSO4−+H2O​

Thus, in this mixture, nitric acid’s function is to act as a base and then as the source of NO2+NO2+ for electrophilic aromatic substitution.​

Detailed explanation of each option

Option A: HNO₃ acts as an acid

Nitric acid is normally classified as a strong acid in aqueous solution, where it donates protons to bases like water or hydroxide. However, in the presence of an even stronger acid, H₂SO₄, it behaves differently: it accepts a proton instead of donating one, which is the defining behavior of a base under the Brønsted–Lowry concept.​

So, in the specific context of the nitrating mixture (conc. HNO₃ + conc. H₂SO₄), calling HNO₃ an acid is incorrect for this particular equilibrium step; the acid in the pair is H₂SO₄, not HNO₃.​

Option B: HNO₃ acts as a base (Correct)

HNO₃ accepts a proton from H₂SO₄ to form the protonated species H2NO3+H2NO3+. Accepting a proton is the defining property of a Brønsted base, so HNO₃ is functioning as a base in this mixture despite being a strong acid in water. After protonation, H2NO3+H2NO3+ decomposes to NO2+NO2+, which then nitrates benzene, showing that HNO₃ also serves as the electrophile precursor.​

Option C: HNO₃ acts as a catalyst

A catalyst should be regenerated at the end of the reaction and not be consumed overall. In benzene nitration, HNO₃ is consumed; one molecule of HNO₃ effectively becomes part of the nitro group in nitrobenzene, and water is formed as a by‑product. Instead, H₂SO₄ is often described as the acid catalyst, because it helps generate NO2+NO2+ and is regenerated after the electrophilic substitution step. Therefore, HNO₃ is not acting as a catalyst.​

Option D: HNO₃ acts as a reducing agent

A reducing agent donates electrons and gets oxidized in the process. Concentrated nitric acid is typically an oxidizing agent, not a reducing agent; it tends to accept electrons and oxidize other substances. In the nitration of benzene, the key role of HNO₃ is acid–base behavior and electrophile generation, not electron donation, so describing it as a reducing agent here is clearly incorrect.​

Summary table of options

This analysis shows that in the nitrating mixture used for benzene nitration, HNO₃ behaves as a base and as the source of the nitronium ion, so the correct answer to the multiple-choice question is option B: base.​