Problem 1 - A Complete Resource Book in Chemistry for JEE Main

Question

Nitrobenzene can be prepared from benzene by using a mixture of concentrated \(\mathrm{HNO}_{3}\) and concentrated \(\mathrm{H}_{2} \mathrm{SO}_{4}\). In the nitrating mixture, nitric acid acts as a (a) base (b) acid (c) reducing agent (d) catalyst

Step-by-Step Solution

Answer

In the nitrating mixture, nitric acid acts as a base (option a).

Step 1: Understanding Nitration Reaction

The process of nitration typically involves introducing a nitro group (\(\mathrm{-NO_2}\)) into an aromatic ring like benzene. This is achieved by using a nitration mixture.

Step 2: Composition of Nitrating Mixture

The nitrating mixture is commonly composed of concentrated nitric acid (\(\mathrm{HNO_3}\)) and concentrated sulfuric acid (\(\mathrm{H_2SO_4}\)). These acids work together to generate the nitronium ion (\(\mathrm{NO_2}^+\)), which is the active species that nitrates benzene.

Step 3: Role of Sulfuric Acid

Sulfuric acid acts as a strong acid and a dehydrating agent in the mixture. It helps to protonate \(\mathrm{HNO_3}\) and facilitates the formation of the nitronium ion by removing water:

Step 4: Reaction of Nitric Acid

Nitric acid acts as a base in this context by accepting protons from sulfuric acid. This protonation then leads to the formation of the nitronium ion (\(\mathrm{NO_2}^+\)) necessary for the nitration reaction with benzene.

Key Concepts

Nitronium Ion Formation

The formation of the nitronium ion, \(\mathrm{NO}_2^+\), is a key step in the nitration reaction. It plays a pivotal role as an electrophile, meaning it accepts electrons. Benzene, an aromatic compound with a rich electron cloud, is perfect for interacting with such an electron-deficient entity.

In the nitration process, the nitronium ion is generated when concentrated nitric acid \( (\mathrm{HNO}_3) \) and concentrated sulfuric acid \( (\mathrm{H}_2\mathrm{SO}_4) \) are combined. This process involves the protonation of the nitric acid followed by the removal of water to form the nitronium ion: \[\mathrm{HNO}_3 + \mathrm{H}_2\mathrm{SO}_4 \rightarrow \mathrm{NO}_2^+ + \mathrm{HSO}_4^- + \mathrm{H}_2\mathrm{O}\]This reaction is crucial for introducing the \(\mathrm{-NO_2}\) group into the benzene ring, resulting in nitrobenzene.

Role of Sulfuric Acid

Sulfuric acid \( (\mathrm{H}_2\mathrm{SO}_4) \) has a critical role in the nitration reaction. It functions as a strong acid and aids in the dehydration process. When combined with nitric acid, it helps in the generation of the powerful nitronium ion \(\mathrm{NO}_2^+\).

One of its primary functions is to protonate nitric acid. This added proton facilitates the transformation of nitric acid into an effective nitrating agent. The sulfuric acid not only donates a proton but also helps absorb the water byproduct of the reaction:\[\mathrm{H}_2\mathrm{SO}_4 + \mathrm{HNO}_3 \rightarrow \mathrm{HSO}_4^- + \mathrm{NO}_2^+ + \mathrm{H}_2\mathrm{O}\]By facilitating this dehydration, sulfuric acid shifts the equilibrium towards the formation of more nitronium ions, enhancing the efficiency of the nitration process.

Nitric Acid Function in Nitration

In the context of the nitration reaction, nitric acid \( (\mathrm{HNO}_3) \) plays a unique and vital role. Although typically thought of as an acid in other contexts, here it behaves somewhat differently. In the nitrating mixture, it acts more like a base or an acceptor of protons when interacting with sulfuric acid.

This interaction with sulfuric acid leads to the formation of the nitronium ion \(\mathrm{NO}_2^+\), the active species necessary for nitration:\[\mathrm{HNO}_3 + \mathrm{H}_2\mathrm{SO}_4 \rightarrow \mathrm{NO}_2^+ + \mathrm{HSO}_4^- + \mathrm{H}_2\mathrm{O}\]Thus, nitric acid in the nitration process is essential for forming the nitronium ion, aiding sulfuric acid by accepting the protons it donates. This protonation step is crucial to liberating the nitronium ion and enabling it to efficiently attack and bond with the benzene ring, leading to successful nitration.