Problem 83 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following statements relating to aniline is not true? (a) aniline on Schotten-Baumann reaction gives benzanilide (b) aniline cannot be prepared by the reduction of benzonitrile with \(\mathrm{LiAlH}_{4}\) (c) on heating with concentrated \(\mathrm{H}_{2} \mathrm{SO}_{4}\) at \(180^{\circ} \mathrm{C}\), aniline gives sulphanilic acid (d) aniline liberates nitrogen on treatment with ice cold nitrous acid

Step-by-Step Solution

Answer

Statement (b) is not true.

Step 1: Understanding the Schotten-Baumann Reaction

The Schotten-Baumann reaction involves the acylation of amines, such as aniline, in the presence of a base like sodium hydroxide to produce amides. In the case of aniline, reacting it with benzoyl chloride under these conditions yields benzanilide. Therefore, statement (a) is true.

Step 2: Reduction of Benzonitrile

Benzonitrile can be reduced by a strong reducing agent like lithium aluminum hydride (\( \mathrm{LiAlH}_{4} \)). This reduction converts the nitrile group (\( \mathrm{-C \equiv N} \)) to an amine group (\( \mathrm{-NH}_{2} \)), forming aniline. Therefore, statement (b) is false, as it's claimed that aniline cannot be prepared this way.

Step 3: Sulphanilic Acid Formation

When aniline is heated with concentrated sulfuric acid (\( \mathrm{H}_{2} \mathrm{SO}_{4} \)) at high temperatures (up to 180°C), it undergoes sulfonation to form sulphanilic acid. Therefore, statement (c) is true.

Step 4: Nitrogen Liberation with Nitrous Acid

When aniline is treated with ice-cold nitrous acid, it forms a diazonium salt, which can decompose to liberate nitrogen gas. This is a typical reaction of primary aromatic amines with nitrous acid. Therefore, statement (d) is true.

Key Concepts

Schotten-Baumann Reaction

The Schotten-Baumann reaction is a popular method for acylating amines to form amides. It typically involves the use of an acid chloride and a base, such as sodium hydroxide. The role of the base here is crucial as it neutralizes the hydrochloric acid produced during the reaction, thereby driving the reaction forward. In the case of aniline, reacting it with benzoyl chloride under Schotten-Baumann conditions yields benzanilide, a process that clearly demonstrates the transformation of aniline into an amide. This reaction falls within organic synthesis techniques and is named after the chemists who conceptualized it.

Reduction of Benzonitrile

Benzonitrile, a compound containing a nitrile group, can be reduced to aniline using a strong reducing agent such as lithium aluminum hydride (\( \mathrm{LiAlH}_{4} \)). This reduction involves breaking the triple bond in the nitrile group, converting it into a primary amine. Although the original statement claims the impossibility of producing aniline from benzonitrile through this method, the reduction is indeed feasible and results in high yields. Understanding this provides insight into how functional groups can be manipulated in organic chemistry to synthesize desired compounds.

Sulphanilic Acid Formation

Heating aniline with concentrated sulfuric acid at a high temperature, like 180°C, leads to the formation of sulphanilic acid. This reaction is a type of sulfonation process. Here, the sulfonic acid group attaches to the aromatic ring of aniline, leading to the formation of sulphanilic acid, a compound widely used in dyes and as an intermediate in various chemical processes. The reaction conditions, particularly the high temperature, are crucial for this transformation as they provide the necessary energy for the sulfonic acid group to attach efficiently.

Nitrous Acid Reaction

Aniline reacts with nitrous acid under cold conditions to form a diazonium salt. This reaction is typical for primary aromatic amines and doesn't stop simply at forming the salt. The diazonium salt can further decompose, especially at low temperatures, to release nitrogen gas. This characteristic liberation of nitrogen is a key identifying reaction in organic chemistry for primary aromatic amines. Understanding the formation and subsequent decomposition of diazonium salts is crucial for anyone studying the reactivity of amine groups in aromatic chemistry.