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: Identify the Statements to Analyze

We are given four statements about aniline, and we need to determine which one is not true. The statements involve: (a) Schotten-Baumann reaction, (b) reduction of benzonitrile, (c) heating with concentrated sulfuric acid, and (d) reaction with nitrous acid.

Step 2: Analyze Statement A

Statement (a) claims that aniline on Schotten-Baumann reaction gives benzanilide. The Schotten-Baumann reaction is an acylation reaction where an amine reacts with an acid chloride in the presence of a base to form an amide. Aniline reacts with benzoyl chloride to give benzanilide. This statement is true.

Step 3: Analyze Statement B

Statement (b) asserts that aniline cannot be prepared by the reduction of benzonitrile with \( \text{LiAlH}_4 \). Benzonitrile can be reduced by \( \text{LiAlH}_4 \) to form benzylamine, not aniline. Aniline does not result from this reduction process, making this statement true.

Step 4: Analyze Statement C

Statement (c) suggests that on heating with concentrated \( \text{H}_2 \text{SO}_4 \) at \(180^\circ \text{C}\), aniline gives sulphanilic acid. Aniline reacts with sulfuric acid to produce sulfanilic acid through sulfonation, a known reaction. This statement is true.

Step 5: Analyze Statement D

Statement (d) claims that aniline liberates nitrogen on treatment with ice cold nitrous acid. When aniline, a primary aromatic amine, reacts with nitrous acid, it forms a diazonium salt, which does release nitrogen \( (N_2) \) when decomposing. Hence, this statement is true.

Step 6: Determine the Incorrect Statement

Having evaluated each statement, all described reactions, except for statement (b), involve correct descriptions. Since aniline is not formed by reducing benzonitrile with \( \text{LiAlH}_4 \), statement (b) is misleading as it suggests the impossibility of any reaction but doesn't match the result.

Key Concepts

Schotten-Baumann Reaction

The Schotten-Baumann reaction is a fundamental method of forming amides from amines. In this process, an amine reacts with an acid chloride in the presence of a base. The role of the base is crucial as it helps to neutralize the hydrochloric acid formed during the reaction.

For instance, when aniline, a primary aromatic amine, reacts with benzoyl chloride in a basic environment like aqueous sodium hydroxide, the product formed is benzanilide, an amide. This reaction is typically conducted at room temperature and is considered quite efficient for producing amides.

Key points to remember:

• Aniline directly reacts with an acid chloride under basic conditions.
• The method is well-suited for aromatic amines.
• The base prevents unwanted side reactions by neutralizing by-products like HCl.

Overall, the Schotten-Baumann reaction is useful for synthesizing amides like benzanilide from aromatic amines such as aniline.

Reduction of Benzonitrile

Benzonitrile reduction is a notable process in organic chemistry, particularly when discussing the preparation of different types of amines. This reaction specifically requires a strong reducing agent, such as lithium aluminium hydride ( LiAlH_4 ). While this agent is effective in reducing nitriles to primary amines, the compound formed from benzonitrile reduction is benzylamine, not aniline, which illustrates an important dichotomy.

Important distinctions include:

• Benzonitrile reduction does not yield aniline.
• Instead, the product is benzylamine, another primary amine but not directly related.
• Aniline is usually synthesized via different pathways, often involving nitrobenzene reduction.

This process highlights the selective formation of compounds in organic synthesis and the specificity of reducing agents like LiAlH_4 .

Sulphonation with Sulfuric Acid

Sulphonation is a chemical reaction where aromatic compounds like aniline are treated with concentrated sulfuric acid, leading to the formation of sulfonic acids. When aniline undergoes sulphonation, sulfanilic acid is produced.

Details of this reaction:

• Aniline reacts notably under heat (around 180°C).
• The process requires concentrated H_2SO_4 , enhancing the electrophilic substitution on the aromatic ring.
• Sulfanilic acid formed is used in various chemical applications, especially in dyes.

The transformation of aniline to sulfanilic acid reflects its utility in producing compounds for industrial uses, such as dye intermediates.

Reaction with Nitrous Acid

The reaction between aniline and nitrous acid is a classic example of diazotization. Here, a primary aromatic amine like aniline reacts with nitrous acid, often generated in-situ by mixing sodium nitrite with hydrochloric acid, at cold temperatures, usually around 0°C.

This leads to the formation of a diazonium salt, which is notorious for its instability at higher temperatures as it tends to decompose, liberating nitrogen gas ( N_2 ).

Highlights include:

• Occurs at cold temperatures to maintain the stability of the diazonium salt.
• Diazonium salts are important intermediates in the synthesis of azo dyes.
• Nitrogen liberation is a characteristic aspect when these diazonium salts are decomposed.

This reaction illustrates the transformation of simple aromatic amines to versatile intermediates used in further synthetic transformations.