Problem 161 - Arihant Objective Approach to Organic Chemistry for IIT JEE and NEET all Medical Entrances Part 4

Question

Hydrolysis of benzonitrile by dilute HCl yields: (a) benzoic acid (b) aniline (c) benzaldehyde (d) benzamide

Step-by-Step Solution

Answer

(a) benzoic acid

Step 1: Understand the Hydrolysis Reaction

Benzonitrile, when subjected to hydrolysis, reacts with water. In the presence of an acid like dilute HCl, this typically results in the conversion of nitriles into carboxylic acids. The reaction involves breaking the C-N triple bond and forming a C-O bond.

Step 2: Identify the Product

The hydrolysis of benzonitrile in the presence of acid yields benzoic acid. The nitrile group (-CN) gets converted to a carboxylic acid group (-COOH).

Step 3: Review Other Options

To ensure understanding, review why other options are incorrect: - Aniline (b) is an amine, not related to nitrile hydrolysis. - Benzaldehyde (c) is an aldehyde, not typically formed from nitrile hydrolysis. - Benzamide (d) can be formed during partial hydrolysis but not the final step in presence of dilute HCl.

Key Concepts

Carboxylic Acids

Carboxylic acids are a key functional group in organic chemistry. They are characterized by having the functional group \[ -COOH \], often known as a carboxyl group. This group consists of a carbonyl (C=O) and a hydroxyl group (OH), both bonded to the same carbon atom. These compounds are known for their acidic properties, allowing them to donate a proton in a reaction. This acidic characteristic comes from the resonance stabilization of the carboxylate ion, which forms when a proton is lost.
Among carboxylic acids, benzoic acid is a common compound, especially relevant in the context of nitrile hydrolysis. Benzonitrile, when hydrolyzed, forms benzoic acid, showcasing the transformation of a nitrile into a carboxylic acid. Understanding this transformation helps in recognizing the formation of functional groups through specific reactions.
Carboxylic acids have significant applications owing to their reactive nature. They're often used in food preservation, pharmaceuticals, and synthetic chemistry. Their ability to form esters and amides expands their utility in various chemical syntheses and industrial applications.

Nitrile Hydrolysis

Nitrile hydrolysis is a chemical reaction where a nitrile compound is converted into a carboxylic acid. The process involves breaking the nitrile group \[ -C \equiv N \], which is characterized by a strong triple bond between carbon and nitrogen, and integrating an oxygen, forming the carboxylic acid group \[ -COOH \].
This reaction can be classified as both acid-catalyzed or base-catalyzed, although the exercise specifically mentions the acid-mediated step involving dilute HCl. In such an acidic environment, the nitrile's triple bond is susceptible to attack by water, leading to the eventual breakdown and transformation into a carboxylic acid. Environments that allow for the protonation of the nitrile nitrogen facilitate this process, making the carbon more electrophilic and thus more reactive towards a nucleophilic attack by water.
Hydrolysis of nitriles is an essential reaction in organic chemistry because it provides a straightforward method to synthesize carboxylic acids, which are otherwise challenging to prepare directly from certain organic compounds.

Reaction Mechanism

Understanding the reaction mechanism provides insight into how benzonitrile hydrolyzes into benzoic acid. Initially, in the presence of an acid such as dilute HCl, the nitrogen in the \[ -C \equiv N \] group of benzonitrile is protonated. This raises the carbon's susceptibility to nucleophilic attack.
The next step involves water acting as a nucleophile. Water's oxygen attacks the electrophilic carbon, prompting the formation of an iminol intermediate. Subsequently, rearrangement and proton transfer steps occur leading to the formation of an amide, often benzamide in partial hydrolysis.
In the fully acidic environment, further protonation and reaction with water lead to breaking the amide bond, yielding the carboxylic acid, benzoic acid. This conversion entails several proton transfer and structural rearrangement steps, depicting an intricate pathway from nitrile to the final acid product. Through each phase, the acid catalyst helps facilitate the progression and the stability of intermediates, steering the reaction to completion.