Problem 131 - A Complete Resource Book in Chemistry for JEE Main

Question

The end product (b) in the following sequence of reactions \(\mathrm{CH}_{3} \mathrm{Cl} \stackrel{\mathrm{KCN}}{\longrightarrow}\) (a) \(\stackrel{\mathrm{H}^{\prime} / \mathrm{H}_{2} \mathrm{O}}{\longrightarrow}\) (b) is (a) \(\mathrm{HCOOH}\) (b) \(\mathrm{CH}_{3} \mathrm{NH}_{2}\) (c) \(\mathrm{CH}_{3} \mathrm{COOH}\) (d) \(\mathrm{CH}_{3} \mathrm{COCH}_{3}\)

Step-by-Step Solution

Answer

The correct option is (c) \( \text{CH}_3\text{COOH} \).

Step 1: Identify the Initial Reaction

The first reaction is between \( \text{CH}_3\text{Cl} \) and \( \text{KCN} \). In this step, \( \text{CH}_3\text{Cl} \) undergoes a nucleophilic substitution reaction where chloride \( \text{Cl}^- \) is replaced by the cyanide ion \( \text{CN}^- \). Thus, the compound changes from \( \text{CH}_3\text{Cl} \) to \( \text{CH}_3\text{CN} \).

Step 2: Convert Nitrile to Amide

The intermediate formed is \( \text{CH}_3\text{CN} \), which is a nitrile. In the presence of an acidic aqueous solution \( (\text{H}^+ / \text{H}_2\text{O}) \), this can further undergo hydrolysis to form an amide. However, without heat or further alteration, this doesn't yet turn into a carboxylic acid directly.

Step 3: Hydrolysis to Acid

Upon continued hydrolysis, the amide converts into a carboxylic acid. Therefore, \( \text{CH}_3\text{CN} \) is hydrolyzed through the amide dereivative to yield \( \text{CH}_3\text{COOH} \), which is acetic acid.

Key Concepts

Nucleophilic Substitution

In organic chemistry, a nucleophilic substitution is a fundamental type of reaction where a nucleophile replaces a leaving group in a molecule. In simpler terms, a nucleophile, which is a molecule or ion with a pair of electrons ready to donate, attacks a carbon atom that is attached to a more electronegative leaving group, such as a halide. This results in the substitution or replacement of the leaving group. In the context of our exercise, the substrate is methyl chloride (\( \text{CH}_3\text{Cl} \)), and the nucleophile is the cyanide ion (\( \text{CN}^- \)). The chloride ion (\( \text{Cl}^- \)) detaches, allowing the cyanide ion to attach and form methyl cyanide (\( \text{CH}_3\text{CN} \)). The cyanide acts as a stronger nucleophile compared to chloride because it can effectively share its electron pair to form a new bond with carbon. As a result, a nucleophilic substitution reaction efficiently transforms the initial reactant into the desired product.

Cyanide Ion

The cyanide ion (\( \text{CN}^- \)) is a negatively charged ion comprising one carbon atom double-bonded to a nitrogen atom. It is known for its strong nucleophilic behavior due to its high electron density on carbon, making it capable of attacking electrophilic centers, such as the carbon in methyl chloride. A characteristic feature of the cyanide ion is its linear shape and stability as a nucleophile. Utilizing cyanide in nucleophilic substitution reactions allows chemists to introduce the cyano group (\( \text{-C} \equiv \text{N} \)) into organic molecules, ultimately allowing these compounds to be further transformed into other functional groups, such as carboxylic acids. In our exercise, the cyanide ion replaces the chloride ion in the initial step, creating a nitrile compound, which sets the stage for subsequent transformations.

Hydrolysis

Hydrolysis is the chemical process where a water molecule splits a compound into two new compounds. Typically, it involves the breaking of bonds by the addition of water. In the pathway from our exercise, hydrolysis plays a crucial role where the nitrile (\( \text{CH}_3\text{CN} \)) is converted to a carboxylic acid (\( \text{CH}_3\text{COOH} \)). Initially, the nitrile undergoes partial hydrolysis in an acidic aqueous solution, forming an amide intermediate. Continued hydrolysis, under the influence of heat or catalysts, transforms the amide into the carboxylic acid. This step is particularly important in turning nitrile groups into carboxylic acids, which are more versatile in chemical reactions and industrial applications. Hydrolysis is thus a powerful tool in organic synthesis, enabling the transformation and modification of organic compounds.

Nitriles

Nitriles are organic compounds that contain a cyano group (\( \text{-C} \equiv \text{N} \)) bonded to an alkyl or aryl group. They are considered important precursors in organic chemistry because of their ability to be converted into various functional groups. In our exercise, the nitrile formed from methyl chloride is methyl cyanide (\( \text{CH}_3\text{CN} \)). Nitriles are often synthesized through nucleophilic substitution, as we saw with the (\( \text{CN}^- \)) ion replacing (\( \text{Cl}^- \)). Their conversion to other functional groups, such as carboxylic acids, primarily through hydrolysis, demonstrates their versatility. This process underscores the importance of nitriles as intermediates, heightening their significance in synthetic chemistry. Nitriles are not only useful in laboratory settings but have practical applications in industrial chemistry as well.

Carboxylic Acids

Carboxylic acids are a group of organic acids characterized by the presence of a carboxyl group (\( \text{-COOH} \)). They are known for their acidic properties due to the donation of a proton (\( \text{H}^+ \)) from the carboxyl group, making them reactive in various chemical reactions. The transformation of nitriles to carboxylic acids, as in the hydrolysis of methyl cyanide to acetic acid (\( \text{CH}_3\text{COOH} \)), plays a significant role in organic synthesis. Carboxylic acids serve as building blocks to synthesize esters, anhydrides, and other derivatives crucial to both academic research and industrial production. Their ability to engage in multiple types of reactions makes them indispensable components in synthetic pathways, ranging from pharmaceuticals to polymers.