Problem 71 - A Complete Resource Book in Chemistry for JEE Main

Question

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

Step-by-Step Solution

Answer

The end product [B] is (d) \(\mathrm{CH}_3\mathrm{COOH}\).

Step 1: Identify Reaction 1

The first reaction involves methyl chloride (\(\mathrm{CH}_3\mathrm{Cl}\)) reacting with potassium cyanide (\(\mathrm{KCN}\)). In this reaction, the chlorine in \(\mathrm{CH}_3\mathrm{Cl}\) is replaced by the cyanide ion (\(\mathrm{CN}^-\)), forming methyl cyanide (acetonitrile, \(\mathrm{CH}_3\mathrm{CN}\)). Thus, [A] is \(\mathrm{CH}_3\mathrm{CN}\).

Step 2: Identify Reaction 2

The second reaction for compound [A] is its hydrolysis in the presence of water and hydrogen ions (\(\mathrm{H}/\mathrm{H}_2\mathrm{O}\)). The hydrolysis of \(\mathrm{CH}_3\mathrm{CN}\) (an organic nitrile) produces acetic acid (\(\mathrm{CH}_3\mathrm{COOH}\)). This involves the nitrile group (\(\mathrm{CN}\)) being converted to a carboxylic acid group (\(\mathrm{COOH}\)). Therefore, [B] is \(\mathrm{CH}_3\mathrm{COOH}\).

Step 3: Compare with Given Options

The result of the sequence ends with the formation of acetic acid (\(\mathrm{CH}_3\mathrm{COOH}\)). Looking at the provided options: (a) \(\mathrm{HCOOH}\), (b) \(\mathrm{CH}_3\mathrm{COCH}_3\), (c) \(\mathrm{CH}_3\mathrm{NH}_2\), (d) \(\mathrm{CH}_3\mathrm{COOH}\), we match [B], acetic acid, with option (d).

Key Concepts

Nucleophilic Substitution

In organic chemistry, nucleophilic substitution is a fundamental type of reaction. It involves the replacement of a leaving group, usually a halide, with a nucleophile. In our exercise, methyl chloride (\(\mathrm{CH}_3\mathrm{Cl}\)) undergoes nucleophilic substitution. The chlorine atom, a fairly good leaving group, is substituted by the cyanide ion (\(\mathrm{CN}^-\)) from potassium cyanide (\(\mathrm{KCN}\)). This results in the formation of methyl cyanide (\(\mathrm{CH}_3\mathrm{CN}\)), also known as acetonitrile.

• Nucleophiles are electron-rich species that seek positive centers.
• The strength of a nucleophile can affect the rate and outcome of the reaction.
• Good leaving groups, like halides, help to facilitate these reactions by exiting the molecule easily.

Understanding nucleophilic substitution is crucial as it forms the basis for many synthetic pathways in organic chemistry.

Hydrolysis

Hydrolysis is a reaction involving the breakdown of a compound by water. Essentially, water molecules cleave bonds in the reacting compound, leading to the formation of new substances. In this particular exercise, hydrolysis occurs in the second part where methyl cyanide (\(\mathrm{CH}_3\mathrm{CN}\)) is transformed into acetic acid (\(\mathrm{CH}_3\mathrm{COOH}\)).

• The hydrolysis of nitriles usually requires an acidic environment to proceed efficiently.
• This reaction replaces the nitrogen in the nitrile group with an oxygen from water, forming a carboxylic acid.
• Carboxylic acids are characterized by the \(\mathrm{C(O)OH}\) group, which is crucial in biological and chemical applications.

Hydrolysis reactions are not confined to nitriles and are essential in various biological processes such as digestion.

Organic Chemistry

Organic chemistry focuses on carbon-containing compounds, exploring their structures, properties, and transformations. The sequence of reactions in this exercise underscores the dynamic nature of organic compounds through alteration and reconfiguration.

• This branch of chemistry is fundamental in developing pharmaceuticals, polymers, and many consumer goods.
• Organic reactions often involve functional group transformations, such as the conversion seen here from a nitrile to a carboxylic acid.
• Understanding reaction mechanisms is key to predicting products and optimizing conditions for desired outcomes.

By grasping these principles, students can tackle complex synthesis problems and appreciate the diversity of organic substances.

Acetic Acid

Acetic acid (\(\mathrm{CH}_3\mathrm{COOH}\)), a simple carboxylic acid, is the end product of the exercise's reaction sequence. It is well known as the main component of vinegar and has various industrial applications.

• It serves as a solvent, a chemical reagent, and a preservative.
• In the laboratory, it is a common reactant for esterification and polymerization reactions.
• Acetic acid's acidic properties arise from the ability of its carboxyl group to donate a proton.

This substance exemplifies the interconnectedness and versatility of organic molecules in both everyday uses and scientific fields.

Cyanide Ion Transformation

The cyanide ion (\(\mathrm{CN}^-\)) plays a critical role in the formation of products in the exercise sequence. Initially acting as a nucleophile, it displaces the chlorine in methyl chloride to form methyl cyanide. This transformation involves significant changes in molecular structure and properties.

• Cyanide is known for its reactivity, making it a useful tool in various synthesis reactions.
• Although toxic, its functional group allows for diverse transformations, particularly in organic synthesis.
• Understanding the behavior of the cyanide group is essential for safely harnessing its utility in chemical reactions.

By converting to other functional groups, the cyanide ion's transformation showcases the transformative power inherent in organic chemistry.