Problem 20 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following gives primary amine on reduction? (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{NO}_{2}\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2}-\mathrm{O}-\mathrm{N}=\mathrm{O}\) (c) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{~N}=\mathrm{NC}_{6} \mathrm{H}_{5}\) (d) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{NC}\).

Step-by-Step Solution

Answer

(a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{NO}_{2}\) on reduction gives a primary amine.

Step 1: Identify the Functional Groups

Determine the functional groups present in each compound to understand what type of organic compounds they are. (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{NO}_{2}\) is a nitroethane with a nitro group \(\mathrm{NO}_{2}\). (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2}-\mathrm{O}-\mathrm{N}=\mathrm{O}\) has an alkoxy group connected to a nitroso group. (c) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{~N}=\mathrm{NC}_{6} \mathrm{H}_{5}\) is an azobenzene derivative with azo linkage \(\mathrm{N=N}\). (d) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{NC}\) is an ethyl isocyanide with an isocyanide group \(\mathrm{NC}\).

Step 2: Recall Reduction Reactions

Focus on what functional groups may yield a primary amine upon undergoing reduction.- Nitro groups \((\mathrm{NO}_{2})\) upon reduction with reducers like \(\mathrm{LiAlH}_{4}\) or \(\mathrm{H}_{2}\) with Pd-C will produce a primary amine.- Nitroso and azo groups typically yield secondary or other types of amines.- Isocyanides \((\mathrm{NC})\) on reduction will yield a primary amine.

Step 3: Assess Each Option for Primary Amine Formation

Evaluate which compound yields a primary amine on reduction:- (a) A nitro group \((\mathrm{NO}_{2})\) in \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{NO}_{2}\) gets reduced to amino group \((\mathrm{NH}_{2})\) producing ethylamine, a primary amine.- (b) A nitroso group generally forms different products, not giving a primary amine.- (c) Azo linkages, when reduced, typically result in hydrazo compounds instead of primary amines.- (d) An isocyanide group reduces to form a primary amine, however it typically involves a more complex reaction sequence.

Step 4: Conclusion

From the above assessment, option (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{NO}_{2}\) on reduction will yield a primary amine, specifically ethylamine.

Key Concepts

Reduction Reactions

Reduction reactions are a vital part of organic chemistry, as they involve the gain of electrons by a molecule or ion. This process usually leads to the decrease of oxidation numbers and can transform various functional groups into different chemical forms. In organic reactions, reduction often involves adding hydrogen or removing oxygen or other electronegative elements.

Common reagents used in reduction reactions include hydrogen gas with a palladium catalyst (H₂/Pd-C), lithium aluminum hydride (LiAlH₄), and other metal hydrides. These reagents are chosen based on the specific functional group present in the compound.

For example, a nitro group (NO₂) can be reduced using these agents to form an amine group, leading to valuable products like primary amines used in many chemical industries. Understanding which reagent is suitable for a particular functional group is crucial for effectively carrying out reduction reactions.

Functional Groups

Functional groups are specific groups of atoms within molecules that dictate the chemical reactions of those molecules. They are the key to understanding organic chemistry, as they determine the reactivity and properties of the compounds.

Some common functional groups include:

• Alcohols (OH)
• Aldehydes (CHO)
• Carboxylic acids (COOH)
• Nitro groups (NO₂)
• Azo groups (N=N)
• Isocyanides (NC)

These functional groups bring unique characteristics to organic molecules, influencing boiling points, solubility, and density. For example, nitro groups are known for their electron-withdrawing properties and ability to engage in reduction reactions to form amines, which are crucial in pharmaceuticals and dyes.

Identifying functional groups in a compound allows chemists to predict the types of reactions that the compound can undergo, leading to the design of specific synthesis pathways.

Primary Amines

Primary amines are organic compounds where a nitrogen atom is attached to one alkyl or aryl group and two hydrogen atoms, designated as RNH₂. They are essential in the formation of more complex molecules and are found in various natural and synthetic products.

Primary amines possess distinct characteristics:

• They are basic, meaning they can accept protons.
• They can participate in hydrogen bonding, affecting their boiling and melting points.
• They are pivotal in forming peptide bonds in proteins.

In reduction reactions, primary amines often form from the reduction of nitro compounds. For instance, reducing nitroethane (CH₃CH₂NO₂) results in the formation of ethylamine, a primary amine. This transformation is significant for creating building blocks in the pharmaceutical and chemical industries.

Nitro Compounds

Nitro compounds are characterized by having one or more nitro groups (NO₂) attached to a carbon atom. These compounds are recognized for their stability and intense color, which makes them useful in dyes, explosives, and pharmaceuticals.

The nitro group is strongly electron-withdrawing, greatly affecting the reactivity and polarity of the compounds. In terms of reactions, nitro compounds are particularly notable for undergoing reduction reactions.

When reduced, nitro compounds commonly convert into amines. For example, nitroethane, upon reduction, yields ethylamine, a simple primary amine. This ability to transform into amines is exploited in synthesizing various products, including drugs like paracetamol and certain herbicides. Understanding the behavior of nitro compounds in these reactions is crucial for designing effective chemical synthesis routes.