Chemistry Class 12 NCERT Solutions Chapter 13: Organic Compounds Containing Nitrogen – Important Questions

Introduction

In Class 12 Chemistry, Chapter 13 of the NCERT textbook focuses on Organic Compounds Containing Nitrogen. This chapter is a critical part of the curriculum, offering students a comprehensive understanding of the various nitrogenous organic compounds, their properties, synthesis, and applications. The chapter is pivotal for preparing for board exams and competitive tests, as it encompasses important topics such as amines, cyanides, isocyanides, and diazonium salts. This article delves into the important questions from this chapter, providing detailed explanations and solutions to help students excel in their exams.

Overview of Organic Compounds Containing Nitrogen

Organic compounds containing nitrogen are essential in various chemical processes and biological systems. The nitrogen atom in these compounds can form different types of bonds, leading to diverse chemical behaviors and properties. The main categories covered in this chapter include:

1. Amines
2. Cyanides and Isocyanides
3. Diazonium Salts

Each category has its own set of important reactions, properties, and applications. Understanding these compounds requires a solid grasp of their structure, reactivity, and the synthesis processes involved.

1. Amines

1.1. Structure and Classification

Amines are organic compounds derived from ammonia (NH₃) where one or more hydrogen atoms have been replaced by alkyl or aryl groups. They can be classified into three types based on the number of carbon groups attached to the nitrogen:

• Primary Amines (R-NH₂): The nitrogen is bonded to one carbon group.
• Secondary Amines (R₂NH): The nitrogen is bonded to two carbon groups.
• Tertiary Amines (R₃N): The nitrogen is bonded to three carbon groups.

1.2. Preparation of Amines

1. From Alkyl Halides: Amines can be synthesized by the reaction of alkyl halides with ammonia or amines. $R-X + NH₃ \rightarrow R-NH₂ + HX$ $R-X + R'-NH₂ \rightarrow R-R'-NH + HX$ $R-X + R'-R''NH \rightarrow R-R'-R''N + HX$

2. Reduction of Nitriles: Nitriles can be reduced to primary amines using reducing agents. $R-CN + 2[H] \rightarrow R-CH₂NH₂$

3. Reduction of Nitro Compounds: Nitro compounds can be reduced to amines using reducing agents like tin and hydrochloric acid. $R-NO₂ + 3[H] \rightarrow R-NH₂$

1.3. Reactions of Amines

1. With Acids: Amines react with acids to form salts. $R-NH₂ + HCl \rightarrow R-NH₃⁺Cl⁻$

2. With Nitroalkanes (Hinsberg Test): Primary amines react with nitroalkanes to form N-alkyl nitroalkylamines. $R-NH₂ + R'-NO₂ \rightarrow R-NH-R' + H₂O$

3. With Carbonyl Compounds: Amines react with carbonyl compounds to form imines. $R-NH₂ + R'-CHO \rightarrow R-CH=N-R'$

1.4. Important Questions

Q1: Describe the preparation of primary amines from alkyl halides. Provide an example.

Solution: Primary amines can be prepared by treating alkyl halides with ammonia in the presence of an alcoholic medium. For instance, methylamine can be synthesized from methyl iodide. $CH₃I + NH₃ \rightarrow CH₃NH₂ + HI$

Q2: Explain the reduction of nitro compounds to amines. Include the role of reducing agents.

Solution: Nitro compounds are reduced to primary amines using reducing agents such as tin (Sn) and hydrochloric acid (HCl). The nitro group (-NO₂) is converted to the amino group (-NH₂) through the addition of hydrogen. $R-NO₂ + 3[H] \rightarrow R-NH₂$

2. Cyanides and Isocyanides

2.1. Structure and Properties

Cyanides (R-CN) and isocyanides (R-NC) are important functional groups containing nitrogen. Cyanides have a cyano group (-C≡N) while isocyanides have an isocyano group (-N≡C).

• Cyanides are generally more stable and are used in various chemical syntheses.
• Isocyanides are less stable and often used in specific organic reactions.

2.2. Preparation of Cyanides and Isocyanides

1. From Alkyl Halides: Cyanides can be prepared by treating alkyl halides with sodium cyanide. $R-X + NaCN \rightarrow R-CN + NaX$

2. From Carboxylic Acids: Cyanides can also be prepared by the dehydration of carboxylic acids with phosphorous pentoxide. $R-COOH + P₂O₅ \rightarrow R-CN + H₂O$

3. From Aldehydes or Ketones (Isocyanides): Isocyanides are synthesized through the reaction of aldehydes or ketones with chloroform in the presence of a base. $R-CHO + CHCl₃ + KOH \rightarrow R-NC + KCl + H₂O$

2.3. Reactions of Cyanides and Isocyanides

1. Hydrolysis: Cyanides hydrolyze to form carboxylic acids and ammonia. $R-CN + 2H₂O \rightarrow R-COOH + NH₃$

2. With Grignard Reagents: Cyanides react with Grignard reagents to form ketones. $R-CN + RMgX \rightarrow R-CO-R'$

3. With Acid Chlorides (For Isocyanides): Isocyanides react with acid chlorides to form amides. $R-NC + R'-COCl \rightarrow R-NH-COR'$

2.4. Important Questions

Q1: How can cyanides be prepared from alkyl halides? Provide a reaction mechanism.

Solution: Cyanides are prepared by treating alkyl halides with sodium cyanide. The mechanism involves the nucleophilic substitution where the cyanide ion displaces the halide ion. $R-X + NaCN \rightarrow R-CN + NaX$

Q2: Explain the hydrolysis of cyanides and its products.

Solution: Cyanides hydrolyze in the presence of water to form carboxylic acids and ammonia. The reaction involves breaking the cyano group and adding water. $R-CN + 2H₂O \rightarrow R-COOH + NH₃$

3. Diazonium Salts

3.1. Structure and Preparation

Diazonium salts are a class of organic compounds with the general formula R-N₂⁺X⁻. They are important intermediates in organic synthesis, especially in azo coupling reactions.

1. Preparation: Diazonium salts are prepared by the diazotization reaction of primary aromatic amines with nitrous acid (generated in situ from sodium nitrite and hydrochloric acid). $R-NH₂ + NaNO₂ + HCl \rightarrow R-N₂⁺Cl⁻ + 2H₂O$

3.2. Reactions of Diazonium Salts

1. Azo Coupling: Diazonium salts react with phenols or aromatic amines to form azo dyes. $R-N₂⁺Cl⁻ + Ar-OH \rightarrow R-N=Ar + HCl$

2. Replacement Reactions: Diazonium salts undergo replacement reactions with various nucleophiles.

   • With water: Forms phenol. $R-N₂⁺Cl⁻ + H₂O \rightarrow R-OH + N₂ + HCl$
   • With potassium iodide: Forms aryl iodides. $R-N₂⁺Cl⁻ + KI \rightarrow R-I + KCl + N₂$

3.3. Important Questions

Q1: Describe the preparation of diazonium salts from primary aromatic amines. Provide an example.

Solution: Diazonium salts are prepared by reacting primary aromatic amines with nitrous acid. For example, aniline (C₆H₅NH₂) reacts with sodium nitrite and hydrochloric acid to form benzenediazonium chloride. $C₆H₅NH₂ + NaNO₂ + HCl \rightarrow C₆H₅N₂⁺Cl⁻ + 2H₂O$

Q2: Explain the azo coupling reaction with an example.

Solution: The azo coupling reaction involves the reaction of diazonium salts with phenols or aromatic amines to form azo compounds. For example, benzenediazonium chloride reacts with phenol to form an azo dye. $C₆H₅N₂⁺Cl⁻ + C₆H₅OH \rightarrow C₆H₅-N=O-C₆H₅ + HCl$

Conclusion

Understanding organic compounds containing nitrogen is crucial for mastering Class 12 Chemistry. This chapter provides foundational knowledge that is essential for further studies in organic chemistry. By focusing on the preparation, reactions, and properties of amines, cyanides, isocyanides, and diazonium salts, students can gain a deep understanding of the subject. This comprehensive guide to important questions from Chapter 13 should aid in reinforcing key concepts and preparing for exams effectively. Regular practice and a thorough review of these topics will ensure a strong grasp of the material and improve overall performance in chemistry.