Comprehensive Amines Class 12 NCERT Solutions with Step-by-Step Explanations

Understanding Amines - Class 12 Chemistry

Question: What are amines and how are they classified?
Answer: Amines are organic compounds derived from ammonia by replacing one or more hydrogen atoms with alkyl or aryl groups. They are classified into three types based on the number of alkyl or aryl groups attached to the nitrogen atom:

• Primary amines (1°): One hydrogen atom is replaced (e.g., methylamine, CH₃NH₂).
• Secondary amines (2°): Two hydrogen atoms are replaced (e.g., dimethylamine, (CH₃)₂NH).
• Tertiary amines (3°): All three hydrogen atoms are replaced (e.g., trimethylamine, (CH₃)₃N).

Question: How are amines prepared in laboratories?
Answer: Amines can be prepared through several methods, including:

• Reduction of nitro compounds: Nitrobenzene is reduced to aniline using Sn/HCl.
• Ammonolysis of alkyl halides: Reaction of alkyl halides with ammonia.
• Reduction of amides: Amides are reduced to amines using LiAlH₄.
• Gabriel Phthalimide Synthesis: This method is used to prepare primary amines from phthalimide and alkyl halides.

Question: Explain the basicity of amines in aqueous and gaseous phases.
Answer: The basicity of amines depends on the availability of a lone pair of electrons on the nitrogen atom. In the gaseous phase, tertiary amines are more basic than secondary and primary due to the electron-releasing effect of alkyl groups. However, in the aqueous phase, secondary amines are the most basic due to hydrogen bonding and solvation effects.

Question: What is the Hinsberg test, and how is it used to distinguish between primary, secondary, and tertiary amines?
Answer: The Hinsberg test helps differentiate amines using benzene sulfonyl chloride:

• Primary amines form a soluble sulfonamide compound that dissolves in alkali.
• Secondary amines form an insoluble sulfonamide compound.
• Tertiary amines do not react.

Question: Why do aromatic amines exhibit lower basicity than aliphatic amines?
Answer: In aromatic amines, the lone pair on nitrogen participates in resonance with the benzene ring, reducing its availability for protonation, making them less basic than aliphatic amines.

Question: How is aniline converted into benzene?
Answer: Aniline is first diazotized using NaNO₂ and HCl at 0-5°C to form benzene diazonium chloride, which is then reduced using H₃PO₂ to produce benzene.

Question: What is the difference between Gabriel Phthalimide synthesis and Hoffmann Bromamide reaction?
Answer: Gabriel Phthalimide synthesis produces only primary amines, while Hoffmann Bromamide reaction produces primary amines with one carbon less than the parent amide.

Question: What is the role of acetylation in aromatic amines?
Answer: Acetylation of aromatic amines reduces their reactivity by introducing an acetyl group, protecting them from oxidation or unwanted side reactions.

Question: What are quaternary ammonium salts, and how are they formed?
Answer: Quaternary ammonium salts are formed when a tertiary amine reacts with an alkyl halide, leading to a positively charged nitrogen center bonded to four alkyl or aryl groups.

Question: How can we distinguish between ethylamine and aniline?
Answer: Ethylamine gives a positive carbylamine test (foul-smelling isocyanide formation), while aniline gives a positive bromine water test (white precipitate of 2,4,6-tribromoaniline).

Question: What is the importance of azo compounds in industry?
Answer: Azo compounds are widely used as dyes due to their vibrant colors and stability in different conditions.

Question: How does aniline react with bromine water?
Answer: Aniline reacts with bromine water to form a white precipitate of 2,4,6-tribromoaniline.

Question: Why is aniline more reactive than benzene in electrophilic substitution reactions?
Answer: The lone pair on nitrogen increases the electron density on the benzene ring, making it more reactive towards electrophilic attack.

Question: How is methylamine converted to ethylamine?
Answer: Methylamine is first converted into a cyanide (by reacting with methyl iodide and KCN), which is then reduced to ethylamine.

Question: How is nitrobenzene converted into aniline?
Answer: Nitrobenzene is reduced using Sn/HCl or Fe/HCl to form aniline.

Question: What happens when aniline is treated with acetic anhydride?
Answer: Aniline undergoes acetylation, forming acetanilide, which reduces the reactivity of the amine group.

Question: How do amines react with nitrous acid?
Answer: Primary amines form diazonium salts, secondary amines form nitrosoamines, and tertiary amines form nitrites.

Question: What is a diazonium salt, and how is it prepared?
Answer: A diazonium salt is an organic compound with a –N₂⁺ group, prepared by reacting primary aromatic amines with NaNO₂ and HCl at low temperatures.

Question: Why are diazonium salts unstable?
Answer: They decompose easily at room temperature, releasing nitrogen gas, making them highly reactive intermediates.

Question: What is the importance of Hoffmann’s bromamide reaction?
Answer: It is useful for converting amides into primary amines with one carbon atom less than the parent compound.

Question: How can we prepare tertiary amines selectively?
Answer: Through exhaustive alkylation of ammonia or by the reduction of tertiary amides.

Question: Why do amines form hydrogen bonds?
Answer: Due to the presence of an electronegative nitrogen atom, amines exhibit hydrogen bonding, affecting their boiling points and solubility.

Question: How does temperature affect the stability of diazonium salts?
Answer: Higher temperatures cause decomposition, leading to the release of nitrogen gas and the formation of phenols or other products.

Question: What is Hofmann’s degradation method?
Answer: It converts amides into primary amines by using bromine and NaOH.

Question: How can aniline be converted to para-bromoaniline?
Answer: By first acetylating aniline, then brominating it, followed by hydrolysis to remove the acetyl group.

Question: Why are quaternary ammonium compounds used in disinfectants?
Answer: They have antimicrobial properties and are effective against bacteria and viruses.

Question: What happens when ethylamine reacts with benzoyl chloride?
Answer: It forms N-ethylbenzamide as a result of acylation.

Question: What is the difference between aliphatic and aromatic amines?
Answer: Aliphatic amines have alkyl groups, while aromatic amines have aryl groups directly attached to the nitrogen atom.

Question: How do amines react with carbonyl compounds?
Answer: They form Schiff bases (imines) when reacted with aldehydes or ketones.

Question: How is aniline converted into phenol?
Answer: By diazotization followed by hydrolysis using warm water.

These are some of the key questions and answers for Class 12 Chemistry Amines NCERT Solutions, covering fundamental concepts and reactions comprehensively.

Recommended Indian Books for 'Amines' - Class 12 NCERT Solutions

"NCERT Solutions Chemistry Class 12th" by Geeta Rastogi
Publisher: Arihant Publication India Limited
Content: This book offers comprehensive solutions to all NCERT textbook questions, including detailed explanations for the 'Amines' chapter. It also provides additional practice questions and conceptual insights to enhance understanding.

"NCERT Solutions Chemistry Class 12" by MTG Editorial Board
Publisher: MTG Learning Media Pvt Ltd
Content: This eBook provides step-by-step solutions to NCERT questions, focusing on clarity and conceptual understanding. The 'Amines' chapter is thoroughly covered with explanations and additional practice problems.

"Chemistry Part II for Class XII" by NCERT
Publisher: National Council of Educational Research and Training
Content: The official NCERT textbook provides in-depth explanations of all topics, including 'Amines'. It includes exercises and exemplar problems to test comprehension and application skills.

"Pradeep's New Course Chemistry for Class XII" by S.C. Kheterpal and S.N. Dhawan
Publisher: Pradeep Publications
Content: This book offers detailed theory, a variety of solved and unsolved questions, and covers competitive exam-oriented problems related to 'Amines'.

"Modern's ABC of Chemistry for Class XII" by S.P. Jauhar
Publisher: Modern Publishers
Content: Known for its clear explanations and extensive question bank, this book covers 'Amines' with illustrative examples and practice exercises.

"Comprehensive Chemistry for Class 12" by Laxmi Publications
Publisher: Laxmi Publications Pvt Ltd
Content: This book provides detailed theory, NCERT solutions, and additional questions for practice, focusing on 'Amines' among other topics.

"Together with Chemistry Class XII" by R.K. Gupta
Publisher: Rachna Sagar Pvt Ltd
Content: This guide includes chapter-wise summaries, NCERT solutions, and a variety of questions for practice, with a dedicated section on 'Amines'.

"Dinesh New Millennium Chemistry Class 12" by P.N. Kapil
Publisher: S. Dinesh & Co.
Content: This book offers detailed explanations, numerous solved examples, and practice questions, covering 'Amines' comprehensively.

"Exemplar Problems: Chemistry Class XII" by NCERT
Publisher: National Council of Educational Research and Training
Content: This book provides additional problems and solutions for deeper understanding, including challenging questions on 'Amines'.

"Chemistry for Class 12" by R.K. Mukherjee
Publisher: Bharati Bhawan Publishers & Distributors
Content: This book offers clear explanations, solved examples, and practice questions, with a focus on conceptual clarity in the 'Amines' chapter.

Amines Class 12 NCERT Solutions: An In-Depth Guide

Amines are a crucial class of organic compounds derived from ammonia by replacing one or more hydrogen atoms with alkyl or aryl groups. They play a significant role in both industrial applications and biological systems. For Class 12 students, mastering the 'Amines' chapter is essential, not only for board examinations but also for competitive exams like JEE and NEET.

Understanding Amines: Classification and Structure

Amines are categorized based on the number of alkyl or aryl groups attached to the nitrogen atom:

• Primary Amines (1°): One hydrogen atom is replaced. Example: Methylamine (CH₃NH₂).
• Secondary Amines (2°): Two hydrogen atoms are replaced. Example: Dimethylamine ((CH₃)₂NH).
• Tertiary Amines (3°): All three hydrogen atoms are replaced. Example: Trimethylamine ((CH₃)₃N).

The nitrogen atom in amines is sp³ hybridized, leading to a pyramidal geometry. The lone pair of electrons on nitrogen significantly influences the chemical properties of amines, particularly their basicity and nucleophilicity.

Preparation Methods of Amines

Understanding the synthesis of amines is vital:

Reduction of Nitro Compounds: Aromatic nitro compounds like nitrobenzene can be reduced to primary amines using reducing agents such as Sn/HCl.

Ammonolysis of Alkyl Halides: Alkyl halides react with ammonia to produce primary amines. However, this method can lead to a mixture of primary, secondary, and tertiary amines due to further alkylation.

Gabriel Phthalimide Synthesis: This method is particularly useful for preparing primary amines. It involves the reaction of potassium phthalimide with alkyl halides, followed by hydrolysis.

Hofmann Bromamide Degradation: Primary amides react with bromine in an alkaline medium to yield primary amines with one less carbon atom.

Physical and Chemical Properties

Physical Properties: Lower aliphatic amines are gases with a fishy odor, while higher members are liquids or solids. Amines can form hydrogen bonds, influencing their boiling points and solubility in water.

Basicity: Amines act as Lewis bases due to the lone pair of electrons on nitrogen. The basicity depends on the alkyl or aryl groups attached. Aliphatic amines are generally more basic than aromatic amines because the lone pair on nitrogen in aromatic amines is delocalized due to resonance.

Acylation and Alkylation: Amines react with acyl chlorides and alkyl halides to form amides and higher amines, respectively.

Reaction with Nitrous Acid: Primary aliphatic amines react with nitrous acid to form alcohols, while aromatic amines form diazonium salts, which are pivotal intermediates in synthetic organic chemistry.

Importance of Diazonium Salts

Aromatic diazonium salts are versatile intermediates used in the synthesis of various aromatic compounds, including phenols, halides, and azo dyes. Their ability to undergo substitution reactions makes them invaluable in organic synthesis.

Expert Recommendations for Mastery

To excel in the 'Amines' chapter:

Conceptual Clarity: Focus on understanding the structure, classification, and properties of amines. Grasp the underlying principles governing their reactions.

Practice Problems: Regularly solve NCERT exercises and exemplar problems. This not only reinforces concepts but also exposes you to various question patterns.

Reference Quality Study Material: Utilize the recommended books listed above. They provide diverse problem sets and detailed explanations, aiding in comprehensive understanding.

Visual Learning: Employ molecular models or online 3D visualization tools to comprehend the spatial arrangement of atoms in amines, which is crucial for understanding their reactivity.

Group Study: Engaging in discussions with peers can offer new perspectives and clarify doubts, leading to a deeper understanding of complex topics.

By integrating these strategies into your study routine, you can develop a robust understanding of amines, paving the