Amines Class 12 Chapter Notes | CBSE Chemistry

Key Points to Remember about Amines

• Amines are derivatives of ammonia (NH₃) where one or more hydrogen atoms are replaced by alkyl or aryl groups.
• They are classified as primary (1°), secondary (2°), or tertiary (3°) based on the number of alkyl/aryl groups attached to the nitrogen atom, not the carbon atom.
• Hofmann Bromamide Degradation Reaction is used to prepare primary amines with one carbon atom less than the parent amide.
• Gabriel Phthalimide Synthesis is ideal for preparing pure primary aliphatic amines, but not aromatic primary amines.
• Amines are basic due to the lone pair of electrons on the nitrogen atom, which can be donated.
• The basicity order of aliphatic amines in the gaseous phase is 3° > 2° > 1° > NH₃, due to +I effect.
• The basicity order of aliphatic amines in aqueous solution is generally 2° > 1° > 3° (for CH₃) or 2° > 3° > 1° (for C₂H₅), due to a combination of inductive effect, steric hindrance, and solvation effects.
• Aromatic amines are generally less basic than aliphatic amines and ammonia due to the resonance stabilization of the lone pair on nitrogen with the benzene ring.
• The Carbylamine reaction (isocyanide test) is a characteristic test for primary amines, producing foul-smelling isocyanides.
• Diazotisation (reaction with HNO₂ at 0-5°C) is crucial for aromatic primary amines, forming diazonium salts which are versatile synthetic intermediates.

Key Definitions

Amines

Organic compounds derived from ammonia (NH₃) by replacing one, two, or three hydrogen atoms with alkyl or aryl groups.

Hofmann Bromamide Degradation

A named reaction for the preparation of primary amines by treating an amide with bromine in an aqueous or ethanolic solution of sodium hydroxide. The product amine has one carbon atom less than the starting amide.

Gabriel Phthalimide Synthesis

A method for the preparation of pure primary aliphatic amines involving the reaction of phthalimide with ethanolic KOH, followed by heating with an alkyl halide and hydrolysis. Not suitable for aromatic amines.

Carbylamine Reaction (Isocyanide Test)

A characteristic test for primary amines (aliphatic and aromatic) where they react with chloroform and alcoholic KOH to form foul-smelling isocyanides (carbylamines).

Diazotisation

The reaction of a primary aromatic amine (like aniline) with nitrous acid (prepared in situ from NaNO₂ and HCl) at 0-5°C to form an arenediazonium salt.

Hinsberg Reagent

Benzene sulphonyl chloride (C₆H₅SO₂Cl), used to distinguish between primary, secondary, and tertiary amines based on their reaction with it and the solubility of the product in alkali.

Basicity of Amines: Factors and Comparison

Amines act as Lewis bases due to the presence of a lone pair of electrons on the nitrogen atom, which they can donate to an electron-deficient species (Lewis acid). The basic strength of an amine is determined by the ease with which it can donate this lone pair. Several factors influence this basicity, leading to variations in their pKb values and equilibrium constants.

In the gaseous phase, the basicity is primarily governed by the +I (inductive) effect of the alkyl groups. Alkyl groups are electron-donating, pushing electron density towards the nitrogen atom, thus increasing the availability of the lone pair for donation. Therefore, the basicity order in the gas phase is tertiary (3°) > secondary (2°) > primary (1°) > ammonia (NH₃). More alkyl groups mean greater electron density on nitrogen, making it a stronger base.

However, in aqueous solution, the scenario is more complex due to the additional factors of solvation effect and steric hindrance. When an amine accepts a proton (forms an ammonium ion), the resulting ion gets stabilized by hydrogen bonding with water molecules. The more hydrogen atoms attached to the positively charged nitrogen, the greater the extent of solvation and hence, greater stabilization. Primary amines (RNH₃⁺) can form three hydrogen bonds, secondary (R₂NH₂⁺) two, and tertiary (R₃NH⁺) only one. This factor favors primary amines.

Simultaneously, steric hindrance becomes important. Larger alkyl groups in tertiary amines hinder the approach of water molecules for solvation and also hinder the approach of the proton, reducing basicity. The net effect of the inductive effect, solvation, and steric hindrance leads to an observed order of basicity in aqueous solution that depends on the nature of the alkyl group. For methyl amines, the order is (CH₃)₂NH (2°) > CH₃NH₂ (1°) > (CH₃)₃N (3°) > NH₃. For ethyl amines, the order is (C₂H₅)₂NH (2°) > (C₂H₅)₃N (3°) > C₂H₅NH₂ (1°) > NH₃.

Aromatic amines, such as aniline, are generally weaker bases than ammonia and aliphatic amines. This is because the lone pair of electrons on the nitrogen atom in aromatic amines is involved in resonance with the benzene ring. This delocalization makes the lone pair less available for protonation, thereby reducing their basic character. Additionally, the anilinium ion (C₆H₅NH₃⁺) formed after protonation is less stabilized by resonance than aniline itself, further contributing to lower basicity.

Distinguishing Primary, Secondary, and Tertiary Amines

Hofmann Bromamide Degradation Reaction

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Exam Traps & Scoring Tips

Examiners often test your understanding of amine basicity, especially the anomalous order in aqueous solutions for methyl and ethyl amines. Always mention the three factors (inductive effect, solvation, steric hindrance) when explaining basicity trends. Named reactions like Hofmann bromamide degradation and Gabriel phthalimide synthesis are frequent targets, so memorize their reactants, products, and specific conditions. Pay close attention to distinguishing tests (Hinsberg test, Carbylamine reaction, reaction with HNO₂) and their characteristic observations (e.g., effervescence, oily layer, foul smell). For aromatic amines, the diazotisation reaction and subsequent coupling reactions are high-scoring topics. Practice writing balanced chemical equations for all key reactions.

Practice Questions with Solutions

• Q: Why is aniline a weaker base than methylamine? A: Aniline's lone pair on nitrogen is delocalized due to resonance with the benzene ring, making it less available for protonation. Methylamine's alkyl group (+I effect) increases electron density on nitrogen, making its lone pair more available.
• Q: Name a reaction used to prepare a primary amine with one carbon atom less than the starting material. A: Hofmann Bromamide Degradation Reaction.
• Q: How can you distinguish between ethylamine and diethylamine using the Hinsberg reagent? A: Ethylamine (1°) reacts with Hinsberg reagent to form an N-ethylbenzenesulphonamide, which is soluble in aqueous KOH. Diethylamine (2°) reacts to form N,N-diethylbenzenesulphonamide, which is insoluble in aqueous KOH.
• Q: What is the significance of maintaining 0-5°C temperature during diazotisation? A: Diazonium salts are unstable at higher temperatures and decompose to produce phenols and nitrogen gas. Maintaining 0-5°C ensures the stability of the diazonium salt for further reactions.

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