Amines in Class 12 Chemistry: Properties, Reactions & Basicity Explained

Introduction to Amines: Structure and Classification

Amines are organic compounds derived from ammonia ($NH_3$) by replacing one or more hydrogen atoms with alkyl or aryl groups. In Class 12 NCERT Chemistry, amines are classified based on the number of alkyl or aryl groups attached to the nitrogen atom:

• Primary amines (1°): One alkyl/aryl group, two hydrogens (e.g., $CH_3NH_2$)
• Secondary amines (2°): Two alkyl/aryl groups, one hydrogen (e.g., $(CH_3)_2NH$)
• Tertiary amines (3°): Three alkyl/aryl groups, no hydrogens (e.g., $(CH_3)_3N$)

The nitrogen atom has a lone pair of electrons, making amines nucleophilic and basic. This lone pair influences their chemical behaviour, including reactions and bonding.

Understanding this classification and structure is essential before exploring their chemical properties and reactions.

Basicity of Amines: Factors Affecting Strength

Amines are basic because the nitrogen atom has a lone pair of electrons that can accept protons ($H^+$). However, their basic strength varies depending on the type of amine and substituents attached.

Key factors affecting amine basicity:

• Electron-donating alkyl groups (+I effect): Alkyl groups push electron density toward nitrogen, increasing basicity.
• Resonance in aromatic amines: In aniline ($C_6H_5NH_2$), the lone pair delocalizes into the benzene ring, reducing availability to accept protons, thus lowering basicity.
• Steric hindrance: Bulky groups around nitrogen can reduce accessibility of the lone pair.
• Solvation effects: In aqueous solutions, solvation stabilizes protonated amines differently, affecting basic strength.

Comparative basicity order (in aqueous solution):

Example formula:

Base dissociation constant $K_b$ and $pK_b$ measure basic strength:

$$ K_b = \frac{[BH^+][OH^-]}{[B]} \quad \text{and} \quad pK_b = -\log K_b $$

Lower $pK_b$ means stronger base.

Chemical Reactions of Amines: Overview and Mechanisms

Amines participate in many important chemical reactions due to the lone pair on nitrogen. Here are the main reactions covered in Class 12 NCERT Chemistry:

• Formation of ammonium salts: Amines react with acids to form ammonium salts.
• Example: $C_2H_5NH_2 + HCl \rightarrow C_2H_5NH_3^+Cl^-$
• These salts can be converted back to amines by treatment with a base like $NaOH$.

• Alkylation: Amines react with alkyl halides to form higher amines.

• Acylation: Reaction with acid chlorides or anhydrides produces amides.

• Carbylamine reaction (Isocyanide test): Primary amines react with chloroform and base to form isocyanides, a test for primary amines.

• Reaction with nitrous acid ($HNO_2$): Different for each amine class:
• Primary aliphatic amines form unstable diazonium salts that decompose to alcohols.
• Aromatic primary amines form stable diazonium salts used in dye synthesis.
• Secondary and tertiary amines give different products or no diazonium salts.

• Hinsberg’s test: Reaction with benzenesulphonyl chloride distinguishes primary, secondary, and tertiary amines based on sulphonamide formation and solubility.

These reactions are essential for identifying amines and synthesizing related compounds.

Aromatic Amines: Special Reactions and Resonance Effects

Aromatic amines, such as aniline ($C_6H_5NH_2$), show unique behaviour due to the interaction of the nitrogen lone pair with the benzene ring.

Resonance and Basicity:

• The lone pair on nitrogen delocalizes into the aromatic ring, reducing its availability to accept protons.
• This resonance lowers the basicity of aniline compared to aliphatic amines.

Electrophilic substitution reactions:

• The amino group is a strong activating and ortho/para-directing group.
• Common reactions include:
• Bromination: Aniline reacts with bromine water to give 2,4,6-tribromoaniline, a white precipitate.
• Nitration: Produces nitroanilines under controlled conditions.
• Sulphonation: Forms sulphonated aniline derivatives.

Protection of amino group:

• To control reactivity during substitution, the amino group can be acetylated to form acetanilide.
• This reduces the activating effect and directs substitution more predictably.

Example reaction:

$$ C_6H_5NH_2 + 3Br_2 \rightarrow C_6H_2Br_3NH_2 + 3HBr $$

Understanding these effects is crucial for synthesizing aromatic amine derivatives.

Distinguishing Amines: Hinsberg’s Test and Other Identification Methods

Hinsberg’s test is a classical method to differentiate primary, secondary, and tertiary amines based on their reaction with benzenesulphonyl chloride (Hinsberg’s reagent).

Procedure and observations:

• Primary amines: React to form sulphonamides soluble in alkali.
• Secondary amines: Form sulphonamides insoluble in alkali.
• Tertiary amines: Do not react; remain insoluble.

Reaction mechanism:

• Primary amines form sulphonamides with an acidic hydrogen, which dissolve in alkali due to salt formation.
• Secondary amines lack this hydrogen and form insoluble sulphonamides.

Other identification methods:

• Carbylamine test: Positive for primary amines.
• Reaction with nitrous acid: Different products based on amine class.

These tests help in practical lab identification and reinforce theoretical understanding.

Worked Example: Comparing Basicity of Amines

Compare the basic strength of the following amines in aqueous solution:

• Aniline ($C_6H_5NH_2$)
• Ammonia ($NH_3$)
• Ethylamine ($C_2H_5NH_2$)

Solution:

• Aniline: Least basic due to resonance delocalization of lone pair into the benzene ring.
• Ammonia: More basic than aniline; lone pair localized on nitrogen.
• Ethylamine: Most basic; alkyl group donates electron density (+I effect) increasing availability of lone pair.

Order of basicity:

$$ C_6H_5NH_2 < NH_3 < C_2H_5NH_2 $$

Explanation:

• Electron donating alkyl groups increase basicity.
• Resonance in aromatic amines decreases basicity.

This example highlights how structure influences amine properties, a key NCERT concept.