Amines Class 12: Complete Guide to Properties and Reactions

Introduction to Amines: Definition and Classification

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

• Primary amines (1°): One alkyl/aryl group attached (e.g., methylamine, $CH_3NH_2$)
• Secondary amines (2°): Two alkyl/aryl groups attached (e.g., dimethylamine, $(CH_3)_2NH$)
• Tertiary amines (3°): Three alkyl/aryl groups attached (e.g., trimethylamine, $(CH_3)_3N$)

Amines can be aliphatic (alkyl groups only) or aromatic (aryl groups attached, e.g., aniline). Understanding this classification helps in predicting their chemical behaviour and properties.

Physical Properties of Amines

Amines exhibit distinct physical properties influenced by their structure:

• State and Appearance: Lower amines are gases or liquids with strong fishy odour. Higher amines are usually oily liquids or solids.
• Solubility: Amines with up to 6 carbon atoms are soluble in water due to hydrogen bonding between $N-H$ and water molecules.
• Boiling Points: Primary and secondary amines have higher boiling points than tertiary amines due to hydrogen bonding. However, all amines have lower boiling points than corresponding alcohols because $N-H$ bonds are less polar than $O-H$ bonds.

These properties affect how amines interact in chemical reactions and their applications.

Basicity of Amines: Factors and Comparison

Amines are basic due to the lone pair of electrons on nitrogen which can accept protons ($H^+$). However, their basic strength varies depending on the substituents and environment.

Factors affecting basicity:

• Electron donating groups (alkyl groups): Increase electron density on nitrogen, enhancing basicity.
• Electron withdrawing groups (aryl groups): Decrease electron density via resonance, reducing basicity.
• Steric hindrance: Bulky groups reduce accessibility of lone pair, lowering basicity.

Order of basicity (typical):

$$\text{C}_6\text{H}_5\text{NH}_2 < NH_3 < \text{C}_6\text{H}_5\text{CH}_2\text{NH}_2 < C_2H_5NH_2 < (C_2H_5)_2NH$$

Understanding basicity helps predict amine behaviour in acid-base reactions.

Chemical Reactions of Amines

Amines undergo various important chemical reactions relevant for Class 12 exams:

• Reaction with acids: Amines form ammonium salts when treated with acids.

$$RNH_2 + HCl \rightarrow RNH_3^+Cl^-$$

• Acylation: Amines react with acyl chlorides to form amides.

$$RNH_2 + R'COCl \rightarrow R'CONHR + HCl$$

• Reaction with nitrous acid: Primary aliphatic amines form alcohols, while primary aromatic amines form diazonium salts.

• Formation of diazonium salts: Aromatic primary amines react with sodium nitrite and HCl at 0-5 °C to form diazonium salts, crucial intermediates for synthesis.

These reactions highlight the versatility of amines in organic synthesis.

Diazonium Salts: Formation and Reactions

Diazonium salts are important derivatives of aromatic amines, especially in dye chemistry. They are formed by the reaction of primary aromatic amines with nitrous acid at low temperature:

$$C_6H_5NH_2 + HNO_2 + HCl \xrightarrow{0-5^\circ C} C_6H_5N_2^+Cl^- + 2H_2O$$

Physical properties:

• Benzenediazonium chloride is a colourless solid, soluble in water but unstable at room temperature.
• Benzenediazonium fluoroborate is more stable and water-insoluble.

Reactions involving displacement of nitrogen:

• Sandmeyer reaction: Replacement of diazonium group by halides using Cu(I) salts.
• Gattermann reaction: Halogenation using halogen acids and copper powder.
• Iodination: Replacement with iodine using KI.
• Fluorination: Formation of aryl fluorides by heating diazonium fluoroborate.
• Reduction: Conversion to arenes with mild reducing agents.
• Hydrolysis: Formation of phenols by heating with water.
• Nitration: Introduction of nitro groups using sodium nitrite and copper.

These reactions enable synthesis of various substituted aromatic compounds.

Coupling Reactions of Diazonium Salts and Azo Dye Formation

Diazonium salts can couple with phenols or aromatic amines to form azo compounds characterized by the azo linkage ($-N=N-$). These azo dyes are brightly coloured and widely used in textile industries.

Example reactions:

• Benzenediazonium chloride + phenol → p-hydroxyazobenzene (orange-red dye)
• Benzenediazonium chloride + aniline → p-aminoazobenzene (yellow dye)

Mechanism: The diazonium cation acts as an electrophile and couples at the para or ortho position of phenols or aromatic amines.

These coupling reactions demonstrate the synthetic importance of diazonium salts in producing dyes and pigments.

Worked example: Write the reaction for coupling benzenediazonium chloride with phenol.

Solution:

$$C_6H_5N_2^+Cl^- + C_6H_5OH \rightarrow C_6H_5-N=N-C_6H_4OH + HCl$$

This forms p-hydroxyazobenzene, an azo dye.