1. Amines (Introduction, Preparation, Physical Properties)

Amines are organic compounds derived from ammonia ($\text{NH}_3$) where one or more hydrogen atoms are replaced by alkyl or aryl groups. They are classified as primary ($\text{RNH}_2$), secondary ($\text{R}_2\text{NH}$), and tertiary ($\text{R}_3\text{N}$) amines based on the number of alkyl/aryl groups attached to the nitrogen atom. Amines are generally basic due to the lone pair of electrons on the nitrogen atom. Their physical properties, such as boiling point and solubility, are influenced by hydrogen bonding.

2. Amines (Chemical Reactions)

Amines exhibit characteristic chemical reactions due to the basicity and nucleophilicity of the nitrogen atom. They react with acids to form ammonium salts. Primary amines can be alkylated and acylated. They also undergo reactions like carbylamine reaction (forming isocyanides) and reactions with nitrous acid, which differentiate between primary, secondary, and tertiary amines. These reactions are crucial for synthesizing complex organic molecules and understanding biological processes.

3. Amines (Diazonium Salts)

Primary aromatic amines react with nitrous acid ($\text{HNO}_2$) at low temperatures (0-5$\degree$C) to form diazonium salts ($\text{Ar-N}_2^+ \text{X}^-$). These salts are highly reactive intermediates and are very useful in organic synthesis. The diazonium group ($\text{-N}_2^+$) can be readily replaced by various functional groups, such as halogens, cyanide, hydroxyl, and hydrogen, through reactions like the Sandmeyer reaction. This versatility makes diazonium salts essential for synthesizing dyes, pharmaceuticals, and agrochemicals.