Physical Properties of Amines
Amines are organic derivatives of ammonia, characterized by the presence of a nitrogen atom bonded to alkyl or aryl groups. The physical properties of amines, such as boiling point and solubility, are heavily influenced by their ability to form Hydrogen Bonds due to the polar $N-H$ bond.
1. Physical State and Odor
- Lower Aliphatic Amines: (e.g., Methylamine, Ethylamine) are gases at room temperature and have a characteristic fishy odor (ammoniacal smell).
- Primary Amines with 3+ Carbons: Are liquids.
- Higher Amines: Are odorless solids.
- Aromatic Amines: (e.g., Aniline) are usually colorless liquids but become brown/black upon storage due to atmospheric oxidation.
2. Boiling Points (Hydrogen Bonding)
Amines have higher boiling points than non-polar hydrocarbons of comparable mass but lower boiling points than alcohols and carboxylic acids. This is because nitrogen is less electronegative (3.0) than oxygen (3.5), making the $N-H$ hydrogen bond weaker than the $O-H$ bond.
Order among Isomeric Amines
The boiling point depends on the number of hydrogen atoms available for H-bonding.
- Primary ($1^\circ$): Two H-atoms available for H-bonding. Strongest intermolecular forces.
- Secondary ($2^\circ$): One H-atom available. Weaker association.
- Tertiary ($3^\circ$): No H-atoms available. Cannot form intermolecular H-bonds with themselves. Lowest boiling point.
Example (Molar Mass ~59)
n-Propylamine ($1^\circ$, BP 48.6°C) > Ethyl methylamine ($2^\circ$, BP 36.7°C) > Trimethylamine ($3^\circ$, BP 2.9°C).
3. Solubility in Water
Lower aliphatic amines are soluble in water because they can form hydrogen bonds with water molecules. The nitrogen atom's lone pair accepts a hydrogen bond from water ($N \cdots H-O-H$).
Trend
Solubility decreases with an increase in the size of the hydrophobic alkyl group (R). Higher amines (R > 6 carbons) are practically insoluble.
- Aromatic Amines (Aniline): Insoluble in water due to the large hydrophobic benzene ring. However, they dissolve in mineral acids ($HCl$) forming soluble ammonium salts.
4. Comparison Table
| Compound Type | Intermolecular Force | Boiling Point Trend |
|---|---|---|
| Carboxylic Acids | Very Strong H-Bond (Dimer) | Highest |
| Alcohols | Strong H-Bond (O-H) | High |
| Primary Amines | Moderate H-Bond (N-H) | Moderate |
| Tertiary Amines | Dipole-Dipole (No H-Bond) | Low |
| Alkanes | Van der Waals | Lowest |
5. Density
Amines are generally lighter than water. Their density increases with molecular mass.
Amines Quiz
Test your concepts on Physical trends. 10 MCQs with explanations.
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