Overview
This lecture covers the physical and chemical properties of amines, including their molecular characteristics, basicity, key reactions, and how these properties are assessed and distinguished.
Physical Properties of Amines
- Amines with 3–5 carbon atoms are usually liquids; higher amines are solids at room temperature.
- Amines typically have a foul, fishy odor.
- Most amines are colorless, but aromatic amines like aniline are colorless until oxidized, forming colored compounds.
- Primary and secondary amines can form hydrogen bonds, while tertiary cannot, affecting solubility and boiling point.
- Lower amines are soluble in water due to hydrogen bonding; solubility decreases with increasing hydrocarbon chain length.
- Amines are more soluble in organic solvents as alkyl chain length increases.
- Alcohols are more soluble in water than amines due to stronger hydrogen bonding from higher electronegativity of oxygen.
- Boiling point order (for similar size/mass): alcohol > primary amine > secondary amine > tertiary amine > alkane.
Basicity of Amines
- Amines act as bases because their nitrogen atom has a lone pair available for protonation or electron donation.
- Basicity (strength as a base) can be explained by inductive effect, resonance, solvation, and steric factors.
- In the gaseous phase, basicity order: tertiary > secondary > primary amine.
- In aqueous phase (water): primary > secondary > tertiary amine, due to increased solvation of smaller ions.
- Aromatic amines (e.g., aniline) are less basic than aliphatic amines due to resonance delocalization of the nitrogen’s lone pair.
- Electron-donating groups increase, and electron-withdrawing groups decrease, basicity of amines.
Key Chemical Reactions of Amines
- Alkylation: Amines react with alkyl halides to form higher amines through nucleophilic substitution.
- Acylation: Primary and secondary amines react with acid chlorides/anhydrides/esters to form amides (tertiary amines do not react due to lack of H).
- Benzoylation: Amines react with benzoyl chloride to form benzamides.
- Carbylamine Test: Only primary amines react with chloroform and alcoholic KOH to form foul-smelling isocyanides (used as a distinguishing test).
- Reaction with Nitrous Acid:
- Primary aliphatic amines form alcohols, Nâ‚‚, and HCl.
- Primary aromatic amines (like aniline) form stable diazonium salts at low temperature.
- Reaction with Aryl Sulfonyl Chloride (Hinsberg Test):
- Only primary and secondary amines react, forming sulfonamides (distinguished by solubility in alkali); tertiary amines do not react.
- Electrophilic Substitution (Aromatic Amines Only):
- Amino group activates benzene ring and is ortho/para-directing, leading to substitution at these positions.
- Excessive reactivity may require protection (e.g., acylation) to control product.
Key Terms & Definitions
- Amines — Organic compounds derived from ammonia by replacement of one or more hydrogen atoms by alkyl or aryl groups.
- Hydrogen Bonding — Attraction between a hydrogen atom attached to electronegative atom (like N or O) and another electronegative atom.
- Inductive Effect — Transmission of charge through a chain of atoms in a molecule, affecting electron density.
- Solvation Effect — Stabilization of ions by surrounding solvent molecules, especially in water.
- Carbylamine Test — Chemical test for the detection of primary amines via formation of isocyanides.
- Diazonium Salt — An intermediate formed from aromatic amines and nitrous acid, useful for further reactions.
- Hinsberg Test — Reaction to distinguish primary, secondary, and tertiary amines using sulfonyl chloride.
Action Items / Next Steps
- Review the solubility and boiling point comparison tables in your NCERT textbook.
- Complete Example 9.4: Arrange given amines in order of decreasing basic strength.
- Prepare for the next lecture on diazonium salt reactions and their applications.