Amines (-NH₂) are one of the functional groups, derivatives of Ammonia (NH₃) in which one or more hydrogen atoms have been replaced by Alkyl groups
Classifications Of Amines ;
- Primary (1′) Amines
- Secondary (2′) Amines
- Tertiary(3′) Amines
1. Common System :
2. Substitute System :
Salts of amines :
- All amines are basic in nature.
- They react with acids to form salts.The salts can be considered as being related to Ammonium (NH₄⁺) salts where the hydrogens of ammonium ion have been replaced by Alkyl groups.
- When the four Hydrogen atom of Ammonim ion have been replaced by Alkyl groups,that compound is called as Quaternary Ammonium Salt
Structure of Amines :
- The C-N(sigma) bond in methylamine is formed by overlap of an sp³ orbital of carbon and an sp³ orbital of nitrogen.
- Each N-H (sigma) bond is formed by the overlap of an sp³ orbital of nitrogen and s orbital of hydrogen.
- Each C-H (sigma) bond id formed by the overlap of an sp³ orbital of carbon and s orbital of hydrogen.
Basicity Of Amines :
Basicities Of Some Amines :
|NAME||pKa of conjugate acid|
Structural Relationship Of Amine Basicity :
Importance of Hybridization :
Methods Of Preparation Of Amines :
1. Reaction of alkyl halides with ammonium or amines :
- Ammonia or amines carries an unshared pair of electrons and can act as a nucleophile in a substitution reactions with Alkyl Halide.The product of this reaction with Ammonia or an amine is an amine salt.
- The free amine can be prepared by treating the amine salt with bases like NaOH.
- It follows an SN² reaction.
2. Gabriel Phthalimide Method :
- This involves the treatment of Phthalimide with potassium hydroxide to form the potassium salt.
- The salt is then heated with an alkyl halide to give N-alkylphthalimide,which in turns react with an potassium hydroxide to form potassium Phthalimide salt and a pure primary amine
3. Reduction Of Nitroalkanes :
- Primary amines can be obtained by reduction of nitroalkanes with H₂ + Pt (or Ni) or Lithium aluminium hydride
4. Reduction of Nitriles :
- The primary amines can be prepared by reduction of nitriles (alkyl cyanide) with H₂+Pt (or Ni) or Lithium aluminium hydride
5. Reduction of amides :
- Primary amines can be obtained by reduction of of the simple amides with Lithium aluminium hydride.
- Notice that the product contains the same number of carbons as the reactant Amide
6. Hoffmann’s Degradation of amides :
- This is good laboratory method for the preparation of the amides to a pure primary amine.
- The amide is warmed with the Bromine and concentrated Aqueous NaOH solutions
7. Curtius Rearrangement :
Physical Properties Of Amines :
- lower amines are gases or low-boiling liquids and possess a characteristic ammonia like smell
- Aniline and other arylamines are generally colorless and transparent
- Secondary amines are more basic when compared with Primary and tertiary amines.Because the two alkyl groups present in the Secondary amine has better stabilize the conjugate acid (inductive effect)
- Primary and secondary amines are capable of intermolecular hydrogen bondng, because the contain N-N bonds.Because the Nitrogen is more electronegative than Oxygen, however intermolecular hydrogen bonds betwee N and H are weaker than those O and H
- Amines are soluble in organic solvents.All amines having < 5Cs are water soluble because they can hydrogen bond with H₂O.Amines having > 5C’s are water insoluble because the nonpolar alkyl portion is too large to dissolve in the polar H₂O solvent
- Tertiary (3′) amines have lower boiling point than 1′ and 2′ amines of comparable molecular weight,because they have no N-H bonds and are incapable of hydrogen bonding.
Chemical Properties Of Amines :
1. Salt Formation :
2. Reaction with Alkyl Halide :
3. Reaction with Acid Chlorides (Acylation) :
- Primary amines react with acid chlorides or acid unhydrides to form N-substituted amides
- Secondary amines react with acid chlorides to form N,N-disubstituted amides
- Tertiary amines do not react with acid chlorides,since they do not have any replaceable hydrogen on the nitrogen.
4. Carbylamine Reaction :
5. Reaction with Carbon disulfide :
To know In-depth details about Amines – Organic Chemistry
Reference : Bahl & Arun Bahl ( Advanced Organic Chemistry)