Aliphatic amines are organic compounds that consist of an alkyl or cycloalkyl group attached to one or more amino groups (-NH2).
They can be primary (one carbon group), secondary (two carbon groups), or tertiary (three carbon groups).
Here are common methods for preparing aliphatic amines:
1. Ammonolysis of Alkyl Halides
Process:
Reacts an alkyl halide (RX) with ammonia (NH₃) to produce primary, secondary, and tertiary amines, along with quaternary ammonium salts.
Reaction:
RX + NH₃ → RNH₂ + HX
Details:
Requires an excess of ammonia to favor amine formation. Can result in a mixture of products, requiring further purification.
2. Reduction of Nitro Compounds
Process:
Reduces aliphatic nitro compounds (RNO₂) to primary amines using reducing agents like hydrogen gas with a catalyst (e.g., palladium on carbon), iron with hydrochloric acid, or tin with hydrochloric acid.
Reaction:
RNO₂ + 3H₂ → RNH₂ + 2H₂O
Details:
Effective for preparing primary amines from nitro compounds.
3. Reduction of Nitriles
Process:
Converts nitriles (RCN) to primary amines using hydrogen with a catalyst (e.g., nickel) or lithium aluminum hydride (LiAlH₄).
Reaction:
RCN + 2H₂ → RCH₂NH₂
Details:
Useful for synthesizing primary amines with an additional carbon atom compared to the original nitrile.
4. Gabriel Synthesis
Process:
Involves the reaction of potassium phthalimide with an alkyl halide to form N-alkylphthalimide, which is then hydrolyzed to release the primary amine.
Reaction:
C₆H₄(CO)₂N⁻K⁺ + RX → C₆H₄(CO)₂NR → Hydrolysis → RNH₂ + C₆H₄(CO)₂OH
Details:
Advantageous for avoiding over-alkylation seen in direct ammonolysis.
5. Reductive Amination
Process:
An aldehyde or ketone reacts with ammonia or an amine to form an imine or iminium ion, which is then reduced to form the amine.
Reaction:
RCHO + NH₃ → RCH=NH → RCH₂NH₂ (after reduction)
Details:
Can synthesize primary, secondary, or tertiary amines depending on the starting materials.
Each method has specific uses and advantages, allowing for the targeted synthesis of different types of aliphatic amines.