1. Reduction of nitro compounds:
Reactants: Nitro compounds
Reagent: H2 in the presence of finely divided nickel, palladium or platinum
Reactants: Nitro compounds
Reagent: Metals in acidic medium. Reduction with iron scrap and hydrochloric acid is preferred because FeCl2 formed gets hydrolysed to release hydrochloric acid during the reaction. Thus, only a small amount of hydrochloric acid is required to initiate the reaction.
Product: Amines
This method cannot be used when the molecule also contains some other easily hydrogenated group, such as a Carbon carbon double bond.
2. Ammonolysis of alkyl halides:
Reactants: alkyl or benzyl halide
Reagent: ethanolic solution of ammonia
Mechanism: nucleophilic substitution reaction
Product: Amines
CH3CH2CH2Br + NH3 → CH3CH2CH2NH2 + HBr
This process of cleavage of the C–X bond by ammonia molecule is known as ammonolysis.
The primary amine thus obtained behaves as a nucleophile and can further react with alkyl halide to form secondary and tertiary amines, and finally quaternary ammonium salt.
The free amine can be obtained from the ammonium salt by treatment with a strong base:
R - NH⁺₃X⁻ + NaOH → R - NH₂ + H₂O + NaX⁻
The order of reactivity of halides with amines is RI > RBr >RCl.
3. Reduction of nitriles: This reaction is used for ascent of amine series, i.e. for preparation of amines containing one carbon atom more than the starting nitrile.
Reactants: Nitriles
Reagent: lithium aluminium hydride (LiAlH4)
Reaction: Reduction
Reactants: Nitriles
Reagent: H2 in presence of catalyst Ni
Reaction: Reduction
Product: Amines
4. Reduction of amides:
Reactants: Amides
Reagent: Lithium aluminium hydride (LiAlH4)
Reaction: Reduction
Reactants: Amides
Reagent: mixture of base and bromine (KOH + Br2)
Product: Hofmann Bromamide reaction.
Product: Amines
The reaction is as follows, RCONH2 + Br2 + 4KOH → RNH2 + K2CO3 + 2KBr + 2H2O
Here, the amine formed has one carbon less than that of the corresponding amide. Due to the loss of carbon atom, this reaction is also called as Hofmann degradation of amides.
Base
abstracts an acidic N-H proton, yielding an anion.
The
anion reacts with bromine in an α-substitution reaction to give an N-bromoamide.
Base
abstraction of the remaining amide proton gives a bromoamide anion.
The bromoamide anion
rearranges as the R group attached to the carbonyl carbon migrates to nitrogen at the same time the bromide ion leaves, giving n isocyanate.
The isocyanate adds water in a nucleophilic addition step to yield a carbamic acid (aka urethane).
The carbamic acid spontaneously loses CO2, yielding the amine product.
Apart from this, amides can be dehydrated by P2O5 to their corresponding nitriles and nitriles can then be reduced.
By this method you are retaining the number of carbon atoms in both amide and the amine
5. Gabriel phthalimide synthesis:
Reactants: Pthalimide and alkyl halide
Reagent: Ethanolic potassium hydroxide
Product: Primary amine
Phthalimide on treatment with ethanolic potassium hydroxide forms potassium salt of phthalimide
Phthalimide on heating with alkyl halide followed by alkaline hydrolysis produces the corresponding primary amine.
Aromatic primary amines cannot be prepared by this method because aryl halides do not undergo nucleophilic substitution with the anion formed by phthalimide.
6. Schmidt reaction (IIT):
Reactants: Hydrozoic acid and carboxylic acid
Reagent: Sulphuric acid
Product: Amines
7. Curtius Reaction (IIT):
Reactants: Acid chloride and sodium azides
Reagent: Heat and water
Product: Amines
The isocyanate formed by reaction of acid chloride with sodium azides is decomposed with treatment of water and amines are obtained.
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