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Preparation of Amines | chemca

Preparation of Amines | chemca
Organic Chemistry

Preparation of Amines

Synthesis from Nitro compounds, Halides, Nitriles, Amides, and Phthalimide.

By chemca Team • Updated Jan 2026

Amines are organic derivatives of ammonia ($NH_3$). They can be prepared by reduction of various functional groups or by nucleophilic substitution reactions.

1. Reduction of Nitro Compounds

Catalytic or Acidic Reduction

Nitro compounds are reduced to amines by passing hydrogen gas in the presence of catalysts ($Ni, Pt, Pd$) or by treatment with metals in acidic medium.

$$ R-NO_2 + 3H_2 \xrightarrow{Pd, \text{Ethanol}} R-NH_2 + 2H_2O $$
Laboratory Method (Scrap Metal + Acid):
$$ R-NO_2 + 6[H] \xrightarrow{Sn/HCl \text{ or } Fe/HCl} R-NH_2 + 2H_2O $$
Note: Reduction with Fe scrap and HCl is preferred because $FeCl_2$ formed hydrolyzes to release $HCl$, so only a small amount of acid is required to initiate the reaction.

2. Ammonolysis of Alkyl Halides

Nucleophilic Substitution

Reaction of alkyl halides with ethanolic solution of ammonia yields a mixture of primary, secondary, and tertiary amines, and finally quaternary ammonium salt.

$$ R-X \xrightarrow{NH_3} R-NH_2 \xrightarrow{R-X} R_2NH \xrightarrow{R-X} R_3N \xrightarrow{R-X} [R_4N]^+X^- $$
Condition for $1^\circ$ Amine: Use excess of Ammonia ($NH_3$) to minimize poly-alkylation.

3. Reduction of Nitriles and Amides

Using Hydrides

A. Nitriles ($R-CN$): Reduced to $1^\circ$ Amines using $LiAlH_4$ or $H_2/Ni$.

$$ R-C \equiv N \xrightarrow{H_2/Ni \text{ or } Na(Hg)/C_2H_5OH} R-CH_2-NH_2 $$

Reduction with Sodium Amalgam and Ethanol is called Mendius Reaction.

B. Amides ($R-CONH_2$): Reduced to amines retaining the same number of carbon atoms.

$$ R-CO-NH_2 \xrightarrow{1. LiAlH_4, 2. H_2O} R-CH_2-NH_2 $$

4. Gabriel Phthalimide Synthesis

Pure Primary Amines

Used specifically for the preparation of Primary Aliphatic Amines. Phthalimide reacts with ethanolic KOH to form potassium phthalimide, which reacts with alkyl halide to form N-alkylphthalimide. Alkaline hydrolysis yields the amine.

Phthalimide $\xrightarrow{KOH}$ K-Phthalimide $\xrightarrow{R-X}$ N-Alkylphthalimide $\xrightarrow{NaOH (aq)}$ $1^\circ$ Amine ($R-NH_2$)
Limitation: Aromatic primary amines (like Aniline) cannot be prepared by this method because aryl halides do not undergo nucleophilic substitution with the phthalimide anion.

5. Hoffmann Bromamide Degradation

Step Down Reaction

An amide reacts with Bromine in an aqueous or ethanolic solution of NaOH/KOH to form a primary amine with one carbon less than the parent amide.

$$ R-CONH_2 + Br_2 + 4KOH \rightarrow R-NH_2 + K_2CO_3 + 2KBr + 2H_2O $$
The alkyl group migrates from Carbon to Nitrogen.

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