MP Board · Class 12 · Chemistry · AminesDiscuss the methods of preparation of primary amines using Reduction reactions, Hofmann bromamide degradation reaction, and Gabriel phthalimide synthesis. Give chemical equations for each.

Introduction\nPrimary amines are crucial organic compounds containing the $-NH_2$ group attached to an alkyl or aryl group. They serve as fundamental building blocks in organic synthesis and pharmaceutical manufacturing. Various industrial and laboratory methods exist for their preparation.

1. Reduction of Nitro Compounds\nNitroalkanes and nitroarenes can be reduced to primary amines by passing hydrogen gas in the presence of finely divided catalyst like nickel, palladium, or platinum, or by reduction with metals in acidic medium such as $Fe/HCl$ or $Sn/HCl$. Iron is preferred over hydrochloric acid because $FeCl_2$ formed gets hydrolysed releasing hydrochloric acid during the reaction, requiring only a small amount of acid to initiate the process.

• Reaction: $R-NO_2 + 3H_2 \xrightarrow{Pd/C} R-NH_2 + 2H_2O$
• Example: $CH_3CH_2-NO_2 + 6[H] \xrightarrow{Sn/HCl} CH_3CH_2-NH_2 + 2H_2O$

2. Hofmann Bromamide Degradation Reaction\nAugustus Wilhelm von Hofmann discovered a method in which an amide is treated with bromine in an aqueous or ethanolic solution of sodium hydroxide to give primary amines with one carbon atom less than the parent amide. This degradation reaction is extremely useful for stepping down a series in carbon chain length.

• General Reaction: $R-CONH_2 + Br_2 + 4NaOH \rightarrow R-NH_2 + Na_2CO_3 + 2NaBr + 2H_2O$
• Key Feature: The alkyl or aryl group migrates directly from the carbonyl carbon of the amide to the nitrogen atom.

3. Gabriel Phthalimide Synthesis\nGabriel synthesis is exclusively used for the preparation of pure aliphatic primary amines. Phthalimide is treated with ethanolic potassium hydroxide to form potassium phthalimide, which is then heated with an alkyl halide to give N-alkylphthalimide. Alkaline hydrolysis of the resulting N-alkylphthalimide yields the corresponding primary amine along with phthalic acid (or phthalate salt).

• Steps Involved:
  1. Phthalimide + $KOH \rightarrow$ Potassium phthalimide $+ H_2O$
  2. Potassium phthalimide $+ R-X \rightarrow$ N-Alkylphthalimide $+ KX$
  3. N-Alkylphthalimide $+ NaOH (aq) \rightarrow$ Primary Amine $+$ Sodium phthalate
• Limitation: Aromatic primary amines cannot be prepared by this method because aryl halides do not undergo nucleophilic substitution with the anion formed by phthalimide under ordinary laboratory conditions.