# 12.1 Preparation of Amines Amines are organic derivatives of ammonia. Primary amines ($RNH_2$) and secondary amines ($R_2NH$) can be synthesized through various chemical reactions depending on the desired structure and available starting materials. This section outlines the primary methods for their preparation. ### 1. Preparation by Reaction of Alkyl Halides with Ammonia (Ammonolysis) This method involves the reaction of an alkyl halide with ammonia. #### Mechanism and Challenges When an ethanolic solution of ammonia ($NH_3$) is heated with an alkyl halide (e.g., bromoethane) in a sealed tube at $100^{\circ}C$, the nitrogen atom in ammonia, being nucleophilic, attacks the electrophilic carbon of the alkyl halide. This initially forms an alkylammonium salt. This salt then reacts with excess ammonia to produce the free primary amine. $$ NH_3 + CH_3CH_2Br \longrightarrow CH_3CH_2NH_3^+Br^- $$ $$ CH_3CH_2NH_3^+Br^- + NH_3 \longrightarrow CH_3CH_2NH_2 + NH_4^+Br^- $$ *ethylamine* A significant challenge with this method is that the reaction does *not* stop at the primary amine stage. The newly formed primary amine is also nucleophilic and can react further with the alkyl halide, leading to a mixture of secondary, tertiary amines, and eventually quaternary ammonium salts. This process is known as **over-alkylation**. * **Formation of Secondary Amine:** The primary amine reacts with another molecule of alkyl halide. $$ CH_3CH_2NH_2 + CH_3CH_2Br \longrightarrow (CH_3CH_2)_2NH_2^+Br^- $$ $$ (CH_3CH_2)_2NH_2^+Br^- + NH_3 \longrightarrow (CH_3CH_2)_2NH + NH_4^+Br^- $$ *(diethylamine)* * **Formation of Tertiary Amine:** The secondary amine then reacts with a third molecule of alkyl halide. $$ (CH_3CH_2)_2NH + CH_3CH_2Br \longrightarrow (CH_3CH_2)_3NH^+Br^- $$ $$ (CH_3CH_2)_3NH^+Br^- + NH_3 \longrightarrow (CH_3CH_2)_3N + NH_4^+Br^- $$ *(triethylamine)* * **Formation of Quaternary Ammonium Salt:** Finally, the tertiary amine reacts with a fourth molecule of alkyl halide to form a quaternary ammonium salt, which is stable and does not react further with ammonia. $$ (CH_3CH_2)_3N + CH_3CH_2Br \longrightarrow (CH_3CH_2)_4N^+Br^- $$ *(tetraethylammonium bromide)* #### Product Separation At the end of the reaction, the addition of a strong alkali, such as $KOH$, liberates the free primary, secondary, and tertiary amines from their respective salts. However, the quaternary ammonium salt is unaffected by the alkali. The mixture of primary, secondary, and tertiary amines can then be separated by **fractional distillation** due to their different boiling points. To favor the production of the primary amine and minimize over-alkylation, a large **excess of ammonia** is typically used. However, even with excess ammonia, the yield of the primary amine is often low. ### 2. Preparation by Reduction of Nitrogen-Containing Functional Groups Compounds containing nitrogen, such as nitriles, nitro compounds, and amides, can be reduced to amino groups using various reducing agents. These methods generally offer better control over the degree of alkylation compared to ammonolysis. #### (a) Reduction of Nitriles The reduction of alkyl or aryl nitriles ($R-C \equiv N$) yields primary amines. This reduction can be achieved using strong reducing agents like **Lithium Aluminum Hydride ($LiAlH_4$)** or by **sodium in ethanol ($Na/C_2H_5OH$)**. $$ R-C \equiv N + 4[H] \xrightarrow{LiAlH_4 \text{ or } Na/C_2H_5OH} R-CH_2-NH_2 $$ #### (b) Catalytic Reduction of Nitriles Nitriles can also be reduced catalytically by reacting with hydrogen gas ($H_2$) in the presence of a metal catalyst, typically **nickel ($Ni$)**, to form primary amines. $$ CH_3CH_2C \equiv N + 2H_2 \xrightarrow{Ni} CH_3CH_2CH_2-NH_2 $$ #### (c) Reduction of Amides When an amide ($RCONH_2$) is treated with a strong reducing agent like **Lithium Aluminum Hydride ($LiAlH_4$)**, the carbonyl group ($-CO-$) of the amide is reduced to a methylene group ($-CH_2-$), forming a primary amine. This is an important method for chain lengthening as it retains all carbon atoms. $$ CH_3CONH_2 + 4[H] \xrightarrow{LiAlH_4} CH_3CH_2NH_2 + H_2O $$ ### 3. Hofmann Bromamide Degradation This reaction is used to convert an amide into a primary amine with **one less carbon atom** than the starting amide. The amide is treated with bromine ($Br_2$) in an aqueous or ethanolic solution of sodium hydroxide ($NaOH$). $$ RCONH_2 + Br_2 + 4NaOH \longrightarrow R-NH_2 + Na_2CO_3 + 2NaBr + 2H_2O $$ ### 4. Reduction of Nitro Compounds Nitro compounds (especially nitrobenzene) are reduced to primary amines using metals in acidic media (like $Sn/HCl$ or $Fe/HCl$) or by catalytic hydrogenation. $$ C_6H_5NO_2 + 6[H] \xrightarrow{Sn/HCl} C_6H_5NH_2 + 2H_2O $$ ### 5. Preparation of Aniline from Benzene Diazonium Chloride Aniline can be prepared by the reduction of benzene diazonium chloride or via the hydrolysis of diazonium salts in specific conditions, though it is more commonly used as a starting material for other derivatives.