Preparation of Amines: Reduction of Nitro Compounds

Amines are an essential part of organic chemistry. In this article, we are going to discuss the preparation of amines. Before reading about the preparation of amines, you should know some important basic concepts such as their physical and chemical properties, their uses, and other related topics. 

Amines can be prepared from several methods, for instance, reduction of nitro compounds to amines. We are going to discuss these processes in detail in this article.

What are nitro compounds?

Nitro compounds contain one or multiple nitro functional groups. They are organic compounds. Nitro compounds are a part of many organic reactions. One of the most critical reactions is the reduction of nitro to an amine. The chemical reaction for the same is: 

RNO2 + 3H2 → RNH2 + 2H2O

This reduction of nitro to amine mechanism will be explained in detail below. Apart from reduction reactions, nitro compounds are also used in condensation reactions and biochemical reactions.

What are amines?

Amines are functional groups and compounds that contain one Lone pair and one basic nitrogen atom. Amines are ammonia derivatives. Amines are used in producing azo dyes, painkillers, and several drugs. It is also used in water purification.

Amines have four types:

Primary amines

In these amines, aryl or alkyl replaces one atom of hydrogen. CH3NH2 is an example of primary amines.

Secondary amines

In these amines, aryl or alkyl replaces two atoms of hydrogen. Dimethylamine or CH3NHCH3 are some key examples of secondary amines.

Tertiary amines

In these amines, aryl or alkyl replaces the three atoms of hydrogen. Trimethylamine is an example of tertiary amines. 

Cyclic amines

Note that secondary and tertiary amines can be cyclic. A typical example of cyclic amines is a three-member ring aziridine.

Methods of preparation of amines

Several methods are used to prepare amines. Here are some of them:

Preparation of amines by reduction of nitro compounds

Nitro compounds are reduced by a chemical reduction, generally by acid and metal or by catalytic hydrogenation by using molecular hydrogen. However, this method of reducing nitro compounds to amine is not used when the molecule has some hydrogenated group like a carbon double bond. Generally, the chemical reduction method is preferred by adding hydrochloric acid to the metal and nitro compound mixture. 

Reduction of nitro compounds can also be made with active metals like Sn, Fe, Zn, and so on with concentrated HCl.

(In the presence of HCl/ Sn)

R- NO2 + 3H2  → R- NH2 + 2H2O   

Preparation of amines by reduction of amides

It is possible to convert amides directly to their corresponding amines. This is done by treating amine with bromine and base mixture (KOH + Br2). The reaction for this is:

RCONH2 + Br2 + 4KOH → RNH2 + K2CO2 + 2KBr + 2H2O

This reaction is known as the Hofmann Bromamide reaction. The amine formed through this reaction has one less carbon than the corresponding amine. Because of this loss of atoms, the reaction is also known as Hofmann degradation of amides.

Gabriel Phthalimide Synthesis 

Gabriel phthalimide synthesis is used for making primary amines. Ethanol is a potassium hydroxide used to treat phthalimide, resulting in phthalimide potassium salts. On heating it with an alkyl halide and then by alkaline hydrolysis, the corresponding primary amine is produced. 

However, this method is not used to prepare primary aromatic amines because aryl halides don’t react with anions that are formed through phthalimide.

Preparation of amines by ammonolysis of alkyl halide

Aliphatic halogen compounds on treatment with an alcoholic or aqueous solution of ammonia are converted to amines.

This reaction is carried out by heating them under specific pressure or making the reactants stand at average room temperature. NH3 displaces halogen, which produces amine salts. From the amine salts, the free amines can be liberated.

In this method, all the three types of amine- primary, secondary and tertiary- can be yielded and ammonium salt.

Physical Properties of Amines

To understand the preparation of amines, it is essential to know their physical properties. Here are the physical properties of amines:

Lower aliphatic amines bear a fishy odor and are found in the gaseous state. Primary amines generally have three to four carbon atoms at average room temperature and are found in a liquified state. However, primary amines possess four carbon atoms and are found in a solid state at average room temperature.

Aryl amines, aniline, for instance, are usually colorless. But they do acquire color if kept open. They gain color due to atmospheric oxidation.

Lower aliphatic amines can quickly form hydrogen bonds with the help of water molecules. This indicates that amines are soluble in water. It is important to note that as the hydrophobic alkyl increases in size, the molar mass of amines also increases, decreasing its solubility in water. This indicated that higher amines are not soluble in water.

In the case of tertiary amines, the intermolecular association is absent because of the lack of free hydrogen atoms. These free hydrogen atoms are important for bonding.

Organic solvents can easily dissolve amines. Amines have lower polarity. Thus, they form strong intermolecular hydrogen bonds.

Conclusion

Preparation of amines from the reduction of aromatic nitro compounds is an important topic in organic chemistry. There are other methods of preparing amines, including reduction of nitro to amine mechanism, Gabriel phthalimide synthesis, and preparation of amines by reduction of amines.