Uses of Amines

What are amines?

In Amines, we introduced you to a new type of organic molecule: amines. These are ammonia derivatives, characterised by a nitrogen atom bonded to at least one organic hydrocarbon R group. Amines can be further divided into three different types:

• Primary amines contain a nitrogen atom bonded to just one R group and have the general formula NH₂R.
• Secondary amines contain a nitrogen atom bonded to two R groups and have the general formula NHR₂.
• Tertiary amines contain a nitrogen atom bonded to three R groups and have the general formula NR₃.

Fig. 1 - Primary, secondary and tertiary amines

You can also get quaternary ammonium cations. These consist of a nitrogen atom bonded to four R groups. The nitrogen atom bonds to the fourth R group using a dative covalent bond. Quaternary ammonium ions are an important part of quaternary ammonium salts.

Fig. 2 - A quaternary ammonium cation

Characteristics of amines

Let's now briefly consider some of the characteristic properties of amines:

• Amines are polar molecules.
• Amines form hydrogen bonds, both with other amine molecules and with water. This means that they have high melting and boiling points, and that shorter-chain amines are soluble in aqueous solutions.
• Amines can act as both nucleophiles and bases. Nucleophiles are electron-pair donors whilst bases are hydrogen ion acceptors. This enables many of their reactions.

Uses of amines in daily life

Now that we know not only what amines are, but also about their characteristics and properties, we can look at some of their applications in daily life. After that, we'll consider their uses in industry.

• Amines are found in every cell in your body in the form of proteins. Proteins are condensation polymers, made up of repeating units called amino acids. Each amino acid has a carboxyl and an amine functional group. Multiple amino acids join together to form a long polymer chain. This chain then folds into a specific 3D shape that is unique to each protein.
• Another type of polymer involving amines is polyamides. These include nylon, Kevlar, and a variety of plastics.
• Many common drugs and pharmaceuticals are amines. These include the analgesic morphine, the decongestant ephedrine, and the antidepressant amoxapine.
• Amines play a role in cosmetics, such as shampoos, soaps, and shaving creams. We'll look at how they are made in just a second.
• The common compound tetramethylammonium chloride, used to disinfect water, is also an amine.
• Amines are the precursor to many dyes and tanning agents.

Uses of amines in industry

Knowing what we use amines for is well and good, but how do we make those products in industry? It is now time to learn about two important industrial applications of amines.

• Quaternary ammonium salts.
• Azo compounds.

Quaternary ammonium salts

The first important use of amines in industry is making quaternary ammonium salts.

Earlier, we learned that a quaternary ammonium ion consists of a nitrogen atom bonded to four organic hydrocarbon R groups. It has a permanent positive charge, which means that it can bond ionically to negatively-charged ions, forming a quaternary ammonium salt.

Fig. 3 - A quaternary ammonium salt

Uses of quaternary ammonium salts

Quaternary ammonium salts have a few uses: conditioners, detergents, and antimicrobial agents. They're suitable because of one particular characteristic: their charge. In conditioners and fabric softeners, the positive charge of the ammonium ion is attracted to the negative charge of wet clothes or hair, and the ammonium ions form a layer on the surface. This helps keep the hair or fabric smooth and glossy. In detergents and antimicrobial agents, the positive charge of the ammonium ion is attracted to the negative charge of bacterial cell walls. This disrupts the wall and damages the cell.

Azo compounds

The second use of amines in industry is making diazonium salts and azo compounds. Diazonium salts contain an -N⁺≡N group, while azo compounds contain an N=N azo group. Producing azo compounds involves a multi-step synthesis:

1. Phenylamine (C₆H₅NH₂) is reacted with nitric(III) acid (HNO₂) at low temperatures to form a diazonium salt.
2. The diazonium salt reacts with another aromatic organic molecule to form an azo compound.

Let's investigate those steps in more detail.

Forming a diazonium salt

Firstly, phenylamine reacts with nitric(III) acid at low temperatures to form a diazonium salt containing the -N≡N⁺ group. Nitric(III) acid is extremely reactive and so must be prepared in situ. To carry out the reaction, we mix phenylamine with a chilled solution of a strong acid, such as hydrochloric acid (HCl), and then add sodium nitrite (NaNO₂). The hydrochloric acid and sodium nitrite first react to form nitric(III) acid and sodium chloride:

$$HCl+NaNO_2\rightarrow HNO_2+NaCl$$

The nitric(III) acid formed then reacts with phenylamine and more hydrochloric acid to form a diazonium salt:

$$C_6H_5NH_2+NHO_2+HCl\xrightarrow{\text{< 10}^\circ \text{C}} C_6H_5N^+N\space Cl^-+2H_2O$$

Here's a diagram to help you understand the structure of the molecules involved.

Fig. 4 - Forming a diazonium salt

Forming an azo compound

The second step of the process involves reacting the diazonium salt with another aromatic organic molecule. This is an example of a coupling reaction, forming an azo compound with the N=N azo functional group. In this reaction, the diazonium ion acts as an electrophile and substitutes into the second molecule's benzene ring.

One example is the reaction of a diazonium salt with phenol. This reaction takes place in a basic solution, typically of sodium hydroxide, and produces an azo compound with two benzene rings, one with an -OH group. These benzene rings are joined by an N=N azo bridge. It also produces an acid, which varies depending on the diazonium salt used.

Here's the equation for the reaction between a diazonium chloride salt and phenol. The structural formulae of these molecules can get a little tricky, so we've used displayed formulae to show you the reaction.

Fig. 5 - Forming an azo compound using phenol

A similar reaction takes place between diazonium salts and phenylamine. This produces another type of azo compound, but this time the second benzene ring has an -NH₂ amine group instead of a phenyl group:

Fig. 6 - Forming an azo compound using phenylamine. This time, the azo product has an amine group instead of a phenyl group

Uses of azo compounds

To conclude this section, we'll consider the uses of azo compounds. Thanks to their two benzene rings, which are full of delocalised pi electrons, azo compounds are very stable and present with vivid colours. Azo compounds, therefore, form the basis of many dyes, including ones such as methyl orange that are used as pH indicators. They're also used in the textile industry and in tattoo inks.

Importance of amines

This article is dedicated to the uses of amines, both in everyday life and in industry, and so you should now have an appreciation of why these molecules are so valuable. However, we'll end by summarising a few of reasons why amines are important to us. Then, we'll take a deep dive into the importance of amines in the human body.

• Amines form the basis of many dyes, cosmetics, and toiletries.
• Approximately 42% of all drugs rely on amines - without them, our first-aid kits would be severely depleted!
• Amines also help us clean up our waste gases. These molecules are used to 'sweeten' polluted gases released by oil refineries and processing plants by removing hydrogen sulfide (H₂S) and carbon dioxide (CO₂). As a result, they help mitigate climate change.
• Furthermore, amines are vital inside the human body, as you'll discover below.