AMINES: STRUCTURE, PROPERTIES, TYPES, USES, EXAMPLES - CHEMISTRY - 2025

The amines are organic compounds derived from ammonia. In them, covalent bonds between carbon and nitrogen occur. Naturally, the nitrogen molecule is kinetically inert; but thanks to biological fixation, it is transformed into ammonia, which in turn undergoes subsequent alkylation reactions.

When ammonia is "alkylated," it replaces one, two, or three of its three hydrogens with carbon atoms. These carbons may well come from an alkyl (R) or aryl (Ar) group. Thus, there are aliphatic amines (linear or branched), and aromatic.

General formula for an amine. Source: MaChe, from Wikimedia Commons.

The general formula for aliphatic amines is shown above. This formula can be used for aromatic amines, considering that R can also be an aryl group Ar. Note the similarity between amine and ammonia, NH ₃. Practically, an H has been replaced by an R side chain.

If R consists of aliphatic chains, you have what is known as an alkylamine; while if R is aromatic in nature, an arylamine. Of the arylamines, the most important of all is alanine: an amino group, -NH ₂, attached to the benzene ring.

When there are oxygenated groups in a molecular structure, such as OH and COOH, the compound is no longer called an amine. In that case, the amine is considered a substituent: the amino group. For example, this happens in amino acids, as well as in other biomolecules of enormous importance for life.

Because nitrogen is found in many of the essential compounds for life, those were considered as vital amines; ie 'vitamins'. However, many of the vitamins are not even amines, and furthermore, not all of them are vital for life. However, this does not negate its great importance in living organisms.

Amines are stronger organic bases than ammonia itself. They are easily extractable from plant matter, and generally have strong interactions with the neuronal matrix of organisms; hence many drugs and drugs consist of amines with complex structures and substituents.

Structure

What is its structure? Although it varies depending on the nature of R, the electronic environment of the nitrogen atom is the same for all of them: tetrahedral. But, since there is a pair of unshared electrons on the nitrogen atom (··), the molecular geometry becomes pyramidal. This is so with ammonia and amines.

Amines can be represented with a tetrahedron, just as with carbon compounds. Thus, NH ₃ and CH ₄ are drawn as tetrahedra, where the pair (··) is located at one of the vertices above the nitrogen.

Both molecules are achiral; however, they begin to show chirality as their H's are replaced by R. The amine R ₂ NH is achiral if the two R's are different. However, it lacks any configuration to differentiate one enantiomer from another (as is the case with chiral carbon centers).

This is because the enantiomers:

R ₂ N-H - H-NR ₂

they are exchanged at such a rate that neither of them can isolate themselves; and therefore the structures of the amines are considered achiral even though all the substituents on the nitrogen atom are different.

Properties of amines

Polarity

Amines are polar compounds, since the NH ₂ amino group, having an electronegative nitrogen atom, contributes to the dipole moment of the molecule. Note that nitrogen has the ability to donate hydrogen bonds, which causes amines to generally have high melting and boiling points.

However, when this property is compared with that of oxygenated compounds, such as alcohols and carboxylic acids, they result in lower magnitudes.

For example, the boiling point of ethylamine, CH ₃ CH ₂ NH ₂ (16.6 ° C) is lower than that of ethanol, CH ₃ CH ₂ OH (78 ° C).

Thus, it is shown that OH hydrogen bonds are stronger than those of NH, even though an amine can form more than one bridge. This comparison is only valid if R has the same molecular weight for the two compounds (CH ₃ CH ₂ -). On the other hand, ethane boils at -89ºC, CH ₃ CH ₃, being a gas at room temperature.

As an amine has less hydrogen, it forms fewer hydrogen bonds and its boiling point is lowered. This is observed by comparing the boiling point of dimethylamine, (CH ₃) ₂ NH (7 ° C), with that of ethylamine (16.6 ° C).

Physical characteristics

In the world of chemistry, when talking about an amine, the involuntary act of holding your nose arises. This is because in general, they tend to have unpleasant odors, some of which come to resemble that of rotten fish.

In addition, liquid amines tend to have yellowish tones, which increase the visual mistrust they generate.

Water solubility

Amines tend to be insoluble in water because, despite being able to form hydrogen bonds with H ₂ O, their majority organic component is hydrophobic. The bulkier or longer the R groups are, the lower their solubility in water.

When there is an acid in the medium, however, the solubility is increased by the formation of what are known as amine salts. In them, nitrogen has a positive partial charge, which electrostatically attracts the anion or conjugate base of the acid.

For example, in a dilute solution of HCl, the amine RNH ₂ reacts as follows:

RNH ₂ + HCl => RNH ₃ ⁺ Cl ^- (primary amine salt)

RNH ₂ was insoluble (or slightly soluble) in water, and in the presence of the acid it forms a salt, whose solvation of its ions favors its solubility.

Why does this happen? The answer lies in one of the main properties of amines: they are polar and basic. Being basic, they will react with acids strong enough to protonate them, according to the Brönsted-Lowry definition.

Basicity

Amines are stronger organic bases than ammonia. The higher the electron density around the nitrogen atom, the more basic it will be; that is, it will deprotonate the acids in the environment more quickly. If the amine is very basic, it can even take the proton out of alcohols.

The R groups contribute electron density to nitrogen by inductive effect; since, we must not forget that it is one of the most electronegative atoms in existence. If these groups are very long or bulky, the inductive effect will be greater, which will also increase the negative region around the electron pair (··).

This causes (··) to accept the H ⁺ ion more quickly. However, if R are very bulky, the basicity decreases by steric effect. Why? For the simple reason that H ⁺ must cross a configuration of atoms before reaching nitrogen.

Another way to reason about the basicity of an amine is by stabilizing its amine salt. Now, the one that decreases by inductive effect can decrease the positive charge N ⁺, it will be a more basic amine. The reasons are the same just explained.

Alkylamines vs arylamines

Alkylamines are much more basic than arylamines. Why? To understand it simply, the structure of aniline is shown:

Aniline molecule. Source: Calvero., via Wikimedia Commons

Above, in the amino group, is the pair of electrons (··). This pair "travels" within the ring in the ortho and para positions with respect to NH ₂. This means that the two upper vertices and the one opposite NH ₂ are negatively charged, while the nitrogen atom is positively charged.

Since the nitrogen is positively charged, ⁺ N, it will repel the H ⁺ ion. And if this is not enough, the electron pair is delocalized within the aromatic ring, making it less accessible to deprotonate acids.

The basicity of aniline can increase if groups or atoms that give it electronic density are linked to the ring, competing with the pair (··) and forcing it to be more likely to be located on the nitrogen atom, ready to act as a base.

Types (primary, secondary, tertiary)

Types of amines. Source: Jü via Wikipedia.

Although not formally presented, reference has been implicitly made to primary, secondary and tertiary amines (top image, left to right).

The primary amines (RNH ₂) are monosubstituted; the secondary ones (R ₂ NH) are disubstituted, with two R alkyl or aryl groups; and the tertiary ones (R ₃ N), are trisubstituted, and it lacks hydrogen.

All existing amines are derived from these three types, so their diversity and interactions with the biological and neuronal matrix are enormous.

In general, tertiary amines could be expected to be the most basic; however, such a claim cannot be made without knowing the structures of R.

Training

Ammonia alkylation

At first it was mentioned that amines are derived from ammonia; therefore, the simplest way to form them is by alkylation. To do this, an excess of ammonia is reacted with an alkyl halide, followed by the addition of a base to neutralize the amine salt:

NH ₃ + RX => RNH ₃ ⁺ X ^- => RNH ₂

Note that these steps lead to a primary amine. Secondary and even tertiary amines can also be formed, whereby the yield for a single product decreases.

Some training methods, such as the Gabriel synthesis, make it possible to obtain primary amines so that other undesirable products are not formed.

Also, ketones and aldehydes can be reduced in the presence of ammonia and primary amines, to give rise to secondary and tertiary amines.

Catalytic hydrogenation

Nitro compounds can be reduced in the presence of hydrogen and a catalyst to their corresponding amines.

ArNO ₂ => ArNH ₂

The nitriles, RC≡N, and the amides, RCONR ₂, are also reduced to give primary and tertiary amines, respectively.

Nomenclature

How are amines named? Most of the time they are named based on R, the alkyl or aryl group. To the name of R, derived from its alkane, the word 'amine' is added at the end.

Thus, CH ₃ CH ₂ CH ₂ NH ₂ is propylamine. On the other hand, it can be named considering only the alkane and not as an R group: propanamine.

The first way of naming them is by far the best known and most used.

When there are two NH ₂ groups, the alkane is named and the positions of the amino groups are listed. Thus, H ₂ NCH ₂ CH ₂ CH ₂ CH ₂ NH ₂ is called: 1,4-butanediamine.

If there are oxygenated groups, such as OH, it must be given priority over NH ₂, which happens to be named as a substituent. For example, HOCH ₂ CH ₂ CH ₂ NH ₂ is called: 3-Aminopropanol.

And regarding secondary and tertiary amines, the letters N are used to indicate the R groups. The longest chain will remain with the name of the compound. Thus, CH ₃ NHCH ₂ CH ₃ is called: N-Methylethylamine.

Applications

Colorants

Primary aromatic amines can serve as a starting material for azo dye synthesis. Initially, the amines react to form diazonium salts, which form the azo compounds by azo coupling (or diazo coupling).

These, due to their intense coloration, are used in the textile industry as a dyeing material; for example: methyl orange, direct brown 138, sunset yellow FCF and ponceau.

Drugs and drugs

Many drugs work with agonists and antagonists of natural amine neurotransmitters. Examples:

-Chloropheniramine is an antihistamine used in the control of allergic processes due to the ingestion of some foods, hay fever, insect bites, etc.

-Chlorpromazine is a sedative agent, not a sleep inducer. It relieves anxiety and is even used in the treatment of some mental disorders.

-Ephedrine and phenylephedrine are used as respiratory decongestants.

-Amitriptyline and imipramine are tertiary amines that are used in the treatment of depression. Due to their structure, they are classified as tricyclic antidepressants.

-Opioid pain relievers such as morphine, codeline, and heroin are tertiary amines.

Gas Treatment

Several amines, including diglycolamine (DGA) and diethanolamine (DEA), are used in the removal of carbon dioxide (CO ₂) and hydrogen sulfide (H ₂ S) gases present in natural gas and in refineries.

Agricultural chemistry

Methylamines are intermediate compounds in the synthesis of chemicals that are used in agriculture as herbicides, fungicides, insecticides, and biocides.

Resin manufacturing

Methylamines are used during the production of ion exchange resins, usable in the deionization of water.

Animal Nutrients

Trimethylamine (TMA) is used primarily in the production of choline chloride, a vitamin B supplement used in the feed of chickens, turkeys and pigs.

Rubber industry

Dimethylamine oleate (DMA) is an emulsifier for use in the production of synthetic rubber. DMA is used directly as a polymerization modifier in the vapor phase of butadiene, and as a stabilizer for natural rubber latex in place of ammonia

Solvents

Dimethylamine (DMA) and monomethylamine (MMA) are used to synthesize the polar aprotic solvents dimethylformamide (DMF), dimethylacetamide (DMAc), and n-methylpyrrolidone (NMP).

Applications for DMF include: urethane coating, acrylic yarn solvent, reaction solvents, and extraction solvents.

DMAc is used in the manufacture of yarn dyes and solvent. Finally, NMP is used in the refining of lubricating oils, paint stripper and enamel coating.

Examples

Cocaine

Cocaine molecule. Source: NEUROtiker, via Wikimedia Commons

Cocaine is used as a local anesthetic in certain types of eye, ear, and throat surgery. As seen, it is a tertiary amine.

Nicotine

Nicotine molecule. Source: Jü, from Wikimedia Commons

Nicotine is the primary agent of tobacco addiction and chemically it is a tertiary amine. The nicotine in tobacco smoke is rapidly absorbed and highly toxic.

Morphine

Morphine molecule. Source: NEUROtiker, from Wikimedia Commons

It is one of the most effective pain relievers to relieve pain, especially cancer. It is, again, a tertiary amine.

Serotonin

Serotonin molecule. Source: Harbin, from Wikimedia Commons

Serotonin is an amine neurotransmitter. In depressed patients the concentration of the main metabolite of serotonin is decreased. Unlike the other amines, this one is primary.

References

1. Graham Solomons TW, Craig B. Fryhle. (2011). Organic Chemistry. Amines. (10 ^th edition.). Wiley Plus.
2. Carey F. (2008). Organic Chemistry. (Sixth edition). Mc Graw Hill.
3. Morrison and Boyd. (1987). Organic chemistry. (Fifth edition). Addison-Wesley Iberoamericana.
4. The Chemours Company. (2018). Methylamines: uses and applications. Recovered from: chemours.com
5. Transparency Market Research. (sf). Amines: important facts and uses. Recovered from: transparencymarketresearch.com
6. Wikipedia. (2019). Amine. Recovered from: en.wikipedia.org
7. Ganong, WF (2003). Medical Physiology. 19th edition. Editorial El Manual Moderno.