AMIDES: TYPES, PROPERTIES, NOMENCLATURE, USES - CHEMISTRY - 2025

The amides, also called acid amines, are organic compounds containing molecules derived from amines or ammonia. These molecules are linked to an acyl group, converting the amides into a derivative of carboxylic acids by substituting the OH group for an NH ₂, NHR or NRR group.

In other words, amides are formed when a carboxylic acid reacts with a molecule of ammonia or an amine in a process called amidation; A molecule of water is removed and the amide is formed with the remaining parts of carboxylic acid and amine.

It is precisely because of this reaction that amino acids in the human body come together in a polymer to form proteins. All amides, except one, are solid at room temperature and their boiling points are higher than the corresponding acids.

They are weak bases (although stronger than carboxylic acids, esters, aldehydes and ketones), have high solvent power and are very common in nature and in the pharmaceutical industry.

They can also bind and form polymers called polyamides, tough materials found in nylon and kevlar in bulletproof vests.

General Formula

An amide can be synthesized in its simplest form from an ammonia molecule, in which a hydrogen atom has been replaced by the acyl group (RCO-).

This single amide molecule is represented as RC (O) NH ₂ and is classified as a primary amide.

This synthesis can occur in various ways, but the simplest method is through the combination of a carboxylic acid with an amine, at high temperatures, to meet its requirement for a high activation energy and to avoid a reaction. reverse the return of the amide to its initial reagents.

There are alternative methods for the synthesis of amides that use the "activation" of the carboxylic acid, which consists of first converting it to one of the ester groups, acyl chlorides and anhydrides.

On the other hand, other methods start from various functional groups that include ketones, aldehydes, carboxylic acids and even alcohols and alkenes in the presence of catalysts and other auxiliary substances.

Secondary amides, which are more numerous in nature, are those that have been obtained from primary amines, and tertiary amides start from secondary amines. Polyamides are those polymers that have units that are linked by amide bonds.

Types

Amides, similar to amines, can be divided into aliphatic and aromatic. Aromatics are those that comply with the aromaticity rules (a cyclic and flat molecule with resonant bonds that demonstrate stability conditions) and with Hückel's rule.

In contrast, aliphatic amides are subdivided into primary, secondary and tertiary, in addition to polyamides, which are another different type of these substances.

Primary amides

Primary amides are all those in which the amino group (-NH ₂) is directly attached only to a carbon atom, which itself represents the carbonyl group.

The amino group of this amide has only one degree of substitution, so it has free electrons and can form hydrogen bonds with other substances (or other amides). They have the structure RC (O) NH ₂.

Secondary amides

Secondary amides are those amides in which the nitrogen of the amino group (-NH ₂) is first attached to the carbonyl group, but also to another R substituent.

These amides are more common and have the formula RC (O) NHR '. They can also form hydrogen bonds with other amides, as well as with other substances.

Tertiary amides

These are amides in which their hydrogens have been completely substituted by the carbonyl group and two substituent chains or functional groups R.

These amides, because they do not have unpaired electrons, cannot form hydrogen bonds with other substances. Still, all amides (primary, secondary, and tertiary) can bond with water.

Polyamides

Polyamides are polymers that use amides as bonds for their repeating units; that is, the units of these polymers have bonds with each side of the chemical formula –CONH ₂, using these as bridges.

Some amides are synthetic, but others are found in nature, such as amino acids. The uses of these substances are explained in a later section.

Amides can also be divided according to their type of bond into ionic or covalent. Ionic (or saline) amides are highly alkaline compounds that form when a molecule of ammonia, an amine, or a covalent amide is treated with a reactive metal such as sodium.

On the other hand, covalent amides are solid (except for formamide, which is liquid), they do not conduct electricity and, in the case of those that are soluble in water, they serve as solvents for organic and inorganic substances. This type of amide has a high boiling point.

Physical and chemical properties

The physical properties of amides include boiling points and solubility, while chemical properties include the acid-base nature and its ability to decompose by reduction, dehydration and hydrolysis.

Furthermore, it is important to note that amides are colorless and odorless under normal conditions.

Melting and boiling points

Amides have high melting and boiling points for the size of their molecules due to their ability to form hydrogen bonds.

Hydrogen atoms in a –NH ₂ group are positive enough to form a hydrogen bond with a free pair of electrons in another molecule.

These bonds formed require a reasonable amount of energy to break, so the melting points of amides are high.

Ethanamide, for example, forms colorless crystals at 82 ° C, despite being a primary amide and a short chain (CH ₃ CONH ₂).

Solubility

The solubility of amides is quite similar to that of esters, but at the same time they are typically less soluble than comparable amines and carboxylic acids, since these compounds can donate and accept hydrogen bonds.

The smaller amides (primary and secondary) are soluble in water because they have the ability to form hydrogen bonds with water molecules; Tertiaries do not have this ability.

Basicity

Compared to amines, amides have little basic strength; still, they are stronger as bases than carboxylic acids, esters, aldehydes, and ketones.

Due to resonance effects and, therefore, by the development of a positive charge, amines can facilitate the transfer of a proton: this makes them behave like a weak acid.

This behavior is evidenced in the reaction of ethanamide and mercuric oxide to form a salt of mercury and water.

Decomposition capacity by reduction, dehydration and hydrolysis

Although they are not commonly reduced, amides can be decomposed (to amines) through catalytic reduction at high temperature and pressure; they can also be reduced to aldehydes without the need for catalytic routes.

They can be dehydrated in the presence of dehydrating agents (such as thionyl chloride or phosphorous pentoxide) to form a nitrile (-C≡N).

Finally, they can be hydrolyzed to convert them into acids and amines; This reaction will require a strong acid or alkali to take place at a faster rate. Without these, the reaction will take place at a very low rate.

Nomenclature

Amides must be named with the suffix "-amide", or "-carboxamide" if the carbon that is part of the amide group cannot be included in the main chain. The prefix used in these molecules is "amido-", followed by the name of the compound.

Those amides that have additional substituents on the nitrogen atom will be treated as in the case of amines: ordered alphabetically and prefixed with "N-", as is the case with NN-dimethylmethanamide.

Industrial uses and in daily life

Amides, beyond the other applications that they can present, are part of the human body, and for this reason they are crucial in life.

They make up amino acids and come together in polymer form to build protein chains. Also, they are found in DNA, RNA, hormones, and vitamins.

In industry they are commonly found in the form of urea (a waste product of animals), in the pharmaceutical industry (for example, as the main component of paracetamol, penicillin and LSD) and as polyamide in the case of nylon and Kevlar.

Examples

- Formamide (CH ₃ NO), a liquid miscible with water may be part of herbicides and pesticides.

- Ethanamide (C ₂ H ₅ NO), an intermediate product between acetone and urea.

- Ethanediamide (CONH ₂) ₂, substitute for urea in fertilizers.

- N-methylethanamide (C ₃ H ₇ NO), corrosive and highly flammable substance.

References

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4. ChemGuide. (sf). Amides. Retrieved from chemguide.co.ukFarmer, PS (sf). Physical Properties of Amides. Retrieved from chem.libretexts.org