Amides: General Formula, Types, Properties, Nomenclature, Uses and Examples

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

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

It is precisely because of this reaction that the 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 join and form polymers called polyamides, resistant materials present in the nylon and kevlar of the bulletproof vests.

General Formula

An amide can be synthesized in its simplest form from a molecule of ammonia, in which a hydrogen atom has been replaced by the acyl group (RCO-).

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

This synthesis can take several forms, but the simplest method is through the combination of a carboxylic acid with an amine, at high temperatures, to meet its requirement of a high activation energy and to avoid a reaction reverse the return of the amide to its initial reactants.

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

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

The secondary amides, which are more numerous in nature, are those that have been obtained from primary amines, and the tertiary amides are derived 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 amides, in addition to polyamides, which are another distinct type of these substances.

Primary amides

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

The amino group of this amide has a single 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

The secondary amides are those amides wherein 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 replaced in their entirety by the carbonyl group and two substituent chains or functional groups R.

These amides, by not having unpaired electrons, can not form hydrogen bridges with other substances. Even so, all amides (primary, secondary and tertiary) can form a 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.

The amides can also be divided according to their type of bond in ionic or covalent. Ionic amides (or salines) are highly alkaline compounds that are formed when treating a molecule of ammonia, an amine or a covalent amide with a reactive metal such as sodium.

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

Physical and chemical properties

Among the physical properties of the amides, the boiling points and the solubility can be named, while the chemical properties have the acid-base nature and their decomposition capacity by reduction, dehydration and hydrolysis.

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

Melting and boiling points

The amides have high melting and boiling points for the size of their molecules because of their ability to form hydrogen bonds.

The hydrogen atoms in a group -NH ₂ they are sufficiently positive to form a hydrogen bond with an electron-free pair in another molecule.

These formed bonds require a reasonable amount of energy to break, so the melting points of the 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 the amides is quite similar to that of the 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 smallest amides (primary and secondary) are soluble in water because they have the ability to form hydrogen bonds with water molecules; the tertiaries do not have this ability.

Basicity

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

By 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 mercury oxide to form a salt of mercury and water.

Decomposition capacity by reduction, dehydration and hydrolysis

Although they are not commonly reduced, the amides can be decomposed (to amines) through a 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 dehydrators (such as thionyl chloride or phosphorus 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 be carried out at a more accelerated rate. Without these, the reaction will be carried out at very low speed.

Nomenclature

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

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

Industrial uses and in daily life

Amides, beyond the other applications that can present, are part of the human body, and for this reason are crucial in life. They make up the amino acids and bind in a polymer form to build the protein chains. In addition, they are found in DNA, RNA, hormones and vitamins.

In the industry they can be commonly found in the form of urea (a waste product of animals), in the pharmaceutical industry (for example, as a 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 that can be part of herbicides and pesticides.

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

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

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

References

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2. Assignment, C. (s.f.). Preparation and Properties of Amides. Retrieved from chemistry-assignment.com
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4. ChemGuide. (s.f.) Amides. Retrieved from chemguide.co.ukFarmer, P. S. (s.f.). Physical Properties of Amides. Retrieved from chem.libretexts.org