Amides

The structure and classification of amides

Amides are derivatives of carboxylic acids. In the original acid molecule, the hydroxyl group is replaced by an amine group. Depending on the degree of substitution of a specific amide, hydrogen atoms, an acid radical or any organic group attach to the nitrogen atom. Therefore, amides are characterised by the presence of as many as two funcional groups in their molecules: a carbonyl group and an amino group. The chemical structure of amides is the most diverse among all carboxylic acid derivatives as amides include compounds as simple as urea and as complex as proteins. The amide bond is therefore one of the more crucial bonds for all living organisms as it is found in polypeptide chains.

Based on the structure of their molecules, amides are divided into:

• Primary amides – they are characterised by one hydrogen atom of the NH₃ molecule replaced by an acyl group. An example of such an amide is acetamide (ethanamide).
• Secondary amides – with two acyl groups in their molecules. Secondary amides are commonly referred to as imides. An example of a secondary amide is N-methylacetamide.
• Tertiary amides – they have three acyl groups attached to a nitrogen atom. They are called triacylimines. An example of this group of amides is N,N-dimethylacetamide.

Imides are a group of amides worth particular attention. They are derivatives of anhydrides of dicarboxylic acids, in which the oxygen atom was replaced by a divalent imide group =NH. Examples of such compounds are succinic acid imide and phthalic acid imide. Imides of dicarboxylic acids are usually obtained by reacting dicarboxylic acid anhydrides with ammonia. An interesting example of an imide is carbonic acid diimide, commonly referred to as urea. Nowadays, it is an important raw material in chemical processing. Its large-scale production is mainly due to its use in the fertiliser industry. In addition, urea is the first organic compound that was obtained by chemical synthesis (outside the human body). This was done by Friedrich Wohler in 1828.

Preparation

There are several basic methods for obtaining carboxylic acid amides.

One of the basic reactions in order to obtain amides is heating certain carboxylic acids with an aqueous solution of ammonia. This leads to obtaining primary amides. An intermediate product of such a reaction is the ammonium salt of a carboxylic acid that produces an amide and a water molecule as a result of pyrolysis. You can also use the reaction that occurs between acid anhydrides and ammonia. A primary amide and ammonium salt of the corresponding acid can also be used. Primary amides can also be obtained using esters and nitriles.

Secondary amides are obtained by reacting certain carboxylic acids with the corresponding primary amides. Such a reaction results in a secondary amide and a water molecule. Moreover, secondary amides can be obtained by using particular esters.

Tertiary amides are obtained in the same manner as secondary amides. In this case, however, carboxylic acid reacts with a secondary amide. Similarly to the cases described above, the products of the transformation are a tertiary amide and a water molecule. But that’s not all: tertiary amides can be obtained by reacting an ester with an aqueous ammonia solution.

Among carboxylic acid amides, acetic acid amide deserves special attention. It is commonly known as acetamide. It is obtained, among others, by dehydration of ammonium acetate when the latter is heated to the decomposition temperature. At the same time, the simplest structure in an amide can be found in benzoic acid amide (benzamide). This substance is obtained by reacting ammonia with benzoyl chloride (benzoic acid chloride). The reaction involves substitution with a functional group of the chloride ion.

Properties

The structure of amide molecules directly determines their properties. The amide bond found in the molecule is planar (flat). Amides are generally neutral in nature (but may be slightly acidic in some cases). Simple amides are well soluble in water. This is related to the structure of amide molecules, and in particular to the absence of a long hydrocarbon chain and the presence of a highly electronegative nitrogen atom, as well as carbon atoms that can form hydrogen bonds. When an aqueous solution of simple amides is heated for prolonged time, they convert to ammonium salts. The simplest amide, namely methanoamide (formamide), has a liquid form at room temperature. The other compounds in this group are solid. They are also characterised by relatively high melting and boiling points. Their values far exceed those specific to their corresponding carboxylic acids. What is more, thanks to their molecular structure, amides demonstrate significant polarity and tendency to association, that is to forming larger clusters from individual molecules (formation of hydrogen bonds).

Considering the low reactivity of amides, the conditions for their chemical reactions must be much more extreme. Amides mainly undergo hydrolysis reactions. Those take place under acidic or alkaline conditions. Depending on the reaction, individual amides are reduced to primary, secondary or tertiary amines. In reactions with thionyl chloride, primary amides convert to nitriles. What is even more important, amides can form polymers, called polyamides. Out of all known polyamides, nylon is the most important. Amides also react with strong acids to produce the corresponding salts. Amides are also known to react with hydrogen. They form amines as a result.

Nylon

An amide that proved most important for industrial development, including chemical processing, is nylon, first obtained in 1935. It is an example of a polyamide, a synthetic large-molecule polymer whose structure includes an amide group.

In order to obtain nylon, you need to start with the so-called Beckmann rearrangement. The first step of this reaction is an amide synthesis involving the rearrangement of oximes. One way to effect this process is to heat the oxime with an acid that has strong protonating properties. Sulphuric (VI) acid is an example of such a protonating acid. The Beckmann rearrangement is used on an industrial scale to produce caprolactam, the basic raw material in the production of nylon (the lactam polymerisation process). The nylon obtained is characterised by a number of properties that make it stand out among other materials. It is primarily known for its high strength, and it is easy to work with at the same time. It is also resistant to chemicals. Its advantages include low weight, insulating and dielectric properties, low abrasion and attenuation characteristics.

Nylon is currently one of the most popular plastics. It is considered one of the most versatile plastics in the world. It is used in virtually every branch of industry, from textiles to aviation. It is produced primarily in the form of very strong fibres, compared to other materials. In this form, it is intended for the production of yarn, fabrics and knitwear. Nylon fibres can also be found in carpets, footwear, protective clothing, basketballs, parachutes, guitar strings, surgical threads, bathing suits, electrical outlet boxes or auto body components.