Nitrogen compounds

Ammonia, NH,, is a simple nitrogen compound and the “parent” of amines, in a primary amine, one of ammonia’s hydrogen atoms is replaced by an alkyl or aryl group (R). Substitution by further groups gives secondary and tertiary amines.

A wide range of organic compounds contain nitrogen, including amines, amides, nitriles, oximes, and nitro compounds. Among the most important are amino acids.

Amines can be considered to be derivatives of ammonia, since they are obtained from the ammonia compound. They are classed as either primary, secondary, or tertiary. The classification depends on the number of hydrogen atoms “replaced” by organic groups. If one hydrogen atom is replaced, the compound is a primary amine; if two hydrogen atoms are replaced, it is a secondary amine; and if three organic groups are attached to the nitrogen atom, it is a tertiary amine. A tertiary amine has no hydrogen atoms connected directly to the nitrogen atom.

Amines play important roles in biochemical systems. They are widely distributed in nature as amino acids (the building blocks of protein), alkaloids (organic bases found in plants), and vitamins. Manufactured amine derivatives are medicinal chemicals (sulfa drugs and anesthetics) and starting substances for synthetic fibers such as nylon. Aniline (aminobenzene), which is highly toxic, is the most important industrial amine. It is widely used in the manufacture of dyestuffs.

Structure and properties of amines

The nitrogen atom in an amine has a lone pair of electrons and can be considered as a base, like ammonia. Amines with less than five carbon atoms dissolve in water, forming basic solutions. Almost all amines can be dissolved in dilute acid. Since acids and bases combine to form salts, such a solution of amines in dilute acid forms soluble ammonium salts.

Primary and secondary amines do not form hydrogen bonds as strongly as do alcohols, so their boiling points lie between those of the corresponding hydrocarbons and alcohols. Tertiary amines have boiling points slightly higher than the corresponding hydrocarbon. Thus, the methylamines are all gases at room temperature. Simple amines smell like rotting fish, whose characteristic odor is caused by amines produced by bacteria.

Preparation and reactions: amines and amides

Formation of primary amines is most important because secondary and tertiary compounds can be made from them. The various synthetic methods of producing primary amines include the treatment of certain compounds with ammonia or the reduction of nitrogen-oxygen compounds. In general, one or more of the three hydrogen atoms of ammonia is replaced with an organic group containing carbon and hydrogen.

Aromatic nitrogen compounds range from an aniline (amino-benzene) to heterocyclic compounds. In these compounds, a nitrogen atom has a place in one of the rings (pyridine to indole).

Primary amines can be converted to secondary amines, but they can also be converted to secondary amides. An amide is an ammonia or amine derivative in which the nitrogen atom is attached to a carbonyl group. Primary amines are also convertible to imines (organic compounds with a carbon-nitrogen double bond). Imines are very reactive. Secondary amines react similarly, but less vigorously. In tertiary amines, each of the hydrogen atoms is replaced by an alkyl group, an organic compound consisting solely of carbon and hydrogen atoms.

Chelating agents, such as EDTA illustrated above, can form coordinate bonds with metals. This produces coordination compounds, also called complexes. The diagram at shows how a molecule of EDTA wraps around and forms coordinate bonds (the dotted lines) with a cobalt ion. Chelating agents are useful in removing metallic poisons, such as lead, from the body.

Heterocyclic nitrogen compounds

Heterocyclic compounds are ring structures that include at least one atom other than carbon in the ring. Aromatic heterocyclic compounds of nitrogen contain a nitrogen atom in the ring. These compounds are important as biochemical intermediates, helping in the formation of complex compounds. In some respects, their chemical properties are similar to benzene derivatives. For example, they may display aromatic character. This means that some of the electrons of heterocyclic nitrogen compounds may be diffused around the ring structure, being shared by all the atoms in the ring. One of the most common of these compounds is pyridine, a six-membered ring with one carbon atom substituted by a nitrogen atom. It reacts as amines do with acids to give salts. It can be extracted from coal tar. Its derivatives are important in nature (for example, nicotinic acid, a B-complex vitamin). Quinine, a natural derivative of pyridine, is used to treat malaria.

Porphyrins form what are termed chelate compounds (compounds that hold on to metal ions). The ions are held between four nitrogen atoms. This is the basis for the red blood pigmerit hemoglobin and the green leaf pigment chlorophyll.

Human blood, as well as the blood of all vertebrates, is able to carry oxygen because of hemoglobin, an iron atom held on four sides by the nitrogen atoms (the porphyrin complex). The porphyrin complex is the heme in hemoglobin. Underneath the four sides, the iron is held by a nitrogen atom from the protein globin. This protein is the globin in hemoglobin. Above the four sides is oxygen, which is carried from the lungs by the hemoglobin to the various cells of the body. Chlorophyll, the green pigment in plants, is essential in photosynthesis—the process by which plant cells make carbohydrates from carbon dioxide and water in the presence of chlorophyll and light Chlorophyll is a complex similar to heme, suggesting a common evolutionary background for animals and plants.

Warning eye-spots on a South American butterfly’s wing can be seen in close-up as a series of overlapping scales. The pigments in the colors belong to a class of substances called pterins. These are compounds with a double-ring system containing four nitrogen atoms and six carbon atoms.

Nitriles

Nitriles are a series of organic cyanides. They contain a cyanide group linked to an alkane fragment by a carbon-carbon bond. Nitriles are weak bases and are highly poisonous.

They are important in synthetic chemistry because they can be used to make carboxylic acids (organic acids), or amines. They thus provide the synthetic chemist with a relatively simple means of adding a new carbon atom to another molecule. Acrylonitrile (a type of organic nitrile) is an important starting material in the manufacture of fabrics, plastics, and rubbers.

Many explosives, including detonators, propellants, and warheads, are organic nitrogen compounds. One of the major high explosives for military use is TNT (trinitrotoluene). The diagram shows an industrial process for the continuous production of TNT from MNT (mononitrotoluene). The process begins in the upper-left corner of the diagram, using a mixture of sulfuric acid (HzSO„) and nitric acid ’11 NO,).

Nitrogen-oxygen compounds (oximes)

An important branch of organic chemistry is concerned with compounds with functional groups containing nitrogen and oxygen. These may be nitro compounds, nitroso compounds, or oximes. Organic nitro compounds are formed by nitration—the addition of a nitro group (a molecule containing one nitrogen atom and two oxygen atoms) to a hydrocarbon, generally by using nitric acid. Nitroso compounds contain a molecule consisting of one nitrogen atom and one oxygen atom. Important related compounds are the ni-trosamines.

An oxime is the product of a condensation reaction between an aldehyde or a ketone (carbonyl group) and hydroxylamine (a colorless, crystalline base similar to ammonia). In a condensation reaction, two molecules link together, usually with the expulsion of a small molecule such as water or ammonia. Oximes may be aldoximes (derived from aldehydes) or ketoximes (derived from ketones). Both have a carbon-nitrogen double bond.

Oximes are used to isolate and identify carbonyl compounds. Some are important industrial chemicals. Aldoximes may be dehydrated to nitriles, and ketoximes may be reduced to primary amines. Oximes may be used to make carbonyl compounds or hydroxylamine salts.

The process used to make these compounds or salts is sometimes difficult, because oxime formation is reversible. To prevent this from happening, formaldehyde is added to give a very stable oxime. Ketoximes may be reacted to make amides. Such a reaction is used in the formation of caprolactam, an intermediate step in the manufacture of Nylon 6.

Amines, and proteins derived from them, occur in the tissues of all animals. In dead fish, particularly, amines reveal their presence through their characteristic “fishy” smell.

Nitro compounds

Organic nitro compounds are derived from hydrocarbons by nitration. Some are relatively acidic and can be used as solvents. They are also important in organic synthesis.

Aromatic nitro compounds, known for more than a century, are used chiefly as dye intermediates and as explosives. They are formed readily by the reaction of aromatic hydrocarbons and nitric acid. Examples are trinitrotoluene (TNT) and picric acid (trinitrophenol). The most important synthetic reaction of the aromatic nitro compounds is reduction to aromatic amines. An example of an aromatic amine is aniline, used in making dyes, perfumes, certain medicines, plastics, and resins.

Nitrobenzene, an important industrial product, is used as a solvent and to synthesize aniline. Nitrotoluene, another important derivative, is used in making the explosive trinitrotoluene (TNT).

Nitroso compounds

Compounds with the nitroso group are rather rare because many primary and secondary nitroso compounds readily change into oximes in an acid or base. In the gas phase, and in a dilute water solution, tertiary derivatives (a third type of nitroso compound) have a bluish color. In the pure state, they are colorless liquids.

Nitrosation, the reaction of amines with nitrous acid, is a complex reaction. In aromatic amines, the nitroso group generally attaches to the ring-carbon atom. With aliphatic amines, the nitroso group joins to the amino nitrogen atom to form an N-nitrosamine.

The effect of nitrosamine compounds on . humans has been extensively studied. Nitro-samines often cause cancer in animals and can cause cancer in humans, so the U.S. Department of Agriculture limits the amount of sodium nitrite allowed in cured meats. These nitrosamine compounds are used because they prevent the growth of microorganisms responsible for the type of food poisoning called botulism. Nitrites are also produced by reduction of nitrates in saliva. Both nitrites and amines are formed by bacterial breakdown of proteins in the intestine. Thus, more nitrosa-mines are probably formed in the body than are eaten.