Physical properties of nitro compounds

Nitro compounds have large dipole moments compared to other simple organic compounds. Due to the polarity, their boiling points are unusually high in comparison with other compounds of same molecular mass.

Lower nitro alkanes are colorless liquids with a pleasant smell. Higher members are solids. Aromatic nitro compounds like nitro benzene has a smell of bitter almonds.

Nitro alkanes are sparingly soluble in water (lower members are relatively more soluble than the higher members). Aromatic nitro compounds are not soluble in water. They are soluble in organic solvents.

The formal positive charge on nitrogen of nitro group makes it a strong electron withdrawing group and exerts a strong pull on neighboring electrons. The presence of nitro group in phenols enhances their acidity. In aromatic compounds, the nitro group deactivates then towards electrophilic substitution.

The nitro group is an ambident group and is capable of being attached to carbon chain through nitrogen as well as through oxygen atom.

Nitro compounds are named by using the prefix nitro before the name of parent hydrocarbon. The position of nitro group and other substituents are indicated by Arabic numerals:

CH₃ NO₂ – NitromethaneВ CH₃ CH₂ NO₂ - Nitroethane

Aliphatic nitro compounds are called nitroalkanes while aromatic nitro compounds are called nitroarenes. The compound in which the -NO₂ group is linked to the alkyl or aryl group through oxygen atom are called nitrites. Nitrites are isomeric with nitro compounds. Amines and amides represent the two largest classes of nitrogen compounds.

Many nitro compounds are commercially produced for use as explosives, solvents, or chemical intermediates (substances valued as raw materials in further chemical processing).

Amine

Amine are organic compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are derivatives of ammonia. Amine bases are classified according to the number of alkyl or aryl groups attached to nitrogen. Nitro compounds may be aliphatic or aromatic compounds according to whether the nitro group is attached to alkyl or aryl groups.

R - NO2 Aliphatic nitro compounds

Ar - NO2 Aromatic nitro compound

The aliphatic nitro compounds may be further classified into primary, secondary, tertiary nitro compounds according as the nitro group is attached to primary, secondary or tertiary carbon atom respectively.

A large and widespread class of naturally occurring amines is known as alkaloids. These are basic organic nitrogen compounds, mostly of plant origin. The structures of the plant alkaloids are extraordinarily complex, yet they are related to the simple amines in being weak nitrogen bases. In fact, the first investigator to isolate an alkaloid in pure form was F. W. A. Sertiirner who, in 1816, described morphine as basic, salt-forming, and ammonialike. He used the term "organic alkali" from which is derived the name alkaloid.

Many of the alkaloids are of polycyclic structures and have other functional groups in addition to basic nitrogen. The nitrogens of alkaloids frequently have tertiary amine functions.

All of the alkaloids are substances with very pronounced physiological action. Indeed, alkaloids in general have been used and abused for centuries as medicinals, drugs, and poisons. However, their structures have become known only in this century, and we are still a long way from understanding the chemistry that leads to their pronounced physiological effects.

Alkaloids include compounds that may be classified as antimicrobial (quinine), as analgesics (morphine, codeine), as hallucinogens (mescaline, LSD), as stimulants (cocaine, atropine, caffeine) and as topical anaesthetics (cocaine). Although some of these compounds are used as natural medicines, an entire industry has developed in an effort to produce synthetic analogs with similar, but safer, medicinal properties. Some of the better known of these synthetic drugs include a group of narcotic substances known as barbiturates, which are used widely as sedatives, anticonvulsants, tranquilizing drugs, synthetic stimulants, and antibiotics.

Basic nitrogen compounds similar to the plant alkaloids also occur in animals, although the description animal alkaloid seldom is used. Certain amines and ammonium compounds play key roles in the function of the central nervous system and the balance of amines in the brain is critical for normal brain functioning. Also, many essential vitamins and hormones are basic nitrogen compounds. Nitrogen bases also are vital constituents of nucleic acid polymers (DNA and RNA) and of proteins.

Everyone who works with organic chemicals should be aware that a number of arenamines are carcinogens. The most dangerous examples are known to induce human bladder cancer. These chemicals were used widely in the chemical industry (mostly in azo dye manufacture) long before they were recognized as hazardous carcinogens. Voluntary action and appropriate legislation now controls the industrial uses of these substances, and there also are some controls for uses in research and teaching. It is important to be aware of the potential hazards of known carcinogens and to recognize that all chemicals, both organic and inorganic, should be treated with great respect if their thermodynamic and physiological properties are not known. Carcinogenic character is just one of many possible hazards.

There is an unresolved problem related to the carcinogenic properties of nitroso compounds. You probably are aware (if you read the labels on food packages) that sodium nitrite is added to many packaged meat products. Sodium nitrite prevents the growth of harmful bacteria, thereby retarding spoilage, and it also enhances the appearance by maintaining the red look of fresh meat. There is a possibility that nitrite may have adverse effects on human health by nitrosating the amino and amide functions of proteins in the presence of acids. This possibility has to be balanced against the alternate threat to human health if the use of nitrite were discontinued, that of increased food spoilage. In any case, it seems clear that the amount of sodium nitrite actually used in most processing is in excess of that needed to retard bacterial decay.