Aromatic Series.

Aromatic Series. This term is applied to a large group of organic chemical compounds, many of which occur in balsams, essential oils, and other substances having an aromatic odour. It was originally limited to the compounds of the benzoic group, but it has now been extended so as to include other series homologous with them, and ranging round the group of hydrocarbons, $C\_nH\_\{2n-6\}$. The simplest of these hydrocarbons is Benzene (q.v.), in which there are six atoms of carbon, the formula being $C\_6H\_6$. Now, such a body is an unsaturated one (see ATOMIC THEORY), and is capable of uniting with monatomic elements such as chlorine to form chlorides, containing from one to six atoms of chlorine. To account for this, Kekulé has devised a structural formula for benzene, which assists one in understanding the complex relations of the aromatic series; but it must be borne in mind that such formulæ do not profess to represent the actual positions of the atoms in the compound, but are only used as convenient stand-points from which to regard them. In Kekulé's formula, the double lines, uniting the atoms of carbon, indicate that each of these atoms can still unite with an atom of hydrogen. From this peculiar construction, it is evident that the compounds of the aromatic series must have distinctive properties, and the number of these compounds be very large. Thus, referring to chlorine, we see

that we may either replace hydrogen by chlorine, or add chlorine to benzene, the resulting bodies having the composition $C\_6Cl\_6$ and $C\_6H\_6Cl\_6$, when the full amount of chlorine has been taken up. So also oxygen may enter into the compound, giving us a series of bodies called phenols, which are monatomic, diatomic, or tetratomic, according to the number of atoms introduced. The phenols correspond to the alcohols of the fatty series (see ALCOHOLS), ordinary phenol having the formula $C\_6H\_5OH$, that of common alcohol being $C\_2H\_5OH$. Hydrogen may also be replaced by amidogen, $NH\_2$, giving rise to Amines (see ALKALOIDS), the best known of which is Phenylamine, or Aniline (q.v.), $C\_6H\_5NH\_2$. The nitro compounds, in which hydrogen is replaced by the group $NO\_2$, include nitrobenzene, or artificial oil of bitter almonds (not to be confounded with the true Oil, q.v.), the formula of which is $C\_6H\_5NO\_2$.

When carbon enters the benzene group, $C\_6H\_5$, it forms many new compounds. Thus hydrogen may be replaced by radicals such as methyl and ethyl, $CH\_3$ and $C\_2H\_5$, giving rise to such compounds as

or, again, more than one molecule of these radicals may be introduced, as in

From these again are derived aldehydes, alcohols, and acids, of which we can only give a single example:

All of which, in their composition and properties, show close analogies to the corresponding fatty compounds. An account of the aromatic series would be incomplete without reference to the isomerism which exists among its members. A reference to the formula for benzene will show that when only one atom of chlorine has been introduced into the molecule, there can only be one monochlor benzene. When two, however, are present, or when one atom of chlorine and one of bromine have replaced hydrogen, as in chlorobromo benzene, the case is different :

Chlorobromo benzene.

Here we see that the atoms of hydrogen replaced may be either contiguous (1'2), separated by one group of CH (1'3), or by two groups (1'4); and that this is not a mere distinction on paper is borne out by experiment, which has succeeded in producing three chlorobromo benzenes, differing in properties, but identical in percentage composition. There are many other isomeric bodies known, but for further information the reader must refer to the article on ISOMERISM, or to a practical treatise on the subject.