Carbon Nucleus. - In this series the carbon atoms are supposed to be linked so as to form a closed chain or chains. The lowest member of the series contains six atoms of carbon, which are so linked that the group has six free affinities, thus:

For convenience' sake, this carbon ring, or nucleus, is often graphically represented simply thus :

Radical. - When five of the free affinities are saturated by hydrogen, the group forms an organic radical with one free affinity, termed phenyl:

. - When this free affinity is also saturated by hydrogen, the group forms phenyl hydride, or benzene, or benzol, which must be carefully distinguished from the benzin already mentioned (p. 762).

Bodies belonging to the aromatic group differ from those of the fatty series in the fact that they do not readily link on other substances to themselves, and so form compounds by addition. They form them rather by substitution.

Alcohol. - When one atom of hydrogen in benzene is replaced by hydroxyl (OH), phenyl-alcohol or carbolic acid is formed :

O - H, or C6H5OH.

The name of phenol has been given to this body as it is more convenient than the names phenyl-alcohol or carbolic acid, and its termination, "ol," indicates that it resembles alcohol in its constitution.

The relations of some of the other members of the aromatic group to each other may be more easily seen if they are put in a tabular form:

Benzene,

H, or C6H5.H.

Phenol,

OH, or C6H5.OH, in which 1 atom of hydroxyl (OH) replaces 1 of H in benzene.

Nitrobenzene, or C6H5.NO2, in which 1 atom of nitroxyl (NO2) replaces 1 of H in benzene.

Amidobenzene, or Anilin in which 1 atom of amidogen (NH2) replaces 1 of H in benzene.

or C6H5.NH2,

Benzoic acid, or Phenylformic acid, or C6H5.CO.OH, in which 1 atom of carboxyl (CO.OH) replaces 1 of H in benzene.

Benzoic acid may also be regarded as formic acid in which one atom of hydrogen is replaced by phenyl, and so it may be called phenyl-formic acid.

As the carbon atoms in the benzene ring or nucleus are supposed to be arranged symmetrically, it does not matter which atom of hydrogen is replaced by another radical if the substitution takes place only in one atom, e.g. in phenol.

If we number the carbon atoms so as to distinguish them from one another, thus : it is evident that phenol is always the same, whether the hy-droxyl is attached to the carbon atom, 1, 2, or 3, etc. etc.:

But this is not the case when substitution occurs at two or more points in the benzene ring.

Thus when substitution in the benzene ring occurs at two points these may take three different positions.

The substitution-products

1 and 6 are the same as 1 and 2, and are usually termed ortho-compounds.

and 3

1 and 5 are the same as 1 and 3, and are usually termed meta-compounds.

and 4

1 and 4 are usually termed para-compounds.

Thus three isomeric forms may occur.

When two atoms of hydrogen are replaced by two of hydroxyl, instead of by one, as in phenol, we have three isomeric substances, differing from one another only in the relative position of the substituted atoms. These three bodies are pyrocatechin, in which the position is 1 and 2, resorcin, 1 and 3, and hydroquinone, 1 and 4. The relative position of the hydroxyl groups in these three bodies is indicated in their formulae by the figures (1:2), (1:3), and (1:4), or by the terms ortho, meta, or para, respectively.

Pyrocatechin.

Ortho-di-hydroxy -benzene.

C6H4.(OH)2(l : 2)