This benzene group forms the foundation of an immense number of bodies known as the aromatic series, to which belong aniline, carbolic acid, picric acid, gallic acid, and a host of other compounds important alike in a scientific and commercial sense, and among which we may pretty safely group the whole of the tannins. Commencing with benzene, we may, by inserting atoms of oxygen, produce a series of alcohols or phenols, of which common phenol (usually but incorrectly called carbolic acid) is the first. The following table gives a general view of some of these, so far as they are known, with their corresponding acids:-- ──────────────┬─────────────────┬────────────────────┬─────────────────── C_{6}H_{6} │ C_{6}H_{5}OH │ C_{6}H_{4}(OH)_{2} │ C_{6}H_{3}(OH)_{3} Benzene. │ Phenol. │ Pyrocatechol │ Pyrogallol, │ │ (or catechol), │ Phloroglucol. │ │ Hydroquinol, │ │ │ Resorcinol. │ ──────────────┼─────────────────┼────────────────────┼─────────────────── {H_{5} │ {H_{4} │ {H_{3} │ {H_{2} C_{6}{CO.OH │ C_{6}{OH │ C_{6}{(OH)_{2} │ C_{6}{(OH)_{2} │ {CO.OH │ {CO.OH │ {CO.OH Benzoic acid.│ Salycylic acid, │ Protocatechuic acid│ Gallic acid, │ Oxybenzoic acid.│ (and 5 other │ &c. │ │ isomeric acids). │ ──────────────┴─────────────────┴────────────────────┴─────────────────── It will be noticed that a large proportion of the formulæ given above represent several compounds identical in composition, but frequently very distinct in their properties. The explanation of these differences lies in the different relative position of the OH and CO.OH groups round the benzene ring. Thus the following diagram represents the relative positions of the pyrocatechol series. It may be noted that each phenol yields two isomeric[E] acids. Miller (C. S. Jour., xli. 398), who has investigated these acids, remarks, "Of the 3 phenols C_{6}H_{4}OH_{2}, catechol alone gives a precipitate with lead acetate, and of the 6 acids, C_{6}N_{5}OH_{2}, CO.OH, none yields precipitates with lead acetate, except the 2 which are obtained from catechol." [Footnote E: _Isomeric_, of similar composition but different structure and properties.] Pyrocatechol 1-2. Resorcinol 1-3. Hydroquinol 1-4. H──C══C──O──H H──C══C──O──H H──C══C──O──H │ │ │ │ │ │ H──C C──O──H H──C C──O──H H──C C-H ║ ║ ║ ║ ║ ║ H──C──C──H H──C──C──O──H H──O──C──C-H