------O------- | | CH_{2}OR·CHOR·CH·CHOR·CHOR·CHOR in which the R represents a complex phenol-acid like tannic acid, or digallic acid. These acids are derivatives of the common phenols, whose constitution will be brought to mind by the following series of formulas: Ordinary phenol Pyrocatechol Resorcinol Hydroquinone /\ /\ /\ /\ / \ / \ / \ / \ | |OH | |OH | |OH | |OH | | | |OH | | HO| | \ / \ / \ / \ / \/ \/ \/ \/ OH Pyrogallol Oxyhydroquinone Phloroglucinol OH OH OH /\ /\ /\ / \ / \ / \ | |OH | |OH | | | |OH | | HO| |OH \ / \ / \ / \/ \/ \/ OH These phenols themselves do not occur as constituents of tannins, although they are often found in other glucosides, gums, etc. The following mono-carboxyl acid derivatives of these phenols are, however, found both free and in glucoside formation as constituents of many of the common tannins. _Pyrocatechuic acid_, derived from pyrocatechol, represented by the formula, OH /\ / \ | |OH | | \ / \/ COOH _Gallic acid_, derived from pyrogallol, and represented by the formula, OH /\ / \ HO| | HO| |COOH \ / \/ COOH In most of the common tannins, however, the characteristic acids are oxy-derivatives of the so-called "tannon" group, represented by the formula, C_{6}H_{5}·CO·O·C_{6}H_{5}. For example, _digallic acid_, which is a constituent of many common tannins, is a tetra-oxy, mono-carboxyl derivative of this group, having the structural formula, /\ /\ / \ / \ HO| |--CO·O--| |COOH HO| | HO| | \ / \ / \/ \/ OH OH