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,
Gallic acid, derived from pyrogallol, and represented by the formula,
In most of the common tannins, however, the characteristic acids are oxy-derivatives of the so-called "tannon" group, represented by the formula, C6H5·CO·O·C6H5. 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,
Ellagic acid, which is an hydrolysis product of many of the pyrogallol tannins (see below) and which produces the characteristic "bloom" on leather tanned by this type of tannins, has the following formula,
