, 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

_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,

-----CO.O-----
/\ /\
/ \__________/ \
HO| | | |OH
HO| |___CO.O___| |OH
\ / \ /
\/ \/

CLASSES OF TANNINS

The tannins are divided into two general classes, known respectively as the
_pyrogallol tannins_ and the _catechol tannins_. These differ in their
characteristic reactions as follows:

Pyrogallol variety Catechol variety
Ferric salts Dark blue Greenish black
Bromine water