Tannic acid

Tannic acid is a term for a mixture of related chemical compounds that are used to make leather. These chemicals are an example of tannins, a larger group of bitter-tasting chemicals that attach to proteins.

 Chemistry 
The name "tannic acid" was introduced in 1796, for an extract of oak bark, though the use of similar chemicals is as old as the leather industry. The exact composition of tannic acid was unknown until the early 20th century, when Emil Fischer identified tannic acid as a mixture of various gallic acid esters of glucose.

Tannic acid is unusual because it does not have any of the  functional groups that normally make organic compounds acidic. Instead, its acidity comes from the  groups, similarly to phenol.

 References 

Category:Organic acids