More important are Kekule's observations that nitrous acid oxidizes pyrocatechol or [I.2]-dioxybenzene, and protocatechuic acid or [3.4]- dioxybenzoic acid to dioxytartaric acid, (C(OH) 2 COOH) 2 (Ann., 1883, 221, p. 230); and 0.
The prism formula also received support from the following data: protocatechuic acid when oxidized by nitrous acid gives carboxytartronic acid, which, on account of its ready decomposition into carbon dioxide and tartronic acid, was considered to be HO C(COOH) 3.
The latter when heated with hydrochloric acid to 170°, or water to 200°, separates carbon with the formation of protocatechuic acid, I.
Conversely, by heating protocatechuic acid with potash and methylene iodide, piperonylic acid was regained.
Alkali fusion of eugenol gives protocatechuic acid.
Oak tannin; whilst others yield protocatechuic acid and phloroglucin, e.g.
Moringa-tannin or maclurin, C1,H1006 H20, found in Morus tinctoria, hydrolyses on fusion with caustic potash to phloroglucin and protocatechuic acid.