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.
Conversely, by heating protocatechuic acid with potash and methylene iodide, piperonylic acid was regained.
These results show that piperonylic acid is the methylene ether of protocatechuic acid.
Alkali fusion of eugenol gives protocatechuic acid.
Hydrolysis by alkaline solutions gives a sugar and caffeic acid; whilst fusion with potassium hydroxide gives protocatechuic acid.
