356 PROTOCATECHUIC ACID, ETC. 



moderately difficultly soluble in cold water (in 58 parts 

 at 21), easily soluble in hot water, alcohol, and ether ; 

 contains no water of crystallization ; fuses at 183, and 

 is decomposed at 210-212 into carbonic anhydride 

 and a mixture of hydroquinone and pyrocatechin. 

 Iron chloride turns its solution deep blue, which be- 

 comes blood-red on a subsequent addition of a little 

 ammonia. The salts are very unstable, and are decom- 

 posed, when left in aqueous solution in contact with 

 the air. 



2. Protocatechuic acid. Is formed by the action 

 of melting caustic potassa on iodoparaoxybenzoic acid, 

 bromanisic acid, para- and ortho-cresol sulphuric acids, 

 piperic acid, catechin and a great many resins (guaia- 

 cum, gum-benzoin, dragonsblood, assafcetida, myrrh, 

 acaroid, etc) ; the production from resins is usually ac- 

 companied by the formation of paraoxybenzoic acid. 

 Crystallizes from water in colorless laminae or needles 

 with one molecule of water of crystallization. Diffi- 

 cultly soluble in cold water, more easily in hot water, 

 in alcohol, and ether. Fuses at 199, and decomposes 

 at a higher temperature into carbonic anhydride and 

 pyrocatechin. Its solution is turned dark green by 

 iron chloride ; this color changes to a beautiful blue on 

 the addition of a small quantity of a dilute solution of 

 sodium carbonate, the addition of more of the latter 

 solution giving rise to a dark red. The solutions of its 

 salts turn violet on the addition of salts of iron sub- 

 oxide. When mixed with bromine it is converted into 

 monobromprotocatechuic acid, C 7 H 5 Br0 4 , which crystal- 

 lizes in fine rhombic needles. 



Dimethyl-protocatechuic acid, C 6 H 3 



Is obtained by heating 1 part of protocatechuic acid, 4 

 parts of methyl iodide, and 1 part of potassium hydrox- 

 ide with methyl alcohol in sealed tubes at 140 about 

 three hours. The mass thus obtained is boiled with 

 caustic soda ; and the acid precipitated by means of sul- 



