Protocatechuic acid Synthesis

Synthesis from meta- or para-Hydroxy Benzoic Acid.—The constitution is proven by its synthesis by sidphonation and then alkali fusion of either meta-hydroxy benzoic acid or para-hydroxy benzoic acid. As this synthesis introduces into each of these acids first a sulphonic acid group and then in place of this a second hydroxyl group the two hydroxyls in the final product, protocatechuic acid, must be in the 3-4 positions as only such positions could be occupied in a product obtained from either the meta or para hydroxy benzoic acid. 

This relationship explains the similarity of the names and the fact that both are obtained from gum catechin. The reverse of the above reaction, the synthesis of protocatechuic acid from pyrocatechinol, may be accomplished by heating the phenol with ammonium carbonate and water to 1400° under pressure, which is a modification of the Kolbe reaction for synthesizing salicyclic acid (p. 716). From its constitution and by reference to the formulas on page 662 we will see its relationship to vanillin, heliotropin, eugenole, safrole, guaiacol, etc. 

Beilstein Handbook Reference) Benzoic acid, 3,4-dihydroxy- BRN 1448841 4-Carboxy-1,2-dihydroxybenzene CCRIS 6291 34-Dihydroxybenzoic xid 4,5-Dihydroxybenzoic acid EINECS 202-760-0 NSC 16631 Protocatechuic acid. Used in chemical synthesis, mp = 200-202° d = 1.524 X.m = 270, 290 nm (e = 2754, 3890, HzO) insoluble in CsHe, slightly soluble in H2O (2 g/100 mi), soluble in Et20, very soluble in EtOH, Dmval. 

---