Protocatechuic acid Constitution

Protocatechuic Acid.—One of the di-hydroxy benzoic acids is related to vanillin, which we have already studied. The acid is known as protocatechuic acid, and derives its name from the fact that it may be obtained from a gum or resin, known as gum catechin by fusion with potash, i.e. by heat and oxidation in presence of an alkali. A large variety of plant products including alkaloids essential oils, gums, resins and tannins yield this acid. The following may be mentioned gum catechin, gum benzoin, guaiac resin, myrrh, piperine or piperic acid, vanillin, cafe-tannic acid. These natural sources at once suggest a relationship to vanillin (p. 661) and heliotropin (p. 662). It is the acid corresponding to protocatechuic aldehyde, 3-4-di-hydroxy benzal-dehyde (p. 661), which explains the relationship just mentioned. Its constitution, is then ... 

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 proof is exactly analogous to that for the constitution of pseudocumene, 1-3-4-tri-methyl benzene from either meta-xylene or para-xylene (p. 490). That the two hydroxyls are ortho to each other is proven by the fact that on heating with lime protocatechuic acid yields p3rrocatechinol, 1-2-di-hydroxy benzene. 

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. 

Tannic Acids.—Closely related to gallic acid and to protocatechuic acid is a group of acids known as tannic acids. While the exact constitution of these is not known it is probable that they are anhydrides of different hydroxy benzoic acids, similar to the di-saccharoses as anhydrides of mono-saccharoses. This is indicated by the fact that on hydrolysis the tannic acids yield hydroxy benzoic acids. The different tannic acids are given names that indicate the hydrolytic products or the natural source. 

Phenolic acids also constitute an important class of phenolic compounds with bioactive functions like the flavonoids, usually found in plant and food products. According to their structure, phenolic acids can be divided in two subgroups the hydroxybenzoic and the hydroxycinnamic acids. The most commonly found hydroxybenzoic acids include gallic, p-hydroxybenzoic, protocatechuic, vanillic and syringic acids caffeic, ferulic, p-coumaric, and sinapic acids are classed in the hydroxycinnamic acids (Bravo, 1998 Martins et al., 2011). 

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