What Is Gallic Acid?
Gallic acid, an organic compound with the molecular formula C7H6O5, belongs to the class of aromatic hydroxycarboxylic acids. Known also as 3,4,5-trihydroxybenzoic acid, it is identified by the CAS registration number 149-91-7.
This compound features a molecular weight of 170.12, melts at 250°C, and forms colorless columnar crystals at room temperature. It has a density of 1.7 g/mL, an acid dissociation constant (pKa) for COOH of 4.5 and for OH of 10, and a solubility in water of 1.1 g/100mL at 20℃. Gallic acid is hygroscopic, forming a monohydrate that loses its crystalline water around 120°C.
Uses of Gallic Acid
Gallic acid is highly valued for its strong reducing power, making it a popular antioxidant in foods, cosmetics, and feed. Its alkaline solutions serve as reducing agents and photo developers. Additionally, it finds application in the production of tannins, blue ink, dyes, culture engineering reagents, plant growth regulators, and as a growth inhibitor.
Its derivatives, including propyl and isoamyl gallic acid esters, are utilized as antioxidants for fats and oils, while epigallocatechin gallate, a type of catechin, exemplifies its use in the pharmaceutical industry as astringents and for hemostatic purposes.
Properties of Gallic Acid
In water, gallic acid forms an alkaline solution with strong reducing properties, readily oxidized by air. Catalytic hydrogenation of gallic acid yields cyclohexane ring derivatives. Heating induces decarboxylation, producing pyrogallol, a valuable reagent in organic synthesis, photography, and dyeing.
Gallic acid is hygroscopic; its monohydrate is stable up to its melting point, where it decomposes into pyrogallol and carbon dioxide.
Types of Gallic Acid
Gallic acid is available as a reagent for R&D and industrial chemical use, in quantities ranging from 10 g to 25 kg. These reagents, including the stable monohydrate, are stored at room temperature and cater to a broad range of industrial applications.
Other Information on Gallic Acid
1. Biosynthesis of Gallic Acid
Gallic acid occurs naturally in many plants, including the Japanese knotweed and the Middle Eastern beech and oak, serving as the backbone of hydrolysable tannins. Its biosynthesis involves the cyclization of 3,5-dihydroshikimic acid, produced from 3-dehydroshikimic acid by shikimic acid dehydrogenase.
2. Ester Derivatives of Gallic Acid
Numerous natural and synthetic ester derivatives of gallic acid are known, such as epigallocatechin gallic acid from tea. The enzyme gallic acid-1-beta-glycosyltransferase facilitates the glycosylation of gallic acid, illustrating its biochemical significance.