Gallic acid

Gallic acid is a naturally occurring phenolic acid with molecular formula C₇H₆O₅. Its three hydroxyl groups make it a building block of hydrolysable tannins and a useful reducing agent.

Phenolic acid: An aromatic carboxylic acid whose structure includes one or more hydroxyl groups attached to an aromatic ring. Gallic acid belongs to this family, combining a carboxyl group with three phenolic hydroxyls.
Hydrolysis: A chemical reaction that cleaves a bond through reaction with water. Hydrolysis releases gallic acid from gallotannins and other ester-bound forms.
Gallic acid equivalent: A reporting unit expressing an assay result as the amount of gallic acid that would produce the same response. Plant-extract phenolic measurements are often reported as milligrams of gallic acid equivalents.
Shikimate pathway: A metabolic pathway in plants, fungi, and microorganisms that produces aromatic amino acids and related compounds. Its intermediates supply the aromatic carbon framework used in plant production of gallic acid.
Benzoic acid: An aromatic carboxylic acid consisting of a carboxyl group attached to a benzene ring. Gallic acid has a benzoic-acid backbone with three additional hydroxyl groups.
Oxidation: A chemical process in which a substance loses electrons, often through reaction with oxygen. Oxidation of gallic acid produces colored compounds and can alter its antioxidant activity.
Folin–Denis method: A colorimetric assay that estimates tannins using a phosphomolybdotungstate reagent. Gallic acid can provide a reference standard for quantifying tannins with this method.
Gallotannin: A hydrolysable tannin built from gallic acid groups esterified to a sugar, usually glucose. Gallotannins are abundant plant forms from which gallic acid can be released by hydrolysis.
Pyrogallol: An aromatic compound with three adjacent hydroxyl groups on a benzene ring. Removing gallic acid’s carboxyl group yields pyrogallol, highlighting its hydroxyl-rich ring.
Iron gall ink: A dark writing ink traditionally made from iron salts, tannins, and a binder. Gallic acid from plant tannins reacts with iron salts in the chemistry of this ink.

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