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Gentisic acid is a dihydroxy benzoic acid. It is a derivative of benzoic acid and a minor (1 %) product of the metabolic break down of aspirin, excreted by the kidneys.
It is also found in the African tree Alchornea cordifolia and in wine.

white to light beige crystalline powder

As a hydroquinone , gentisic acid is readily oxidized and is used as an antioxidant excipient in some pharmaceutical preparations. In the laboratory, it is used as a sample matrix in matrix-assisted laser desorption/ionization (MALDI) mass spectrometry , and has been shown to conveniently detect peptides incorporating the boronic acid moiety by MALDI .

ChEBI: A dihydroxybenzoic acid having the two hydroxy groups at the 2- and 5-positions.

Gentisic acid is produced by carboxylation of hydroquinone.
C₆H₄(OH)₂ + CO2 → C₆H₃(CO₂H)(OH)₂
This conversion is an example of a Kolbe–Schmitt reaction.

Journal of the American Chemical Society, 72, p. 3292, 1950 DOI: 10.1021/ja01163a521

Crystallise gentisic acid from hot water or *benzene/acetone. Dry it in a vacuum desiccator over silica gel. [Beilstein 10 H 384, 10 IV 1441.]

[1] RAVI JOSHI. Antioxidant activity and free radical scavenging reactions of gentisic acid: in-vitro and pulse radiolysis studies.[J]. Free Radical Research, 2012, 46 1: 11-20. DOI: 10.3109/10715762.2011.633518
[2] KEIKO ASHIDATE . Gentisic acid, an aspirin metabolite, inhibits oxidation of low-density lipoprotein and the formation of cholesterol ester hydroperoxides in human plasma[J]. European journal of pharmacology, 2005, 513 3: Pages 173-179. DOI: 10.1016/j.ejphar.2005.03.012
[3] J. F. CUNHA. The mechanism of gentisic acid-induced relaxation of the guinea pig isolated trachea: the role of potassium channels and vasoactive intestinal peptide receptors.[J]. Brazilian Journal of Medical and Biological Research, 2001, 4 1: 381-388. DOI: 10.1590/s0100-879x2001000300013
[4] SIMEI SUN. Gentisic acid prevents the transition from pressure overload-induced cardiac hypertrophy to heart failure.[J]. Scientific Reports, 2019: 3018. DOI: 10.1038/s41598-019-39423-8
[5] STEVEN MINK. Gentisic acid sodium salt, a phenolic compound, is superior to norepinephrine in reversing cardiovascular collapse, hepatic mitochondrial dysfunction and lactic acidemia in Pseudomonas aeruginosa septic shock in dogs.[J]. Intensive Care Medicine Experimental, 2016, 4 1: 24. DOI: 10.1186/s40635-016-0095-0