Gallic acid (also known as 3,4,5-trihydroxybenzoic acid) is a
trihydroxybenzoic acid with the formula
C6H2(
OH)3CO2H. It is classified as a
phenolic acid. It is found in
gallnuts,
sumac,
witch hazel,
tea leaves,
oak bark, and other
plants.[1] It is a white solid, although samples are typically brown owing to partial oxidation.
Salts and
esters of gallic acid are termed "gallates".
Its name is derived from
oak galls, which were historically used to prepare
tannic acid. Despite the name, gallic acid does not contain
gallium.
Isolation and derivatives
Gallic acid is easily freed from
gallotannins by acidic or alkaline
hydrolysis. When heated with concentrated
sulfuric acid, gallic acid converts to
rufigallol. Hydrolyzable tannins break down on hydrolysis to give gallic acid and
glucose or ellagic acid and glucose, known as
gallotannins and
ellagitannins, respectively.[2]
Alkaline solutions of gallic acid are readily oxidized by air. The oxidation is catalyzed by the enzyme
gallate dioxygenase, an enzyme found in Pseudomonas putida.
Oxidative coupling of gallic acid with arsenic acid, permanganate, persulfate, or iodine yields
ellagic acid, as does reaction of methyl gallate with
iron(III) chloride.[5] Gallic acid forms intermolecular esters (
depsides) such as
digallic and cyclic ether-esters (
depsidones).[5]
Hydrogenation
Hydrogenation of gallic acid gives the cyclohexane derivative hexahydrogallic acid.[6]
Gallic acid is an important component of
iron gall ink, the standard European writing and drawing ink from the 12th to 19th centuries, with a history extending to the Roman empire and the
Dead Sea Scrolls.
Pliny the Elder (23-79 AD) describes the use of gallic acid as a means of detecting an adulteration of
verdigris[7] and writes that it was used to produce dyes. Galls (also known as oak apples) from oak trees were crushed and mixed with water, producing
tannic acid. It could then be mixed with
green vitriol (
ferrous sulfate) — obtained by allowing sulfate-saturated water from a spring or mine drainage to evaporate[citation needed] — and
gum arabic from acacia trees; this combination of ingredients produced the ink.[8]
Gallic acid was one of the substances used by
Angelo Mai (1782–1854), among other early investigators of
palimpsests, to clear the top layer of text off and reveal hidden manuscripts underneath. Mai was the first to employ it, but did so "with a heavy hand", often rendering manuscripts too damaged for subsequent study by other researchers.[9]
Gallic acid was first studied by the Swedish chemist
Carl Wilhelm Scheele in 1786.[10] In 1818, French chemist and pharmacist
Henri Braconnot (1780–1855) devised a simpler method of purifying gallic acid from galls;[11] gallic acid was also studied by the French chemist
Théophile-Jules Pelouze (1807–1867),[12] among others.
When mixed with
acetic acid, gallic acid had uses in early types of photography, like the
calotype to make the silver more sensitive to light; it was also used in developing photographs.[13]
Gallate esters are
antioxidants useful in food preservation, with propyl gallate being the most commonly used. Their use in human health is scantly supported by evidence.
^Albert W. Burgstahler and Zoe J. Bithos (1962). "Hexahydrogallic Acid and Hexahydrogallic Acid Triacetate". Organic Syntheses. 42: 62.
doi:
10.15227/orgsyn.042.0062.
^Pliny the Elder with John Bostock and H.T. Riley, trans., The Natural History of Pliny (London, England: Henry G. Bohn, 1857), vol. 6,
p. 196. In Book 34, Chapter 26 of his Natural History, Pliny states that verdigris (a form of copper acetate (Cu(CH3COO)2·2Cu(OH)2), which was used to process leather, was sometimes adulterated with copperas (a form of iron(II) sulfate (FeSO4·7H2O)). He presented a simple test for determining the purity of verdigris. From p. 196: "The adulteration [of verdigris], however, which is most difficult to detect, is made with copperas; … The fraud may also be detected by using a leaf of papyrus, which has been steeped in an infusion of nut-galls; for it becomes black immediately upon the genuine verdigris being applied."
^L.D. Reynolds and N.G. Wilson, "Scribes and Scholars" 3rd Ed. Oxford: 1991, pp 193–4.
^Carl Wilhelm Scheele (1786)
"Om Sal essentiale Gallarum eller Gallåple-salt" (On the essential salt of galls or gall-salt), Kongliga Vetenskaps Academiens nya Handlingar (Proceedings of the Royal [Swedish] Academy of Science), 7: 30–34.
^Hodgson JM, Morton LW, Puddey IB, Beilin LJ, Croft KD (2000). "Gallic acid metabolites are markers of black tea intake in humans". Journal of Agricultural and Food Chemistry. 48 (6): 2276–80.
doi:
10.1021/jf000089s.
PMID10888536.
^Pathak, S. B.; Niranjan, K.; Padh, H.; Rajani, M.; et al. (2004). "TLC Densitometric Method for the Quantification of Eugenol and Gallic Acid in Clove". Chromatographia. 60 (3–4): 241–244.
doi:
10.1365/s10337-004-0373-y.
S2CID95396304.
^Gálvez, Miguel Carrero; Barroso, Carmelo García; Pérez-Bustamante, Juan Antonio (1994). "Analysis of polyphenolic compounds of different vinegar samples". Zeitschrift für Lebensmittel-Untersuchung und -Forschung. 199: 29–31.
doi:
10.1007/BF01192948.
S2CID91784893.