Gallic acid
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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.<ref name=Haslam_and_Cai>Template:Cite journal</ref> 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 derivativesEdit
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.<ref>Template:Citation</ref>
BiosynthesisEdit
Gallic acid is formed from 3-dehydroshikimate by the action of the enzyme shikimate dehydrogenase to produce 3,5-didehydroshikimate. This latter compound aromatizes.<ref>Gallic acid pathway on metacyc.org</ref><ref>Template:Cite journal</ref>
ReactionsEdit
Oxidation and oxidative couplingEdit
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.<ref name="ull-hcaa" /> Gallic acid forms intermolecular esters (depsides) such as digallic and cyclic ether-esters (depsidones).<ref name="ull-hcaa">Template:Citation</ref>
HydrogenationEdit
Hydrogenation of gallic acid gives the cyclohexane derivative hexahydrogallic acid.<ref>Template:Cite journal</ref>
DecarboxylationEdit
Heating gallic acid gives pyrogallol (1,2,3-trihydroxybenzene). This conversion is catalyzed by gallate decarboxylase.
EsterificationEdit
Many esters of gallic acid are known, both synthetic and natural. Gallate 1-beta-glucosyltransferase catalyzes the glycosylation (attachment of glucose) of gallic acid.
Historical context and usesEdit
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<ref>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."</ref> 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 evaporateTemplate:Citation needed—and gum arabic from acacia trees; this combination of ingredients produced the ink.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
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.<ref>L.D. Reynolds and N.G. Wilson, "Scribes and Scholars" 3rd Ed. Oxford: 1991, pp 193–4.</ref>
Gallic acid was first studied by the Swedish chemist Carl Wilhelm Scheele in 1786.<ref>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.</ref> In 1818, French chemist and pharmacist Henri Braconnot (1780–1855) devised a simpler method of purifying gallic acid from galls;<ref>Template:Cite journal</ref> gallic acid was also studied by the French chemist Théophile-Jules Pelouze (1807–1867),<ref>J. Pelouze (1833) "Mémoire sur le tannin et les acides gallique, pyrogallique, ellagique et métagallique," Annales de chimie et de physique, 54: 337–365 [presented February 17, 1834].</ref> 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.<ref>Template:Cite book</ref>
OccurrenceEdit
Gallic acid is found in a number of land plants, such as the parasitic plant Cynomorium coccineum,<ref>Template:Cite journal</ref> the aquatic plant Myriophyllum spicatum, and the blue-green alga Microcystis aeruginosa.<ref name="Nakai">Template:Cite journal</ref> Gallic acid is also found in various oak species,<ref>Template:Cite journal</ref> Caesalpinia mimosoides,<ref name="Kilburn">Template:Cite journal</ref> and in the stem bark of Boswellia dalzielii,<ref>Template:Cite journal</ref> among others. Many foodstuffs contain various amounts of gallic acid, especially fruits (including strawberries, grapes, bananas),<ref name="pmid26251571">Template:Cite journal</ref><ref>Template:Cite journal</ref> as well as teas,<ref name="pmid26251571" /><ref>Template:Cite journal</ref> cloves,<ref name="tdmq">Template:Cite journal</ref> and vinegars.<ref>Template:Cite journal</ref>Template:Clarify Carob fruit is a rich source of gallic acid (24–165 mg per 100 g).<ref name="Goulas">Template:Cite journal</ref>
EstersEdit
Also known as galloylated esters:
- Methyl gallate
- Ethyl gallate, a food additive with E number E313
- Propyl gallate, or propyl 3,4,5-trihydroxybenzoate, an ester formed by the condensation of gallic acid and propanol
- Octyl gallate, the ester of octanol and gallic acid
- Dodecyl gallate, or lauryl gallate, the ester of dodecanol and gallic acid
- Epicatechin gallate, a flavan-3-ol, a type of flavonoid, present in green tea
- Epigallocatechin gallate (EGCG), also known as epigallocatechin 3-gallate, the ester of epigallocatechin and gallic acid, and a type of catechin
- Gallocatechin gallate (GCG), the ester of gallocatechin and gallic acid and a type of flavan-3ol
- Theaflavin-3-gallate, a theaflavin derivative
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.
Spectral dataEdit
| colspan="2" Template:Chemical datatable header | UV-Vis | |
|---|---|
| Lambda-max: | 220, 271 nm (ethanol) File:Gallic acid spectrum.PNG Spectrum of gallic acid |
| Extinction coefficient (log ε) | |
| colspan="2" Template:Chemical datatable header | IR | |
| Major absorption bands | ν : 3491, 3377, 1703, 1617, 1539, 1453, 1254 cm−1 (KBr) |
| colspan="2" Template:Chemical datatable header | NMR | |
| Proton NMR
|
δ : 7.15 (2H, s, H-3 and H-7) |
| Carbon-13 NMR
|
δ : 167.39 (C-1), |
| Other NMR data | |
| colspan="2" Template:Chemical datatable header | MS | |
| Masses of main fragments |
ESI-MS [M-H]- m/z : 169.0137 ms/ms (iontrap)@35 CE m/z product 125(100), 81(<1) |
<ref name="Kilburn" />
See alsoEdit
- Benzoic acid
- Catechol
- Hydrolyzable tannin
- Pyrogallol
- Syringol
- Syringaldehyde
- Syringic acid
- Shikimic acid