Gallic acid (also known as 3,4,5-trihydroxybenzoic acid) is a trihydroxybenzoic acid, a type of phenolic acid, found in gallnuts, sumac, witch hazel, tea leaves, oak bark, and other plants. The chemical formula of gallic acid is C6H2(OH)3COOH. It is found both free and as part of hydrolyzable tannins. The gallic acid groups are usually bonded to form dimers such as ellagic acid. Hydrolyzable tannins break down on hydrolysis to give gallic acid and glucose or ellagic acid and glucose, known as gallotannins and ellagitannins, respectively.
|Preferred IUPAC name
3D model (JSmol)
|Molar mass||170.12 g/mol|
|Appearance||White, yellowish-white, or|
pale fawn-colored crystals.
|Density||1.694 g/cm3 (anhydrous)|
|Melting point||260 °C (500 °F; 533 K)|
|1.19 g/100 mL, 20°C (anhydrous)|
1.5 g/100 mL, 20 °C (monohydrate)
|Solubility||soluble in alcohol, ether, glycerol, acetone |
negligible in benzene, chloroform, petroleum ether
|Acidity (pKa)||COOH: 4.5, OH: 10.|
|Safety data sheet||External MSDS|
|Lethal dose or concentration (LD, LC):|
LD50 (median dose)
|5000 mg/kg (rabbit, oral)|
|Benzoic acid, Phenol, Pyrogallol|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
|what is ?)(|
Gallic acid is commonly used in the pharmaceutical industry as a standard for determining the phenol content of various analytes by the Folin-Ciocalteau assay; results are reported in gallic acid equivalents. Gallic acid can also be used as a starting material in the synthesis of the psychedelic alkaloid mescaline.
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 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 — and gum arabic from acacia trees; this combination of ingredients produced the ink.
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.
Gallic acid was first studied by the Swedish chemist Carl Wilhelm Scheele in 1786. In 1818, French chemist and pharmacist Henri Braconnot (1780–1855) devised a simpler method of purifying gallic acid from galls; gallic acid was also studied by the French chemist Théophile-Jules Pelouze (1807–1867), among others.
Gallic acid is formed from 3-dehydroshikimate by the action of the enzyme shikimate dehydrogenase to produce 3,5-didehydroshikimate. This latter compound tautomerizes to form the redox equivalent gallic acid, where the equilibrium lies essentially entirely toward gallic acid because of the coincidentally occurring aromatization.
- gallate + O2 → (1E)-4-oxobut-1-ene-1,2,4-tricarboxylate.
Gallate decarboxylase is another enzyme in the degradation of gallic acid.
Gallate 1-beta-glucosyltransferase is an enzyme that uses UDP-glucose and gallate, whereas its two products are UDP and 1-galloyl-beta-D-glucose.
It is a weak carbonic anhydrase inhibitor. One study indicated that gallic acid has an effect on amyloid protein formation by modifying the properties of alpha-synuclein, a protein associated with the onset of neurodegenerative diseases.
Gallic acid is found in a number of land plants, such as the parasitic plant Cynomorium coccineum, the aquatic plant Myriophyllum spicatum, and the blue-green alga Microcystis aeruginosa. Gallic acid is also found in various oak species, Caesalpinia mimosoides, and in the stem bark of Boswellia dalzielii, among others. Many foodstuffs contain various amounts of gallic acid, especially fruits (including strawberries, grapes, bananas), as well as teas, cloves, and vinegars.[clarification needed]
Gallic acid is easily freed from gallotannins by acidic or alkaline hydrolysis. When gallic acid is heated with concentrated sulfuric acid, rufigallol is produced by condensation. Oxidation with arsenic acid, permanganate, persulfate, or iodine yields ellagic acid, as does reaction of methyl gallate with iron(III) chloride.
|Lambda-max:||220, 271 nm (ethanol)|
|Extinction coefficient (log ε)|
|Major absorption bands||ν : 3491, 3377, 1703, 1617, 1539, 1453, 1254 cm−1 (KBr)|
7.15 (2H, s, H-3 and H-7)
|Other NMR data|
|ESI-MS [M-H]- m/z : 169.0137 ms/ms (iontrap)@35 CE m/z product 125(100), 81(<1)|
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
- Andrew Pengelly (2004), The Constituents of Medicinal Plants (2nd ed.), Allen & Unwin, pp. 29–30
- Edwin Ritzer; Rudolf Sundermann (2007), "Hydroxycarboxylic Acids, Aromatic", Ullmann's Encyclopedia of Industrial Chemistry (7th ed.), Wiley, p. 6
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- 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."
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