Gallic acid (also known as 3,4,5-trihydroxybenzoic acid) is a

trihydroxybenzoic acid Trihydroxybenzoic acid may refer to the following phenolic acids: * Gallic acid (3,4,5-trihydroxybenzoic acid) * Phloroglucinol carboxylic acid (2,4,6-trihydroxybenzoic acid) O-methylated trihydroxybenzoic acids are: * Eudesmic acid * Syringic aci ...

with the formula C₆ H₂( OH)₃CO₂H. It is classified as a

phenolic acid Phenolic acids or phenolcarboxylic acids ? are phenolic compounds and types of aromatic acid compounds. Included in that class are substances containing a phenolic ring and an organic carboxylic acid function (C6-C1 skeleton). Two important nat ...

. It is found in

gallnut Galls (from the Latin , 'oak-apple') or ''cecidia'' (from the Greek , anything gushing out) are a kind of swelling growth on the external tissues of plants. Plant galls are abnormal outgrowths of plant tissues, similar to benign tumors or warts ...

sumac Sumac or sumach ( , )—not to be confused with poison sumac—is any of the roughly 35 species of flowering plants in the genus ''Rhus'' (and related genera) of the cashew and mango tree family, Anacardiaceae. However, it is '' Rhus coriaria ...

, witch hazel,

tea Tea is an aromatic beverage prepared by pouring hot or boiling water over cured or fresh leaves of '' Camellia sinensis'', an evergreen shrub native to East Asia which probably originated in the borderlands of south-western China and nor ...

leaves,

oak bark Tanbark is the bark of certain species of trees, traditionally used for tanning hides into leather. The words "tannin", " tanning", " tan," and " tawny" are derived from the Medieval Latin , "to convert into leather." Bark mills are horse- ...

, and other

plant Plants are the eukaryotes that form the Kingdom (biology), kingdom Plantae; they are predominantly Photosynthesis, photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with c ...

s. It is a white solid, although samples are typically brown owing to partial oxidation.

Salt In common usage, salt is a mineral composed primarily of sodium chloride (NaCl). When used in food, especially in granulated form, it is more formally called table salt. In the form of a natural crystalline mineral, salt is also known as r ...

s and

ester In chemistry, an ester is a compound derived from an acid (either organic or inorganic) in which the hydrogen atom (H) of at least one acidic hydroxyl group () of that acid is replaced by an organyl group (R). These compounds contain a distin ...

s of gallic acid are termed "gallates". Its name is derived from

oak gall An oak apple or oak gall is a large, round, vaguely apple-like gall commonly found on many species of oak. Oak apples range in size from in diameter and are caused by chemicals injected by the larva of certain kinds of gall wasp in the family ...

s, which were historically used to prepare

tannic acid Tannic acid is a specific form of tannin, a type of polyphenol. Its weak acidity (Acid dissociation constant, pKa around 6) is due to the numerous phenol groups in the structure. The chemical formula for commercial tannic acid is often given as ...

. Despite the name, gallic acid does not contain

gallium Gallium is a chemical element; it has Chemical symbol, symbol Ga and atomic number 31. Discovered by the French chemist Paul-Émile Lecoq de Boisbaudran in 1875, elemental gallium is a soft, silvery metal at standard temperature and pressure. ...

Isolation and derivatives

Gallic acid is easily freed from

gallotannin A gallotannin is any of a class of molecules belonging to the hydrolysable tannins. Gallotannins are polymers formed when gallic acid, a polyphenol monomer, esterifies and binds with the hydroxyl group of a polyol carbohydrate such as glucose. Met ...

s by acidic or alkaline

hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...

. When heated with concentrated

sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid (English in the Commonwealth of Nations, Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen, ...

, gallic acid converts to rufigallol. Hydrolyzable tannins break down on hydrolysis to give gallic acid and

glucose Glucose is a sugar with the Chemical formula#Molecular formula, molecular formula , which is often abbreviated as Glc. It is overall the most abundant monosaccharide, a subcategory of carbohydrates. It is mainly made by plants and most algae d ...

or ellagic acid and glucose, known as

s and

ellagitannin image:Castalagin.svg, 130px, Castalagin is a representative ellagitannin, characterized by coupled gallic acid substituents The ellagitannins are a diverse class of hydrolyzable tannins, a type of polyphenol formed primarily from the oxidative link ...

s, respectively.

Biosynthesis

Gallic acid is formed from 3-dehydroshikimate by the action of the enzyme

shikimate dehydrogenase In enzymology, a shikimate dehydrogenase () is an enzyme that catalysis, catalyzes the chemical reaction :shikimate + NADP+ \rightleftharpoons 3-dehydroshikimate + NADPH + H+ Thus, the two substrate (biochemistry), substrates of this enzyme are ...

to produce 3,5-didehydroshikimate. This latter compound aromatizes.

Reactions

Oxidation and oxidative coupling

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 ''Pseudomonas putida'' is a Gram-negative, rod-shaped, saprophytic soil bacterium. It has a versatile metabolism and is amenable to genetic manipulation, making it a common organism used in research, bioremediation, and synthesis of chemicals and ...

''. Oxidative coupling of gallic acid with arsenic acid, permanganate, persulfate, or iodine yields

ellagic acid Ellagic acid is a polyphenol found in numerous fruits and vegetables. It is the dilactone of hexahydroxydiphenic acid. Name The name comes from the French term ''acide ellagique'', from the word ''galle'' spelled backward because it can be o ...

, as does reaction of methyl gallate with

iron(III) chloride Iron(III) chloride describes the inorganic compounds with the formula (H2O)x. Also called ferric chloride, these compounds are some of the most important and commonplace compounds of iron. They are available both in anhydrous and in hydrated f ...

. Gallic acid forms intermolecular esters ( depsides) such as digallic and cyclic ether-esters (

depsidone Depsidones (+ " depside" + "one") are chemical compounds that are sometimes found as secondary metabolites in lichens. They are esters that are both depsides and cyclic ethers. An example is norstictic acid Norstictic acid is a depsidone produ ...

s).

Hydrogenation

Hydrogenation Hydrogenation is a chemical reaction between molecular hydrogen (H2) and another compound or element, usually in the presence of a catalyst such as nickel, palladium or platinum. The process is commonly employed to redox, reduce or Saturated ...

of gallic acid gives the cyclohexane derivative hexahydrogallic acid.

Decarboxylation

Heating gallic acid gives

pyrogallol Pyrogallol is an organic compound with the formula C6H3(OH)3. It is a water-soluble, white solid although samples are typically brownish because of its sensitivity toward oxygen. It is one of three isomers of benzenetriols. Production and reac ...

(1,2,3-trihydroxybenzene). This conversion is catalyzed by gallate decarboxylase.

Esterification

Many esters of gallic acid are known, both synthetic and natural. Gallate 1-beta-glucosyltransferase catalyzes the

glycosylation Glycosylation is the reaction in which a carbohydrate (or ' glycan'), i.e. a glycosyl donor, is attached to a hydroxyl or other functional group of another molecule (a glycosyl acceptor) in order to form a glycoconjugate. In biology (but not ...

(attachment of glucose) of gallic acid.

Historical context and uses

Gallic acid is an important component of

iron gall ink Iron gall ink (also known as common ink, standard ink, oak gall ink or iron gall nut ink) is a purple-black or brown-black ink made from iron salts and tannic acids from vegetable sources. It was the standard ink formulation used in Europe for t ...

, 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 The Dead Sea Scrolls, also called the Qumran Caves Scrolls, are a set of List of Hebrew Bible manuscripts, ancient Jewish manuscripts from the Second Temple period (516 BCE – 70 CE). They were discovered over a period of ten years, between ...

Pliny the Elder Gaius Plinius Secundus (AD 23/24 79), known in English as Pliny the Elder ( ), was a Roman Empire, Roman author, Natural history, naturalist, and naval and army commander of the early Roman Empire, and a friend of the Roman emperor, emperor Vesp ...

(23–79 AD) describes the use of gallic acid as a means of detecting an adulteration of

verdigris Verdigris () is a common name for any of a variety of somewhat toxic copper salt (chemistry), salts of acetic acid, which range in colour from green to a blue-green, bluish-green depending on their chemical composition.H. Kühn, Verdigris and Cop ...

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

. It could then be mixed with

green vitriol Iron(II) sulfate or ferrous sulfate (British English: sulphate instead of sulfate) denotes a range of salts with the formula Fe SO4·''x''H2O. These compounds exist most commonly as the heptahydrate (''x'' = 7), but several values for ...

(

ferrous sulfate Iron(II) sulfate or ferrous sulfate (British English: sulphate instead of sulfate) denotes a range of salts with the formula Fe SO4·''x''H2O. These compounds exist most commonly as the heptahydrate (''x'' = 7), but several values for ...

)—obtained by allowing sulfate-saturated water from a spring or mine drainage to evaporate—and

gum arabic Gum arabic (gum acacia, gum sudani, Senegal gum and by other names) () is a tree gum exuded by two species of '' Acacia sensu lato:'' '' Senegalia senegal,'' and '' Vachellia seyal.'' However, the term "gum arabic" does not indicate a partic ...

from acacia trees; this combination of ingredients produced the ink. Gallic acid was one of the substances used by

Angelo Mai Angelo Mai (''Latin'' Angelus Maius; 7 March 17828 September 1854) was an Italian Cardinal and philologist. He won a European reputation for publishing for the first time a series of previously unknown ancient texts. These he was able to discov ...

(1782–1854), among other early investigators of

palimpsest In textual studies, a palimpsest () is a manuscript page, either from a scroll or a book, from which the text has been scraped or washed off in preparation for reuse in the form of another document. Parchment was made of lamb, calf, or kid ski ...

s, 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 Carl Wilhelm Scheele (, ; 9 December 1742 – 21 May 1786) was a Swedish Pomerania, German-Swedish pharmaceutical chemist. Scheele discovered oxygen (although Joseph Priestley published his findings first), and identified the elements molybd ...

in 1786. In 1818, French chemist and pharmacist

Henri Braconnot Henri Braconnot (; 29 May 178013 January 1855) was a French chemist and pharmacist. He was born in Commercy, his father being a counsel at the local parliament. At the death of his father, in 1787, Henri began his instruction in an elementary ...

(1780–1855) devised a simpler method of purifying gallic acid from galls; gallic acid was also studied by the French chemist

Théophile-Jules Pelouze Théophile-Jules Pelouze (also known as Jules Pelouze), ; 26 February 180731 May 1867) was a French chemist. Life He was born at Valognes, and died in Paris. His father, Edmond Pelouze, was an industrial chemist and the author of several tech ...

(1807–1867), among others. When mixed with

acetic acid Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main compone ...

, gallic acid had uses in early types of photography, like the

calotype Calotype or talbotype is an early photographic process introduced in 1841 by William Henry Fox Talbot, using paper coated with silver iodide. Paper texture effects in calotype photography limit the ability of this early process to record low ...

to make the silver more sensitive to light; it was also used in developing photographs.

Occurrence

Gallic acid is found in a number of

land plant The embryophytes () are a clade of plants, also known as Embryophyta (Plantae ''sensu strictissimo'') () or land plants. They are the most familiar group of photoautotrophs that make up the vegetation on Earth's dry lands and wetlands. Embryophyt ...

s, such as the

parasitic plant A parasitic plant is a plant that derives some or all of its nutritional requirements from another living plant. They make up about 1% of angiosperms and are found in almost every biome. All Parasite, parasitic plants develop a specialized organ ...

'' Cynomorium coccineum'', the

aquatic plant Aquatic plants, also referred to as hydrophytes, are vascular plants and Non-vascular plant, non-vascular plants that have adapted to live in aquatic ecosystem, aquatic environments (marine ecosystem, saltwater or freshwater ecosystem, freshwater ...

Myriophyllum spicatum ''Myriophyllum spicatum'' (Eurasian watermilfoil or spiked water-milfoil) is a submerged perennial aquatic plant which grows in still or slow-moving water. Eurasian watermilfoil is native to Europe, Asia, and North Africa, but has a wide geograph ...

'', and the blue-green

alga Algae ( , ; : alga ) is an informal term for any organisms of a large and diverse group of photosynthetic organisms that are not plants, and includes species from multiple distinct clades. Such organisms range from unicellular microalgae, suc ...

Microcystis aeruginosa ''Microcystis aeruginosa'' is a species of freshwater cyanobacteria that can form harmful algal blooms of economic and ecological importance. They are the most common toxic cyanobacterial bloom in eutrophic fresh water. Cyanobacteria produce neu ...

''. 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

s, cloves, and

vinegar Vinegar () is an aqueous solution of diluted acetic acid and trace compounds that may include flavorings. Vinegar typically contains from 5% to 18% acetic acid by volume. Usually, the acetic acid is produced by a double fermentation, converting ...

Carob The carob ( ; ''Ceratonia siliqua'') is a flowering evergreen tree or shrub in the Caesalpinioideae sub-family of the legume family, Fabaceae. It is widely cultivated for its edible fruit, which takes the form of seed pods, and as an ornam ...

fruit is a rich source of gallic acid (24–165 mg per 100 g).

Esters

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 There are two isomers of propanol. * 1-Propanol, ''n''-propanol, or propan-1-ol: CH3CH2CH2OH, the most common meaning *2-Propanol, isopropyl alcohol, isopropanol, or propan-2-ol: (CH3)2CHOH See also * Propanal (propionaldehyde) differs in spel ...

* Octyl gallate, the ester of octanol and gallic acid * Dodecyl gallate, or lauryl gallate, the ester of dodecanol and gallic acid *

Epicatechin gallate Epicatechin gallate (ECG, (−)-epicatechin-3-gallate) is a flavan-3-ol, a type of flavonoid, primarily found in green tea (''Camellia sinensis''), with smaller amounts in cocoa, grapes, and other plants. It is also reported in buckwheat and in g ...

, a flavan-3-ol, a type of flavonoid, present in green tea *

Epigallocatechin gallate Epigallocatechin gallate (EGCG), also known as epigallocatechin-3-gallate, is the ester of epigallocatechin and gallic acid, and is a type of catechin. EGCG – the most abundant catechin in tea – is a polyphenol under basic research for it ...

(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 Theaflavin-3-gallate is a theaflavin derivative. It can be found in abundance in black tea and is produced during fermentation. It has been studied as a cancer-fighting chemical when combined with cisplatin against ovarian cancer cells. Consuming ...

, a theaflavin derivative Gallate esters are

antioxidant Antioxidants are Chemical compound, compounds that inhibit Redox, oxidation, a chemical reaction that can produce Radical (chemistry), free radicals. Autoxidation leads to degradation of organic compounds, including living matter. Antioxidants ...

s useful in food preservation, with propyl gallate being the most commonly used. Their use in human health is scantly supported by evidence.

Spectral data

References

{{DEFAULTSORT:Gallic Acid Antioxidants Astringent flavors Chelating agents Gallotannins Pyrogallols Reducing agents Trihydroxybenzoic acids