Gallic acid (also known as 3,4,5-trihydroxybenzoic acid) is a trihydroxybenzoic acid with the formula C₆ H₂( OH)₃CO₂H. It is classified as a

phenolic acid Phenolic acids or phenolcarboxylic acids are 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 naturally occurring types of ...

. 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, fungi, or animals. Plant galls are abnormal outgrowths of plant tissues, similar to ...

sumac Sumac ( or ), also spelled sumach, is any of about 35 species of flowering plants in the genus ''Rhus'' and related genera in the cashew family ( Anacardiaceae). Sumacs grow in subtropical and temperate regions throughout the world, including E ...

witch hazel Witch-hazels or witch hazels (''Hamamelis'') are a genus of flowering plants in the family Hamamelidaceae, with three species in North America ('' H. ovalis'', '' H. virginiana'', and '' H. vernalis''), and one each in Japan ('' H.&nb ...

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 southwestern China and north ...

leaves, oak bark, and other

plant Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae ex ...

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

Salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quantiti ...

s and

ester In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glyceride ...

s of gallic acid are termed "gallates".

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. Meta ...

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, elimination, and solvation reactions in which water is the nucleophile. Biological hydrolys ...

. When heated with concentrated

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

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

glucose Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, u ...

or ellagic acid and glucose, known as

s and

ellagitannin The ellagitannins are a diverse class of hydrolyzable tannins, a type of polyphenol formed primarily from the oxidative linkage of galloyl groups in 1,2,3,4,6-pentagalloyl glucose. Ellagitannins differ from gallotannins, in that their galloyl gr ...

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 catalyzes the chemical reaction :shikimate + NADP+ \rightleftharpoons 3-dehydroshikimate + NADPH + H+ Thus, the two substrates of this enzyme are shikimate and NADP+, whereas its ...

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, saprotrophic soil bacterium. Based on 16S rRNA analysis, ''P. putida'' was taxonomically confirmed to be a ''Pseudomonas'' species (''sensu stricto'') and placed, along with several other ...

''. 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 backwards because it can be ...

, as does reaction of methyl gallate with

iron(III) chloride Iron(III) chloride is the inorganic compound with the formula . Also called ferric chloride, it is a common compound of iron in the +3 oxidation state. The anhydrous compound is a crystalline solid with a melting point of 307.6 °C. The co ...

. 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 ester In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one ...

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 reduce or saturate orga ...

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 re ...

(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 In enzymology, a gallate 1-beta-glucosyltransferase () is an enzyme that catalyzes the chemical reaction :UDP-glucose + gallate \rightleftharpoons UDP + 1-galloyl-beta-D-glucose Thus, the two substrates of this enzyme are UDP-glucose and gall ...

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 no ...

(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 ...

, 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 the Qumran Caves Scrolls) are ancient Jewish and Hebrew religious manuscripts discovered between 1946 and 1956 at the Qumran Caves in what was then Mandatory Palestine, near Ein Feshkha in the West Bank, on the ...

Pliny the Elder Gaius Plinius Secundus (AD 23/2479), called Pliny the Elder (), was a Roman author, naturalist and natural philosopher, and naval and army commander of the early Roman Empire, and a friend of the emperor Vespasian. He wrote the encyclopedic ...

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

verdigris Verdigris is the common name for blue-green, copper-based pigments that form a patina on copper, bronze, and brass. The technical literature is ambiguous as to its chemical composition. Some sources refer to "neutral verdigris" as copper(II) ac ...

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 Tannic acid is a specific form of tannin, a type of polyphenol. Its weak acidity ( pKa around 6) is due to the numerous phenol groups in the structure. The chemical formula for commercial tannic acid is often given as C76H52O46, which corresp ...

. It could then be mixed with green vitriol (

ferrous sulfate Iron(II) sulfate (British English: iron(II) sulphate) or ferrous 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 x are know ...

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

gum arabic Gum arabic, also known as gum sudani, acacia gum, Arabic gum, gum acacia, acacia, Senegal gum, Indian gum, and by other names, is a natural gum originally consisting of the hardened sap of two species of the ''Acacia'' tree, ''Senegalia sen ...

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 discove ...

(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 so that the page can be reused for 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 German pharmaceutical chemist. Scheele discovered oxygen (although Joseph Priestley published his findings first), and identified molybdenum, tungsten, barium, hydroge ...

in 1786. In 1818, French chemist and pharmacist

Henri Braconnot Henri Braconnot (29 May 1780 – 13 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 elementa ...

(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, Théophile Pelouze, Theo Pelouze, or T. J. 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 a ...

(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 compon ...

, 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

The name is derived from oak galls, which were historically used to prepare

. Despite the name, gallic acid does not contain

gallium Gallium is a chemical element with the symbol Ga and atomic number 31. Discovered by French chemist Paul-Émile Lecoq de Boisbaudran in 1875, Gallium is in group 13 of the periodic table and is similar to the other metals of the group (alumin ...

. Gallic acid is found in a number of

land plant The Embryophyta (), or land plants, are the most familiar group of green plants that comprise vegetation on Earth. Embryophytes () have a common ancestor with green algae, having emerged within the Phragmoplastophyta clade of green algae as sist ...

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 parasitic plants develop a specialized organ called the ...

'' Cynomorium coccineum'', the

aquatic plant Aquatic plants are plants that have adapted to living in aquatic environments ( saltwater or freshwater). They are also referred to as hydrophytes or macrophytes to distinguish them from algae and other microphytes. A macrophyte is a plant that ...

Myriophyllum spicatum ''Myriophyllum spicatum'' (Eurasian watermilfoil or spiked water-milfoil) is native to Europe, Asia, and north Africa, but has a wide geographic and climatic distribution among some 57 countries, extending from northern Canada to South Africa. It ...

'', and the blue-green

alga Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular mi ...

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 neur ...

''. Gallic acid is also found in various oak species, '' Caesalpinia mimosoides,'' and in the stem bark of ''

Boswellia dalzielii ''Boswellia dalzielii'' is a tree species in the genus ''Boswellia ''Boswellia'' is a genus of trees in the order Sapindales, known for its fragrant resin. The biblical incense frankincense is an extract from the resin of the tree ''Boswelli ...

,'' 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 acetic acid and trace compounds that may include flavorings. Vinegar typically contains 5–8% acetic acid by volume. Usually, the acetic acid is produced by a double fermentation, converting simple sugars to et ...

s. Carob 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 s ...

Octyl gallate Octyl gallate is the ester of 1-octanol and gallic acid. As a food additive, it is used under the E number E311 as an antioxidant and preservative. Properties Octyl gallate is a white powder with a characteristic odor. It is very slightly sol ...

, the ester of octanol and gallic acid *

Dodecyl gallate Dodecyl gallate, or lauryl gallate, is the ester of dodecanol and gallic acid. As a food additive it is used under the E number E312 as an antioxidant and preservative A preservative is a substance or a chemical that is added to products such ...

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

Epicatechin gallate Epicatechin gallate (ECG) is a flavan-3-ol, a type of flavonoid, present in green tea. It is also reported in buckwheat and in grape. The tea component epicatechin gallate is being researched because in vitro experiments showed it can reverse me ...

, 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 its ...

(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

antioxidant Antioxidants are compounds that inhibit oxidation, a chemical reaction that can produce free radicals. This can lead to polymerization and other chain reactions. They are frequently added to industrial products, such as fuels and lubric ...

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

References

Appendix

Spectral data

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