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 ph ...

. It is found in gallnuts,

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 Eas ...

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 exclu ...

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 quant ...

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. Glycerides a ...

s of gallic acid are termed "gallates".

Isolation and derivatives

Gallic acid is easily freed from gallotannins 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, 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 gallotannins 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 ...

s, respectively.

Biosynthesis

Gallic acid is formed from 3-dehydroshikimate by the action of the enzyme shikimate dehydrogenase 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, as does reaction of methyl gallate with iron(III) chloride. Gallic acid forms intermolecular esters ( depsides) such as digallic and cyclic ether-esters ( depsidones).

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 organ ...

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 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 (attachment of glucose) of gallic acid.

Historical context and uses

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 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 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 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 ...

(

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 hepta hydrate (''x'' = 7) but several values for x are kn ...

) — 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 se ...

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

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 skin an ...

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, hyd ...

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 elementar ...

(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 an ...

(1807–1867), 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.

Occurrence

The name is derived from oak galls, which were historically used to prepare tannic acid. Despite the name, gallic acid does not contain gallium. 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 '' Myriophyllum spicatum'', 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 mic ...

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 ne ...

''. Gallic acid is also found in various oak species, ''

Caesalpinia mimosoides ''Hultholia mimosoides'' is a liana species and the sole species in the genus ''Hultholia''. It was formerly placed in the genus ''Caesalpinia'' but phylogenetic studies identified the group to be polyphyletic, leading the placement of ''Caesalp ...

,'' and in the stem bark of ''

Boswellia dalzielii ''Boswellia dalzielii'' is a tree species in the genus ''Boswellia'' found in Africa. The bark of this plant is used in traditional medicines. Gallic and protocatechuic acids were isolated as the main antibacterial and antioxidant principles of ...

,'' among others. Many foodstuffs contain various amounts of gallic acid, especially fruits (including strawberries, grapes, bananas), as well as

s, cloves, and vinegars. 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 Isopropyl alcohol (IUPAC name propan-2-ol and also called isopropanol or 2-propanol) is a c ...

* 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.

References

Appendix

Spectral data

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