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 ...

with the formula C₆ H₂( OH)₃CO₂H. It is classified as a phenolic acid. It is found in gallnuts, sumac, witch hazel, tea 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 ''tannare'', "to convert into leather." Bark mills are ...

, and other plants. It is a white solid, although samples are typically brown owing to partial oxidation. Salts and esters 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. 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 formu ...

, gallic acid converts to

rufigallol Rufigallol or 1,2,3,5,6,7-hexahydroxy-9,10-anthraquinone is an organic compound with formula , which can be viewed as a derivative of anthraquinone through the replacement of six hydrogen atoms (H) by hydroxyl groups (OH). The compound is solub ...

. Hydrolyzable tannins break down on hydrolysis to give gallic acid and glucose or ellagic acid and glucose, known as

s and ellagitannins, 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 Gallate dioxygenase (, ''GalA'') is an enzyme with systematic name ''gallate:oxygen oxidoreductase''. This enzyme catalyses the following chemical reaction A chemical reaction is a process that leads to the IUPAC nomenclature for organic tran ...

, 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 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 col ...

. Gallic acid forms intermolecular esters (

depsides A depside is a type of polyphenolic compound composed of two or more monocyclic aromatic units linked by an ester bond. Depsides are most often found in lichens, but have also been isolated from higher plants, including species of the Ericaceae, L ...

) such as digallic and cyclic ether-esters ( depsidones).

Hydrogenation

Hydrogenation of gallic acid gives the cyclohexane derivative hexahydrogallic acid.

Decarboxylation

Heating gallic acid gives pyrogallol (1,2,3-trihydroxybenzene). This conversion is catalyzed by

gallate decarboxylase The enzyme gallate decarboxylase () catalyzes the chemical reaction :3,4,5-trihydroxybenzoate \rightleftharpoons pyrogallol + CO2 This enzyme belongs to the family of lyases, specifically the carboxy-lyases, which cleave carbon-carbon bonds. T ...

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 al ...

(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. Pliny the Elder (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. 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 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 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. 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 component ...

, 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 co ...

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

Occurrence

The name is derived from

oak gall Oak apple or oak gall is the common name for 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 ...

s, which were historically used to prepare tannic acid. 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 (aluminiu ...

. Gallic acid is found in a number of land plants, such as the parasitic plant ''

Cynomorium coccineum ''Cynomorium'' is a genus of parasitic perennial flowering plants in the family Cynomoriaceae. The genus consists of only one species, ''Cynomorium coccineum'' (although one of its subspecies is sometimes treated as a separate species). Its plac ...

'', 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 '' Microcystis aeruginosa''. 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 ''Caesa ...

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

Esters

Also known as galloylated esters: *

Methyl gallate Methyl gallate is a phenolic compound. It is the methyl ester of gallic acid. Natural occurrences It is found in ''Terminalia myriocarpa'', ''Bergenia ciliata'' (hairy Bergenia) and ''Geranium niveum''. It is found in the fruit extract of '' Pa ...

Ethyl gallate Ethyl gallate is a food additive with E number E313. It is the ethyl ester of gallic acid. Ethyl gallate is added to food as an antioxidant. Though found naturally in a variety of plant sources including walnuts ''Terminalia myriocarpa'' or c ...

, a food additive with E number E313 *

Propyl gallate Propyl gallate, or propyl 3,4,5-trihydroxybenzoate is an ester formed by the condensation of gallic acid and propanol. Since 1948, this antioxidant has been added to foods containing oils and fats to prevent oxidation. As a food additive, it is ...

, or propyl 3,4,5-trihydroxybenzoate, an ester formed by the condensation of gallic acid and propanol *

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 solubl ...

, 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 su ...

, 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 Gallocatechin gallate (GCG) is the ester of gallocatechin and gallic acid and a type of catechin. It is an epimer of epigallocatechin gallate (EGCG). In a high temperature environment, an epimerization In stereochemistry, an epimer is one of a ...

(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 lubricant ...

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