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. It is found in gallnuts, sumac, witch hazel, tea leaves, oak bark, 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 galls, which were historically used to prepare tannic acid. 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 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 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 gallotannins 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 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''. 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 redox, reduce or Saturated ...

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.

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 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. 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 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'', 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. 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 * 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.

Spectral data

References

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