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Pyrogallol
Pyrogallol
Pyrogallol
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Pyrogallol
Skeletal formula
Skeletal formula
Ball-and-stick model
Ball-and-stick model
Names
Preferred IUPAC name
Benzene-1,2,3-triol
Other names
1,2,3-Trihydroxybenzene
Pyrogallic acid
1,2,3-Benzenetriol[1]
Identifiers
3D model (JSmol)
ChEBI
ChEMBL
ChemSpider
ECHA InfoCard 100.001.603 Edit this at Wikidata
EC Number
  • 201-762-9
KEGG
RTECS number
  • UX2800000
UNII
UN number 2811
  • InChI=1S/C6H6O3/c7-4-2-1-3-5(8)6(4)9/h1-3,7-9H checkY
    Key: WQGWDDDVZFFDIG-UHFFFAOYSA-N checkY
  • InChI=1/C6H6O3/c7-4-2-1-3-5(8)6(4)9/h1-3,7-9H
    Key: WQGWDDDVZFFDIG-UHFFFAOYAT
  • Oc1cccc(O)c1O
Properties
C6H6O3
Molar mass 126.11 g/mol
Density 1.453 g/cm3 (4 °C)[1]
Melting point 125.5 °C (257.9 °F; 398.6 K)[1]
Boiling point 307 °C (585 °F; 580 K)[1]
1.561 (134 °C)[1]
Structure[2]
Monoclinic
P21/n
a = 12.1144(11) Å, b = 3.7765(3) Å, c = 13.1365(12) Å
α = 90°, β = 115.484(1)°, γ = 90°
4
Hazards
GHS labelling:
GHS07: Exclamation markGHS08: Health hazard
Warning
H302, H312, H332, H341, H412
P201, P202, P261, P264, P270, P271, P273, P280, P281, P301+P312, P302+P352, P304+P312, P304+P340, P308+P313, P312, P322, P330, P363, P405, P501
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
checkY verify (what is checkY☒N ?)

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.[3] It is one of three isomers of benzenetriols.

Production and reactions

[edit]

It is produced in the manner first reported by Scheele in 1786: heating gallic acid to induce decarboxylation.[3]

Gallic acid is also obtained from tannin. Many alternative routes have been devised. One preparation involves treating para-chlorophenoldisulfonic acid with potassium hydroxide,[4] a variant on the time-honored route to phenols from sulfonic acids.[5]

Polyhydroxybenzenes are relatively electron-rich. One manifestation is the easy C-acetylation of pyrogallol.[6]

Uses

[edit]

It was once used in hair dyeing, dyeing of suturing materials. It also has antiseptic properties.

In alkaline solution, pyrogallol undergoes deprotonation. Such solutions absorb oxygen from the air, turning brown. This conversion can be used to determine the amount of oxygen in a gas sample, notably by the use of the Orsat apparatus. Alkaline solutions of pyrogallol have been used for oxygen absorption in gas analysis.

Use in photography

[edit]

Pyrogallol was also used as a developing agent in the 19th and early 20th centuries in black-and-white developers. Hydroquinone is more commonly used today. Its use is largely historical except for special purpose applications. It was still used by a few notable photographers including Edward Weston. In those days it had a reputation for erratic and unreliable behavior, due possibly to its propensity for oxidation. It experienced a revival starting in the 1980s due largely to the efforts of experimenters Gordon Hutchings and John Wimberley. Hutchings spent over a decade working on pyrogallol formulas, eventually producing one he named PMK for its main ingredients: pyrogallol, Metol, and Kodalk (the trade name of Kodak for sodium metaborate). This formulation resolved the consistency issues, and Hutchings found that an interaction between the greenish stain given to film by pyro developers and the color sensitivity of modern variable-contrast photographic papers gave the effect of an extreme compensating developer. From 1969 to 1977, Wimberley experimented with the Pyrogallol developing agent. He published his formula for WD2D in 1977 in Petersen's Photographic. PMK and other modern pyro formulations are now used by many black-and-white photographers. The Film Developing Cookbook has examples.[7]

Another developer mainly based on pyrogallol was formulated by Jay DeFehr. The 510-pyro,[8] is a concentrate that uses triethanolamine as alkali, and pyrogallol, ascorbic acid, and phenidone as combined developers in a single concentrated stock solution with long shelf life. This developer has both staining and tanning properties and negatives developed with it are immune to the callier effect. It can be used for small and large negative formats.

The Darkroom Cookbook (Alternative Process Photography) has examples.[9]

Safety

[edit]

Pyrogallol use, e.g. in hair dye formulations, is declining because of concerns about its toxicity.[10] Its LD50 (oral, rat) is 300 mg/kg.[3]

Pure pyrogallol was found to be extremely genotoxic when inserted into cultured cells, but α amylase proteins protect against its toxicity during everyday exposure.[11][12]

See also

[edit]

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Pyrogallol

View on Grokipedia
from Grokipedia
Pyrogallol, also known as 1,2,3-trihydroxybenzene or pyrogallic acid, is an with the molecular formula C₆H₆O₃ and a molecular weight of 126.11 g/mol. It is a benzenetriol featuring three hydroxyl groups attached to adjacent carbon atoms on a ring, appearing as an odorless white to gray solid that melts at 131–134 °C and boils at 309 °C. Highly soluble in (507 mg/mL at 25 °C), , and , pyrogallol serves as a potent due to its ability to undergo oxidation and in alkaline solutions. Historically introduced as one of the earliest photographic developers in the mid-19th century, pyrogallol reduces silver halides to metallic silver in black-and-white film processing, producing fine grain and high contrast images while also acting as a tanning agent for emulsions. Beyond photography, it finds applications as an in oils and drug formulations to prevent oxidative damage, a in for oxygen absorption and metal complexation, and in the dyeing of hair, leather, and suturing materials due to its polyphenolic structure. Pyrogallol occurs naturally as a metabolite in , anthocyanins, and other compounds, contributing to its biological roles as a phenolic donor. Despite its utility, pyrogallol exhibits significant , with an oral LD50 of 300 mg/kg in mice, causing to , eyes, and upon exposure; it is also a potential skin sensitizer and has shown equivocal carcinogenic evidence in animal studies. Its handling requires precautions, including protective equipment, due to risks of via ingestion, inhalation, or dermal contact, and it is classified as harmful to aquatic life with long-lasting effects.

Properties

Physical properties

Pyrogallol has the molecular formula C₆H₆O₃ and a molecular weight of 126.11 g/mol. It appears as a white to slightly yellow crystalline powder or solid that darkens upon exposure to air and light. The compound has a of 131–134 °C and a of 309 °C, at which it decomposes. Pyrogallol exhibits high solubility in water, 507 g/L at 25 °C, and is very soluble in ethanol and diethyl ether, while being slightly soluble in benzene and chloroform. Its density is 1.45 g/cm³ at 20 °C. The vapor pressure is low, at 4.79 × 10⁻⁴ mmHg at 25 °C.

Chemical properties

Pyrogallol, chemically known as benzene-1,2,3-triol, is a benzenetriol with the molecular formula C₆H₆O₃, featuring three hydroxyl groups attached to adjacent carbon atoms (positions 1, 2, and 3) on a benzene ring. This structure is also referred to by synonyms such as 1,2,3-trihydroxybenzene and pyrogallic acid. The ortho arrangement of the three hydroxyl groups facilitates intramolecular hydrogen bonding, forming five- or six-membered rings that stabilize the and influence its tautomerism, predominantly favoring the form over keto tautomers. This configuration enhances the acidity of the phenolic protons, with reported pKa values of 9.03 for the first dissociation and 11.63 for the second at 25°C. As a , pyrogallol exhibits strong properties, attributed to the low oxidation potential of its phenolic hydroxyl groups, which readily donate electrons or hydrogen atoms in reactions. Spectroscopically, pyrogallol in displays an ultraviolet-visible absorption maximum at 266 nm, corresponding to π–π* transitions in the aromatic ring influenced by the hydroxyl substituents. In , the O-H stretching vibrations appear as broad bands around 3400 cm⁻¹, shifted lower due to extensive hydrogen bonding among the adjacent hydroxyl groups.

Synthesis

Industrial production

Pyrogallol is industrially produced primarily through the thermal of (3,4,5-trihydroxybenzoic acid), where the precursor is heated to 180–220 °C under controlled conditions, often in an or high-pressure vessel, to yield pyrogallol and via the reaction: \ceC6H2(OH)3COOH−>[heat]C6H3(OH)3+CO2\ce{C6H2(OH)3COOH ->[heat] C6H3(OH)3 + CO2} This process is energy-intensive due to the high temperatures required but is economically viable given the availability of gallic acid. Gallic acid itself is derived from the of natural present in plant materials such as nutgalls (from trees) or tara pods ( spinosa), which are abundant and cost-effective feedstocks. After , the crude pyrogallol is purified through under reduced pressure to remove volatile impurities or by sublimation, which exploits its volatility to separate it from non-volatile byproducts, routinely achieving purities exceeding 98%. Global production capacity for pyrogallol is on the order of several thousand tons per year, concentrated in and , where it serves mainly as an intermediate in and pharmaceuticals.

Laboratory preparation

Pyrogallol is commonly prepared in the laboratory by the thermal of , which involves heating the dried precursor to approximately 200–220 °C, resulting in the loss of to form the trihydroxybenzene structure. This method is straightforward for small-scale synthesis and typically achieves yields of around 81% when facilitated by catalysts such as at lower temperatures (e.g., 135 °C for 1–2 hours), though dry heating alone may require higher temperatures and longer reaction times of 1–2 hours. Alternative synthetic routes suitable for settings include the oxidation of 2,3-dihydroxybenzaldehyde, where the precursor undergoes selective oxidation to introduce hydroxyl group, yielding pyrogallol in a controlled manner. Another approach is the selective of using , which adds a hydroxyl group at the appropriate position to produce pyrogallol, though this method requires careful control to minimize side products. Modern laboratory preparations increasingly utilize microbial fermentation with genetically engineered bacteria, such as , to convert glucose into as an intermediate, followed by enzymatic to pyrogallol. This biosynthetic pathway enables gram-per-liter production under mild conditions (e.g., 30 °C, 5.5–7.0) and achieves yields of 70–90% from the gallic acid intermediate, offering a sustainable alternative to thermal methods.

Reactions

Oxidation reactions

Pyrogallol undergoes rapid auto-oxidation in the presence of air, especially under alkaline conditions, yielding purpurogallin as a prominent product. This proceeds via semiquinone intermediates, where the initial step involves a one-electron oxidation of pyrogallol to form a phenoxyl radical (semiquinone), which subsequently dimerizes, undergoes further oxidation, cyclizes, and decarboxylates to purpurogallin from two molecules of pyrogallol and molecular oxygen. This transformation is significantly accelerated by trace metal ions, such as copper(II), which facilitate the generation of like that propagate the chain reaction. Electrochemical studies reveal that pyrogallol exhibits a low oxidation potential in alkaline media, with the first anodic peak observed at approximately +0.15 V versus the (SCE), indicative of the facile one-electron transfer to the semiquinone stage. In alkaline environments, oxidation of pyrogallol initially produces galloquinone (the ortho-quinone derivative) through dehydrogenation of the vicinal trihydroxy groups, which then rapidly polymerizes via nucleophilic addition and further redox processes, resulting in complex polyphenolic structures responsible for the characteristic browning observed in solutions.

Other reactions

Pyrogallol undergoes esterification with acetic anhydride in the presence of a base such as pyridine to form triacetylpyrogallol, a common derivative used for protection of its hydroxyl groups. This reaction proceeds via nucleophilic acyl substitution at each of the three phenolic hydroxyls, yielding the triester and acetic acid as a byproduct. The balanced equation for this transformation is: \ceC6H3(OH)3+3(CH3CO)2O−>C6H3(OCOCH3)3+3CH3COOH\ce{C6H3(OH)3 + 3 (CH3CO)2O -> C6H3(OCOCH3)3 + 3 CH3COOH} Alkylation of pyrogallol occurs through reaction with alkyl halides under basic conditions, following the mechanism, where the deprotonated phenoxide ions act as nucleophiles to form alkyl aryl ethers. This method is particularly useful for selective protection of hydroxyl groups, as demonstrated by the preparation of pyrogallol monomethyl ether using methyl iodide or in alkaline media. Pyrogallol forms coordination complexes with metal ions through its hydroxyl groups, notably chelating iron(III) to produce intensely colored species suitable for qualitative analysis. These complexes arise from the binding of the trihydroxybenzene moiety to the metal center, resulting in violet to blue hues depending on the and .

Applications

Photography

Pyrogallol serves as a in alkaline developers for films, where it converts exposed (AgBr) to metallic silver (Ag) through a reaction. Pyrogallol reduces exposed silver ions to metallic silver while undergoing oxidation itself. This process selectively amplifies the formed by light exposure on the , producing visible metallic silver grains. Introduced to in the mid-19th century, pyrogallol was first noted for its rapid silver deposition from silver salts in 1832 and formally adopted as an organic developer by Henri-Victor Regnault in 1851. It played a pivotal role in early processes, including Frederick Scott Archer's wet method starting in 1851, where it was used to develop exposed plates immediately after . Pyrogallol also contributed to the transition to early dry plates in the 1870s and 1880s, enabling more reliable in professional and astronomical , such as in the Cape Photographic Durchmusterung project. Its advantages include high developing activity, which allows for quick , and the production of fine grain structures with enhanced edge sharpness due to tanning effects from its oxidation products. However, pyrogallol's instability in alkaline solutions leads to rapid oxidation, necessitating fresh preparation of working solutions to avoid fogging or uneven development. By the late , it began to be supplanted by more stable alternatives like , discovered in 1880, which offered greater reliability and reduced preparation demands. Despite this decline, pyrogallol persists in contemporary alternative processes, such as kallitype printing, where its properties enhance tonal depth in iron-silver emulsions.

Analytical and industrial uses

Pyrogallol is employed as an for the determination of oxygen in gases, where its alkaline solution absorbs oxygen through rapid autooxidation, resulting in a characteristic color change from colorless to dark brown due to the formation of oxidation products. This property enables its use in gas for qualitative detection and quantitative measurement by monitoring oxygen uptake or color intensity. Additionally, pyrogallol functions as a reductant in titrations. In hair dyes and , pyrogallol undergoes oxidation to form colored polymers that contribute to tinting and coloring effects. Its use in hair dyes has been restricted or prohibited in many regions due to concerns. It is banned in the (since 1976) and not reported in use in recent U.S. assessments (as of 2024). Where permitted, such as in , concentrations are limited to 5% (at 5) with warnings; typical levels are below 1%, and in Korea to less than 2% (as of 2022). Pyrogallol finds industrial applications as an , particularly in inks where it reacts with iron salts to produce deep blue-black colors, forming the basis for traditional writing inks. It also serves as an in processes to prevent by removing dissolved oxygen. In biochemical research, pyrogallol is used to generate radicals through , facilitating studies on and the activity of enzymes like .

Safety and toxicology

Health hazards

Pyrogallol is acutely toxic upon , with an oral LD50 of approximately 800 mg/kg in rats. Acute exposure can cause gastrointestinal symptoms such as and , as well as headache and . Dermal contact leads to rapid absorption, resulting in and , while inhalation irritates the respiratory tract. Additionally, pyrogallol exposure is associated with , which manifests as and breathing difficulties due to impaired oxygen transport in the blood. Chronic exposure to pyrogallol may induce sensitization, leading to allergic reactions upon repeated contact. It causes serious eye irritation, potentially resulting in redness and discomfort. Pyrogallol is suspected of causing genetic defects based on and . Animal studies provide equivocal evidence of carcinogenicity, with no evidence in rats but increased incidence of tumors in some dermal studies in mice. Its tendency to auto-oxidize, as noted in chemical reactivity profiles, contributes to health risks through the generation of . The primary mechanism of pyrogallol's toxicity involves auto-oxidation, which produces that damage cells and tissues. In high doses, this can lead to through red blood cell . Exposure occurs mainly via ingestion, which affects the gastrointestinal and systemic systems; dermal absorption, which is efficient and causes local and systemic effects; and inhalation, which primarily irritates the .

Regulatory status

In the , pyrogallol is classified under the Classification, Labelling and Packaging ( as causing serious eye irritation (Eye Irrit. 2), skin irritation (Skin Irrit. 2), and may cause an allergic skin reaction (Skin Sens. 1). It is also classified as toxic to aquatic life with long-lasting effects (Aquatic Chronic 2). Its use in is prohibited under Annex II of (EC) No 1223/2009, with the ban effective since 1992. In the United States, pyrogallol is not assigned a (PEL) by the (OSHA), indicating it is not specifically listed as a hazardous substance under that standard. It is, however, listed on the Toxic Substances Control Act (TSCA) Inventory as an active substance. Recommended (PPE) for handling includes chemical-resistant gloves and safety goggles to prevent skin and eye contact. For safe handling, pyrogallol should be stored in a tightly closed container under an inert atmosphere, such as , in a cool, dry place to prevent oxidation and discoloration upon exposure to air. Disposal must follow U.S. Environmental Protection Agency (EPA) guidelines for , with generators required to assess and classify discarded material accordingly. Internationally, the use of pyrogallol in hair dyes has declined since the 1970s following U.S. (FDA) warnings on hair dyes, which require caution statements and patch testing for products containing such ingredients to mitigate potential risks. It remains exempt from color additive certification for specific uses like suture coloring when combined with ferric ammonium citrate, provided it meets safety criteria.

References

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