Catechol ( or ), also known as pyrocatechol or 1,2-dihydroxybenzene, is a toxic organic compound with the molecular formula . It is the ''ortho''
isomer of the three isomeric
benzenediol
In organic chemistry, dihydroxybenzenes (benzenediols) are organic compounds in which two hydroxyl groups () are substituted onto a benzene ring (). These aromatic compounds are classed as phenols. There are three structural isomers: 1,2-dihydrox ...
s. This colorless compound occurs naturally in trace amounts. It was first discovered by
destructive distillation
Destruction may refer to:
Concepts
* Destruktion, a term from the philosophy of Martin Heidegger
* Destructive narcissism, a pathological form of narcissism
* Self-destructive behaviour, a widely used phrase that ''conceptualises'' certain kind ...
of the plant extract
catechin
Catechin is a flavan-3-ol, a type of secondary metabolite providing antioxidant roles in plants. It belongs to the subgroup of polyphenols called flavonoids.
The name of the catechin chemical family derives from ''catechu'', which is the tanni ...
. About 20,000 tonnes of catechol are now synthetically produced annually as a commodity organic chemical, mainly as a precursor to pesticides, flavors, and fragrances.
Catechol occurs as feathery white crystals that are very rapidly soluble in water.
Isolation and synthesis
Catechol was first isolated in 1839 by Edgar Hugo Emil Reinsch (1809–1884) by
distilling
Distillation, or classical distillation, is the process of separating the components or substances from a liquid mixture by using selective boiling and condensation, usually inside an apparatus known as a still. Dry distillation is the heating ...
it from the solid
tannic preparation
catechin
Catechin is a flavan-3-ol, a type of secondary metabolite providing antioxidant roles in plants. It belongs to the subgroup of polyphenols called flavonoids.
The name of the catechin chemical family derives from ''catechu'', which is the tanni ...
, which is the residuum of
catechu
( or ) is an extract of acacia trees used variously as a food additive, astringent, tannin, and dye. It is extracted from several species of '' Acacia'', but especially '' Senegalia catechu'' (''Acacia catechu''), by boiling the wood in w ...
, the boiled or concentrated juice of ''Mimosa catechu'' (''
Acacia catechu
''Senegalia catechu'' is a deciduous, thorny tree which grows up to in height. The plant is called ''khair''
in H ...
''). Upon heating catechin above its decomposition point, a substance that Reinsch first named ''Brenz-Katechusäure'' (burned catechu acid)
sublimated as a white
efflorescence
In chemistry, efflorescence (which means "to flower out" in French) is the migration of a salt to the surface of a porous material, where it forms a coating. The essential process involves the dissolving of an internally held salt in water, or ...
. This was a thermal decomposition product of the
flavanols
Flavan-3-ols (sometimes referred to as flavanols) are a subgroup of flavonoids. They are derivatives of flavans that possess a 2-phenyl-3,4-dihydro-2''H''-chromen-3-ol skeleton. Flavan-3-ols are structurally diverse and include a range of comp ...
in catechin. In 1841, both
Wackenroder
Wilhelm Heinrich Wackenroder (13 July 1773 – 13 February 1798) was a German jurist and writer. With Ludwig Tieck and the Schlegel brothers, he has co-founded the German Romanticism.
Life
Wackenroder was born in Berlin. He was a close friend o ...
and Zwenger independently rediscovered catechol; in reporting on their findings, ''
Philosophical Magazine
The ''Philosophical Magazine'' is one of the oldest scientific journals published in English. It was established by Alexander Tilloch in 1798;John Burnett"Tilloch, Alexander (1759–1825)" Oxford Dictionary of National Biography, Oxford Univer ...
'' coined the name ''pyrocatechin''. By 1852,
Erdmann realized that catechol was
benzene
Benzene is an organic chemical compound with the molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar ring with one hydrogen atom attached to each. Because it contains only carbon and hydrogen atoms ...
with two oxygen atoms added to it; in 1867,
August Kekulé
Friedrich August Kekulé, later Friedrich August Kekule von Stradonitz ( , ; 7 September 1829 – 13 July 1896), was a German organic chemist. From the 1850s until his death, Kekulé was one of the most prominent chemists in Europe, especially ...
realized that catechol was a
diol of benzene, so by 1868, catechol was listed as ''pyrocatechol''. In 1879, the ''
Journal of the Chemical Society
The ''Journal of the Chemical Society'' was a scientific journal established by the Chemical Society in 1849 as the ''Quarterly Journal of the Chemical Society''. The first editor was Edmund Ronalds. The journal underwent several renamings, spli ...
'' recommended that catechol be called "catechol", and in the following year, it was listed as such.
Catechol has since been shown to occur in free form naturally in
kino
Kino may refer to:
Arts, entertainment and media Broadcasters
* KINO, a radio station in Arizona, U.S.
* Kino FM (98.0 FM – Moscow), a Russian music radio station
* KinoTV, now Ruutu+ Leffat ja Sarjat, a Finnish TV channel
Fictional entiti ...
and in
beechwood tar. Its
sulfonic acid has been detected in the
urine
Urine is a liquid by-product of metabolism in humans and in many other animals. Urine flows from the kidneys through the ureters to the urinary bladder. Urination results in urine being excreted from the body through the urethra.
Cellular ...
of horses and humans.
Catechol is produced industrially by the
hydroxylation of
phenol
Phenol (also called carbolic acid) is an aromatic organic compound with the molecular formula . It is a white crystalline solid that is volatile. The molecule consists of a phenyl group () bonded to a hydroxy group (). Mildly acidic, it ...
using
hydrogen peroxide
Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscous than water. It is used as an oxidizer, bleaching agent, and antiseptic, usually as a dilute solution (3%â ...
.
[Fiegel, Helmut ''et al.'' (2002) "Phenol Derivatives" in ''Ullmann's Encyclopedia of Industrial Chemistry'', Wiley-VCH: Weinheim. .]
:
C6H5OH + H2O2 -> C6H4(OH)2 + H2O
It can be produced by reaction of
salicylaldehyde
Salicylic aldehyde (2-hydroxybenzaldehyde) is the organic compound with the formula (C7 H6 O2) C6H4CHO-2-OH. Along with 3-hydroxybenzaldehyde and 4-hydroxybenzaldehyde, it is one of the three isomers of hydroxybenzaldehyde. This colorless oily ...
with base and hydrogen peroxide (
Dakin oxidation
The Dakin oxidation (or Dakin reaction) is an organic chemistry, organic redox, redox reaction in which an ''arene substitution patterns, ortho''- or ''arene substitution patterns, para''-hydroxylated phenyl group, phenyl aldehyde (2-hydroxyben ...
),
as well as the
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 ...
of 2-substituted phenols, especially
2-chlorophenol, with hot aqueous solutions containing alkali metal hydroxides. Its methyl ether derivative,
guaiacol
Guaiacol () is an organic compound with the formula C6H4(OH)(OCH3). It is a phenolic compound containing an methoxy functional group. Guaiacol appears as a viscous colorless oil, although aged or impure samples are often yellowish. It occurs wid ...
, converts to catechol via hydrolysis of the bond as promoted by
hydroiodic acid
Hydroiodic acid (or hydriodic acid) is an aqueous solution of hydrogen iodide (HI). It is a strong acid, one that is ionized completely in an aqueous solution. It is colorless. Concentrated solutions are usually 48% to 57% HI.
Reactions
Hy ...
(HI).
Reactions
Organic chemistry
Like other difunctional benzene derivatives, catechol readily
condenses
Condensation is the change of the state of matter from the gas phase into the liquid phase, and is the reverse of vaporization. The word most often refers to the water cycle. It can also be defined as the change in the state of water vapor to ...
to form
heterocyclic compounds
A heterocyclic compound or ring structure is a cyclic compound that has atoms of at least two different elements as members of its ring(s). Heterocyclic chemistry is the branch of organic chemistry dealing with the synthesis, properties, and ...
. Cyclic
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 are formed upon treatment with dichloro
electrophile
In chemistry, an electrophile is a chemical species that forms bonds with nucleophiles by accepting an electron pair. Because electrophiles accept electrons, they are Lewis acids. Most electrophiles are positively charged, have an atom that carrie ...
s. For example, using
phosphorus trichloride
Phosphorus trichloride is an inorganic compound with the chemical formula PCl3. A colorless liquid when pure, it is an important industrial chemical, being used for the manufacture of phosphites and other organophosphorus compounds. It is toxi ...
or
phosphorus oxychloride
Phosphoryl chloride (commonly called phosphorus oxychloride) is a colourless liquid with the formula . It hydrolyses in moist air releasing phosphoric acid and fumes of hydrogen chloride. It is manufactured industrially on a large scale from pho ...
gives the cyclic chloro
phosphonite
In organic chemistry, phosphonites are organophosphorus compounds with the formula P(OR)2R. They are found in some pesticides and are used as ligands.
Preparation
Although they are derivatives of phosphonous acid (RP(OH)2), they are not prepar ...
or chloro
phosphonate
In organic chemistry, phosphonates or phosphonic acids are organophosphorus compounds containing groups (where R = alkyl, aryl, or just hydrogen). Phosphonic acids, typically handled as salts, are generally nonvolatile solids that are poorly ...
, respectively;
sulfuryl chloride
Sulfuryl chloride is an inorganic compound with the formula SO2Cl2. At room temperature, it is a colorless liquid with a pungent odor. Sulfuryl chloride is not found in nature, as can be inferred from its rapid hydrolysis.
Sulfuryl chloride is ...
gives the
sulfate
The sulfate or sulphate ion is a polyatomic anion with the empirical formula . Salts, acid derivatives, and peroxides of sulfate are widely used in industry. Sulfates occur widely in everyday life. Sulfates are salts of sulfuric acid and many ...
; and
phosgene () gives the
carbonate
A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the carbonate ...
:
:
C6H4(OH)2 + XCl2 -> C6H4(O2X) + 2 HCl where X = PCl or POCl; ; CO
With metal ions
Basic solutions of catechol react with iron(III) to give the red .
Ferric 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 ...
gives a green coloration with the aqueous solution, while the alkaline solution rapidly changes to a green and finally to a black color on exposure to the air. Iron-containing
dioxygenase
Dioxygenases are oxidoreductase enzymes. Aerobic life, from simple single-celled bacteria species to complex eukaryotic organisms, has evolved to depend on the oxidizing power of dioxygen in various metabolic pathways. From energetic adenosine tri ...
enzyme
Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...
s
catalyze
Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
the
cleavage of catechol.
Redox chemistry
Catechols convert to the semiquinone radical. At , this conversion occurs at 100 mV:
:
C6H4(OH)2 -> C6H4(O)(OH) + 1/2 H2
For the redox of the semiquinone radical to the catecholate dianion, the potential ranges from 530 to 43 mV as the pH varies from 7 to 13.5:
:
C6H4(OH)2 -> C6H4O2^2- + H+
Catechol is produced by a reversible two-electron, two-proton
reduction of
1,2-benzoquinone
1,2-Benzoquinone, also called ''ortho''-benzoquinone, is an organic compound with formula . It is one of the two isomers of quinone, the other being 1,4-benzoquinone. It is a red volatile solid that is soluble in water and ethyl ether. It i ...
( vs
SHE; vs SHE).
The
redox
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...
series catecholate dianion, monoanionic semiquinonate, and benzoquinone are collectively called dioxolenes. Dioxolenes can function as
ligand
In coordination chemistry, a ligand is an ion or molecule ( functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's elec ...
s for metal ions.
Natural occurrences
Small amounts of catechol occur naturally in
fruits and
vegetables
Vegetables are parts of plants that are consumed by humans or other animals as food. The original meaning is still commonly used and is applied to plants collectively to refer to all edible plant matter, including the flowers, fruits, stems ...
, along with the enzyme
polyphenol oxidase
Polyphenol oxidase (PPO; also polyphenol oxidase i, chloroplastic), an enzyme involved in fruit browning, is a tetramer that contains four atoms of copper per molecule.
PPO may accept monophenols and/or ''o''-diphenols as substrates. The ...
(also known as
catecholase, or
catechol oxidase
Catechol oxidase is a copper oxidase that contains a type 3 di-copper cofactor and catalyzes the oxidation of ortho-diphenols into ortho-quinones coupled with the reducing agent, reduction of molecular oxygen to water. It is present in a variet ...
). Upon mixing the enzyme with the substrate and exposure to oxygen (as when a
potato
The potato is a starchy food, a tuber of the plant ''Solanum tuberosum'' and is a root vegetable native to the Americas. The plant is a perennial in the nightshade family Solanaceae.
Wild potato species can be found from the southern Unit ...
or
apple
An apple is an edible fruit produced by an apple tree (''Malus domestica''). Apple trees are cultivated worldwide and are the most widely grown species in the genus ''Malus''. The tree originated in Central Asia, where its wild ancestor, ' ...
is cut and left out), the colorless catechol
oxidizes to reddish-brown
melanoid pigments, derivatives of
benzoquinone Benzoquinone (C6H4O2) is a quinone with a single benzene ring. There are 2 (out of 3 hypothetical) benzoquinones:
* 1,4-Benzoquinone, most commonly, right image (also ''para''-benzoquinone, ''p''-benzoquinone, ''para''-quinone, or just quinone)
* 1 ...
. The enzyme is inactivated by adding an
acid, such as
citric acid
Citric acid is an organic compound with the chemical formula HOC(CO2H)(CH2CO2H)2. It is a colorless weak organic acid. It occurs naturally in citrus fruits. In biochemistry, it is an intermediate in the citric acid cycle, which occurs in ...
contained in
lemon juice
The lemon (''Citrus limon'') is a species of small evergreen trees in the flowering plant family Rutaceae, native to Asia, primarily Northeast India (Assam), Northern Myanmar or China.
The tree's ellipsoidal yellow fruit is used for culina ...
. Excluding oxygen also prevents the
browning reaction. However, the activity of the enzyme increases in cooler temperatures. Benzoquinone is said to be
antimicrobial
An antimicrobial is an agent that kills microorganisms or stops their growth. Antimicrobial medicines can be grouped according to the microorganisms they act primarily against. For example, antibiotics are used against bacteria, and antifungals ar ...
, a property that slows the
spoilage of damaged fruits and other plant parts.
Catechol derivatives
DHSA.svg, 3,4-dihydroxy-9,10-secoandrosta-1,3,5(10)-triene-9,17-dione, a metabolite of cholesterol
Cholesterol is any of a class of certain organic molecules called lipids. It is a sterol (or modified steroid), a type of lipid. Cholesterol is biosynthesized by all animal cells and is an essential structural component of animal cell mem ...
[; ]
Catechin structure.svg, Catechin
Catechin is a flavan-3-ol, a type of secondary metabolite providing antioxidant roles in plants. It belongs to the subgroup of polyphenols called flavonoids.
The name of the catechin chemical family derives from ''catechu'', which is the tanni ...
, a component of tea.
Piceatannol.svg, Piceatannol
Piceatannol is the organic compound with the formula . Classified as a stilbenoid and a phenol, it is a white solid, although samples often are yellow owing to impurities.
Natural occurrences
Piceatannol and its glucoside, astringin, are found i ...
, an antioxidant found in some red wines.
Urushiol.svg, urushiol
Urushiol is an oily mixture of organic compounds with allergenic properties found in plants of the family Anacardiaceae, especially '' Toxicodendron'' ''spp.'' (e.g., poison oak, Chinese lacquer tree, poison ivy, poison sumac), ''Comoclad ...
s, the active agent in poison ivy (R = (CH2)14CH3, (CH2)7CH=CHCH2CH=CHCH2CH=CH2, and others)
catecholamine.svg, catecholamine
A catecholamine (; abbreviated CA) is a monoamine neurotransmitter, an organic compound that has a catechol (benzene with two hydroxyl side groups next to each other) and a side-chain amine.
Catechol can be either a free molecule or a su ...
s, drugs imitating them (such as MDMA
3,4-Methylenedioxymethamphetamine (MDMA), commonly seen in tablet form (ecstasy) and crystal form (molly or mandy), is a potent empathogen–entactogen with stimulant properties primarily used for recreational purposes. The desire ...
), hormone
A hormone (from the Greek participle , "setting in motion") is a class of signaling molecules in multicellular organisms that are sent to distant organs by complex biological processes to regulate physiology and behavior. Hormones are require ...
s/ neurotransmitters
Dopamine.svg, Dopamine, derived from the amino acid tyrosine, an adhesive used by mussels.
Quercetin.svg, Quercetin
Quercetin is a plant flavonol from the flavonoid group of polyphenols. It is found in many fruits, vegetables, leaves, seeds, and grains; capers, red onions, and kale are common foods containing appreciable amounts of it. It has a bitter flavor ...
, which is found in many foods.
Catechol derivatives are found widely in nature. They often arise by hydroxylation of phenols.
Arthropod
Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chiti ...
cuticle consists of
chitin linked by a catechol
moiety to
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
. The cuticle may be strengthened by
Cross-link
In chemistry and biology a cross-link is a bond or a short sequence of bonds that links one polymer chain to another. These links may take the form of covalent bonds or ionic bonds and the polymers can be either synthetic polymers or natural ...
ing (
tanning
Tanning may refer to:
*Tanning (leather), treating animal skins to produce leather
*Sun tanning, using the sun to darken pale skin
**Indoor tanning, the use of artificial light in place of the sun
**Sunless tanning, application of a stain or dye t ...
and
sclerotization Sclerotization is a biochemical process that produces the rigid shell of sclerotin that comprises an insect's chitinous exoskeleton. It is prominent in the thicker, armored parts of insects and arachnids, especially in the biting mouthparts and s ...
), in particular, in
insect
Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body ( head, thorax and abdomen), three ...
s, and of course by
biomineralization
Biomineralization, also written biomineralisation, is the process by which living organisms produce minerals, often to harden or stiffen existing tissues. Such tissues are called mineralized tissues. It is an extremely widespread phenomenon; ...
.
4-tert-Butylcatechol
4-''tert''-Butylcatechol (TBC) is an organic chemical compound which is a derivative of catechol. TBC is available in the form of a solid crystal flake and 85% solution in methanol or water.
Uses
It is added as a stabilizer and polymerisati ...
, which is synthetic, not natural, is used as an
antioxidant and
polymerisation inhibitor Polymerisation inhibitors (US: polymerization inhibitors) are chemical compounds added to monomers to prevent their auto-polymerisation. Unsaturated monomers such as acrylates, vinyl chloride, butadiene and styrene require inhibitors for both proces ...
.
Uses
Approximately 50% of the synthetic catechol is consumed in the production of
pesticides, the remainder being used as a precursor to fine chemicals such as perfumes and pharmaceuticals.
[ It is a common building block in organic synthesis. Several industrially significant flavors and fragrances are prepared starting from catechol. ]Guaiacol
Guaiacol () is an organic compound with the formula C6H4(OH)(OCH3). It is a phenolic compound containing an methoxy functional group. Guaiacol appears as a viscous colorless oil, although aged or impure samples are often yellowish. It occurs wid ...
is prepared by methylation of catechol and is then converted to vanillin
Vanillin is an organic compound with the molecular formula . It is a phenolic aldehyde. Its functional groups include aldehyde, hydroxyl, and ether. It is the primary component of the extract of the vanilla bean. Synthetic vanillin is now u ...
on a scale of about 10M kg per year (1990). The related monoethyl ether of catechol, guethol, is converted to ethylvanillin, a component of chocolate
Chocolate is a food made from roasted and ground cacao seed kernels that is available as a liquid, solid, or paste, either on its own or as a flavoring agent in other foods. Cacao has been consumed in some form since at least the Olmec civ ...
confectioneries. 3-''trans''-Isocamphylcyclohexanol, widely used as a replacement for sandalwood oil
Sandalwood oil is an essential oil obtained from the steam distillation of chips and billets cut from the heartwood of various species of sandalwood trees, mainly ''Santalum album'' (Indian sandalwood) and ''Santalum spicatum'' (Australian sandal ...
, is prepared from catechol via guaiacol and camphor. Piperonal
Piperonal, also known as heliotropin, is an organic compound which is commonly found in fragrances and flavors.Karl-Georg Fahlbusch, Franz-Josef Hammerschmidt, Johannes Panten, Wilhelm Pickenhagen, Dietmar Schatkowski, Kurt Bauer, Dorothea Garbe a ...
, a flowery scent, is prepared from the methylene diether of catechol followed by condensation with glyoxal
Glyoxal is an organic compound with the chemical formula OCHCHO. It is the smallest dialdehyde (a compound with two aldehyde groups). It is a crystalline solid, white at low temperatures and yellow near the melting point (15 °C). The liquid ...
and decarboxylation.
Catechol is used as a black-and-white photographic developer
In the processing of photographic films, plates or papers, the photographic developer (or just developer) is one or more chemicals that convert the latent image to a visible image. Developing agents achieve this conversion by reducing the silve ...
, but, except for some special purpose applications, its use is largely historical. It is rumored to have been used briefly in Eastman Kodak's HC-110 developer and is rumored to be a component in Tetenal's Neofin Blau developer. It is a key component of Finol from Moersch Photochemie in Germany. Modern catechol developing was pioneered by noted photographer Sandy King. His "PyroCat" formulation is popular among modern black-and-white film photographers. King's work has since inspired further 21st-century development by others such as Jay De Fehr with Hypercat and Obsidian Acqua developers, and others.
Nomenclature
Although rarely encountered, the officially "preferred IUPAC
The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...
name" (PIN) of catechol is ''benzene-1,2-diol''. The trivial name ''pyrocatechol'' is a retained IUPAC name, according to the ''1993 Recommendations for the Nomenclature of Organic Chemistry''.
See also
* Enol
In organic chemistry, alkenols (shortened to enols) are a type of reactive structure or intermediate in organic chemistry that is represented as an alkene ( olefin) with a hydroxyl group attached to one end of the alkene double bond (). The t ...
* 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 ...
* Thiotimoline
Thiotimoline is a fictitious chemical compound conceived by American biochemist and science fiction author Isaac Asimov. It was first described in a spoof scientific paper titled "The Endochronic Properties of Resublimated Thiotimoline" in 1 ...
References
External links
International Chemical Safety Card 0411
IUPAC Nomenclature of Organic Chemistry
(online version of the "''Blue Book''")
{{Authority control
Antioxidants
Chelating agents
Enediols
IARC Group 2B carcinogens
Photographic chemicals
Reducing agents