Pyrithione is the common name of an

organosulfur compound Organosulfur compounds are organic compounds that contain sulfur. They are often associated with foul odors, but many of the sweetest compounds known are organosulfur derivatives, e.g., saccharin. Nature abounds with organosulfur compounds—sulf ...

with

molecular formula In chemistry, a chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, ...

, chosen as an abbreviation of pyridinethione, and found in the Persian shallot. It exists as a pair of

tautomer Tautomers () are structural isomers (constitutional isomers) of chemical compounds that readily interconvert. The chemical reaction interconverting the two is called tautomerization. This conversion commonly results from the relocation of a hy ...

s, the major form being the thione 1-hydroxy-2(1''H'')-pyridinethione and the minor form being the

thiol In organic chemistry, a thiol (; ), or thiol derivative, is any organosulfur compound of the form , where R represents an alkyl or other organic substituent. The functional group itself is referred to as either a thiol group or a sulfhydryl gro ...

2-mercaptopyridine ''N''-oxide; it crystallises in the thione form. It is usually prepared from either 2-bromopyridine, 2-chloropyridine, or 2-chloropyridine ''N''-oxide, and is commercially available as both the neutral compound and its sodium

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 ...

. It is used to prepare zinc pyrithione, which is used primarily to treat

dandruff Dandruff is a skin condition that mainly affects the scalp. Symptoms include flaking and sometimes mild itchiness. It can result in social or self-esteem problems. A more severe form of the condition, which includes inflammation of the skin ...

and seborrhoeic dermatitis in medicated shampoos, though is also an

anti-fouling agent Fouling is the accumulation of unwanted material on solid surfaces. The fouling materials can consist of either living organisms (biofouling) or a non-living substance (inorganic or organic). Fouling is usually distinguished from other surf ...

in paints.

Preparation

The preparation of pyrithione was first reported in 1950 by Shaw and was prepared by reaction of 2-chloropyridine ''N''-oxide with

sodium hydrosulfide Sodium hydrosulfide is the chemical compound with the formula NaHS. This compound is the product of the half-neutralization of hydrogen sulfide () with sodium hydroxide (NaOH). NaSH and sodium sulfide are used industrially, often for similar purpos ...

followed by acidification, or more recently with

sodium sulfide Sodium sulfide is a chemical compound with the formula Na2 S, or more commonly its hydrate Na2S·9 H2O. Both the anhydrous and the hydrated salts in pure crystalline form are colorless solids, although technical grades of sodium sulfide are gene ...

. 2-chloropyridine ''N''-oxide itself can be prepared from 2-chloropyridine using

peracetic acid Peracetic acid (also known as peroxyacetic acid, or PAA) is an organic compound with the formula CH3CO3H. This peroxy acid is a colorless liquid with a characteristic acrid odor reminiscent of acetic acid. It can be highly corrosive. Peracetic ...

. Another approach involves treating the same starting ''N''-oxide with

thiourea Thiourea () is an organosulfur compound with the formula and the structure . It is structurally similar to urea (), except that the oxygen atom is replaced by a sulfur atom (as implied by the ''thio-'' prefix); however, the properties of urea a ...

to afford pyridyl-2-isothiouronium chloride ''N''-oxide which undergoes base hydrolysis to pyrithione. 2-Bromopyridine can be oxidised to its ''N''-oxide using a suitable

peracid A peroxy acid (often spelled as one word, peroxyacid, and sometimes called peracid) is an acid which contains an acidic –OOH group. The two main classes are those derived from conventional mineral acids, especially sulfuric acid, and the peroxy ...

(as per 2-chloropyridine), both approaches being analogous to that reported in ''

Organic Syntheses ''Organic Syntheses'' is a peer-reviewed scientific journal that was established in 1921. It publishes detailed and checked procedures for the synthesis of organic compounds. A unique feature of the review process is that all of the data and exp ...

'' for the oxidation of

pyridine Pyridine is a basic heterocyclic organic compound with the chemical formula . It is structurally related to benzene, with one methine group replaced by a nitrogen atom. It is a highly flammable, weakly alkaline, water-miscible liquid w ...

to its ''N''-oxide. A

substitution reaction A substitution reaction (also known as single displacement reaction or single substitution reaction) is a chemical reaction during which one functional group in a chemical compound is replaced by another functional group. Substitution reactions ar ...

using either sodium dithionite () or sodium sulfide with

sodium hydroxide Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations and hydroxide anions . Sodium hydroxide is a highly caustic base and al ...

will allow the replacement of the bromo substituent with a

functional group In organic chemistry, a functional group is a substituent or moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions regardless of the r ...

The alternative strategy is to form the mercaptan before introducing the ''N''-oxide

moiety Moiety may refer to: Chemistry * Moiety (chemistry), a part or functional group of a molecule ** Moiety conservation, conservation of a subgroup in a chemical species Anthropology * Moiety (kinship), either of two groups into which a society is ...

. 2-Mercaptopyridine was originally synthesized in 1931 by heating 2-chloropyridine with calcium hydrosulfide, an approach similar that first used to prepare pyrithione. The analogous thiourea approach via a uronium salt was reported in 1958 and provides a more convenient route to 2-mercaptopyridine. Oxidation to the ''N''-oxide can then be undertaken. Dipyrithione

Pyrithione is found as a

natural product A natural product is a natural compound or substance produced by a living organism—that is, found in nature. In the broadest sense, natural products include any substance produced by life. Natural products can also be prepared by chemical sy ...

in the ''

Allium stipitatum ''Allium stipitatum'', Persian shallot, is an Asian species of Allium, onion native to central and southwestern Asia. Some sources regard ''Allium stipitatum'' and ''Allium hirtifolium, A. hirtifolium'' as the same species, while others treat ...

'' plant, an

Asia Asia (, ) is one of the world's most notable geographical regions, which is either considered a continent in its own right or a subcontinent of Eurasia, which shares the continental landmass of Afro-Eurasia with Africa. Asia covers an are ...

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriat ...

onion An onion (''Allium cepa'' L., from Latin ''cepa'' meaning "onion"), also known as the bulb onion or common onion, is a vegetable that is the most widely cultivated species of the genus '' Allium''. The shallot is a botanical variety of the on ...

, also known as the Persian shallot. Its presence was detected using positive ion

mass spectrometry Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a '' mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is u ...

using a

DART ion source In mass spectrometry, direct analysis in real time (DART) is an ion source that produces electronically or vibronically excited-state species from gases such as helium, argon, or nitrogen that ionize atmospheric molecules or dopant molecules. The ...

and the

disulfide In biochemistry, a disulfide (or disulphide in British English) refers to a functional group with the structure . The linkage is also called an SS-bond or sometimes a disulfide bridge and is usually derived by the coupling of two thiol groups. In ...

(2,2'-disulfanediylbis(pyridine)-1,1'-dioxide) has been reported from the same species. Dipyrithione can be prepared in a laboratory by oxidation of pyrithione with

chlorine Chlorine is a chemical element with the symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine i ...

in the presence of

: ::2 + + 2 NaOH → + 2 NaCl + 2 Dipyrithione is used as a

fungicide Fungicides are biocidal chemical compounds or biological organisms used to kill parasitic fungi or their spores. A fungistatic inhibits their growth. Fungi can cause serious damage in agriculture, resulting in critical losses of yield, quality ...

and

bactericide A bactericide or bacteriocide, sometimes abbreviated Bcidal, is a substance which kills bacteria. Bactericides are disinfectants, antiseptics, or antibiotics. However, material surfaces can also have bactericidal properties based solely on their ...

, and has been reported to possess novel cytotoxic activity by inducing

apoptosis Apoptosis (from grc, ἀπόπτωσις, apóptōsis, 'falling off') is a form of programmed cell death that occurs in multicellular organisms. Biochemical events lead to characteristic cell changes ( morphology) and death. These changes in ...

Properties

Pyrithione exists as a pair of prototropes, a form of

tautomerism Tautomers () are structural isomers (constitutional isomers) of chemical compounds that readily interconvert. The chemical reaction interconverting the two is called tautomerization. This conversion commonly results from the relocation of a hydr ...

whereby the rapid interconversion of

constitutional isomer In chemistry, a structural isomer (or constitutional isomer in the IUPAC nomenclature) of a compound is another compound whose molecule has the same number of atoms of each element, but with logically distinct bonds between them. The term meta ...

s involves the shift of a single proton, in this case between the sulfur and oxygen atoms (shown in the infobox above).

Salts In chemistry, a salt is a chemical compound consisting of an ionic assembly of positively charged cations and negatively charged anions, which results in a compound with no net electric charge. A common example is table salt, with positively c ...

of the

conjugate base A conjugate acid, within the Brønsted–Lowry acid–base theory, is a chemical compound formed when an acid donates a proton () to a base—in other words, it is a base with a hydrogen ion added to it, as in the reverse reaction it loses a ...

of pyrithione can also be considered to exhibit tautomerism by notionally associating the sodium ion with whichever heteroatom bears the negative charge of the anion (as opposed to the

formal charge In chemistry, a formal charge (F.C. or q), in the covalent view of chemical bonding, is the charge assigned to an atom in a molecule, assuming that electrons in all chemical bonds are shared equally between atoms, regardless of relative electro ...

s associated with the ''N''-oxide); however, considering the anion alone, this could also be described as an example of

resonance Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied periodic force (or a Fourier component of it) is equal or close to a natural frequency of the system on which it acts. When an oscil ...

. Pyrithione is a weak acid with p''K''_a values of −1.95 and +4.6 (thiol proton), but is a markedly stronger acid than either of its parent compounds (pyridine-''N''-oxide and pyridine-2-thiol), both of which have p''K''_a > 8. It is only slightly soluble in water (2.5 g L⁻¹) but is soluble in many organic solvents (including

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 ...

chloroform Chloroform, or trichloromethane, is an organic compound with formula C H Cl3 and a common organic solvent. It is a colorless, strong-smelling, dense liquid produced on a large scale as a precursor to PTFE. It is also a precursor to various ...

dichloromethane Dichloromethane (DCM or methylene chloride, methylene bichloride) is an organochlorine compound with the formula . This colorless, volatile liquid with a chloroform-like, sweet odour is widely used as a solvent. Although it is not miscible wit ...

dimethylformamide Dimethylformamide is an organic compound with the formula ( CH3)2NC(O)H. Commonly abbreviated as DMF (although this initialism is sometimes used for dimethylfuran, or dimethyl fumarate), this colourless liquid is miscible with water and the majo ...

dimethylsulfoxide Dimethyl sulfoxide (DMSO) is an organosulfur compound with the formula ( CH3)2. This colorless liquid is the sulfoxide most widely used commercially. It is an important polar aprotic solvent that dissolves both polar and nonpolar compounds and ...

, and

ethyl acetate Ethyl acetate ( systematically ethyl ethanoate, commonly abbreviated EtOAc, ETAC or EA) is the organic compound with the formula , simplified to . This colorless liquid has a characteristic sweet smell (similar to pear drops) and is used in glues ...

) and slight solubility in others (

diethyl ether Diethyl ether, or simply ether, is an organic compound in the ether class with the formula , sometimes abbreviated as (see Pseudoelement symbols). It is a colourless, highly volatile, sweet-smelling ("ethereal odour"), extremely flammable li ...

ethanol Ethanol (abbr. EtOH; also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound. It is an alcohol with the chemical formula . Its formula can be also written as or (an ethyl group linked to a ...

, methyl ''tert''-butyl ether, and

tetrahydrofuran Tetrahydrofuran (THF), or oxolane, is an organic compound with the formula (CH2)4O. The compound is classified as heterocyclic compound, specifically a cyclic ether. It is a colorless, water- miscible organic liquid with low viscosity. It is ...

). Pyrithione can be used as a source of

hydroxyl radical The hydroxyl radical is the diatomic molecule . The hydroxyl radical is very stable as a dilute gas, but it decays very rapidly in the condensed phase. It is pervasive in some situations. Most notably the hydroxyl radicals are produced from the ...

in organic synthesis as it photochemically decomposes to HO^• and (pyridin-2-yl)sulfanyl radical.

Applications

The

of pyrithione (pyrithionate ion) is an

anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...

containing two

donor atom A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as '' ligands'' or complexing agents. Man ...

s, a sulfur atom and an oxygen atom each bearing a negative

; the nitrogen atom remains formally positively charged. The thiolate anion can be formed by reaction with sodium carbonate, and zinc pyrithione is formed when

zinc chloride Zinc chloride is the name of inorganic chemical compounds with the formula ZnCl2 and its hydrates. Zinc chlorides, of which nine crystalline forms are known, are colorless or white, and are highly soluble in water. This salt is hygroscopic ...

is added. The anion can act as either a monodentate or bidentate

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 elect ...

and forms a 1:2

complex Complex commonly refers to: * Complexity, the behaviour of a system whose components interact in multiple ways so possible interactions are difficult to describe ** Complex system, a system composed of many components which may interact with each ...

with a zinc(II) metal centre. Zinc pyrithione has been used since the 1930s though its preparation was not disclosed until a 1955 British patent in which pyrithione was reacted directly with hydrated

zinc sulfate Zinc sulfate is an inorganic compound. It is used as a dietary supplement to treat zinc deficiency and to prevent the condition in those at high risk. Side effects of excess supplementation may include abdominal pain, vomiting, headache, and ti ...

in ethanol. In its

monomer In chemistry, a monomer ( ; '' mono-'', "one" + '' -mer'', "part") is a molecule that can react together with other monomer molecules to form a larger polymer chain or three-dimensional network in a process called polymerization. Classification ...

ic form, zinc pyrithione has two of the anions

chelate Chelation is a type of bonding of ions and molecules to metal ions. It involves the formation or presence of two or more separate coordinate bonds between a polydentate (multiple bonded) ligand and a single central metal atom. These ligands are ...

d to a zinc centre with a tetrahedral geometry. In the solid state, it forms a

dimer Dimer may refer to: * Dimer (chemistry), a chemical structure formed from two similar sub-units ** Protein dimer, a protein quaternary structure ** d-dimer * Dimer model, an item in statistical mechanics, based on ''domino tiling'' * Julius Dimer ( ...

in which each zinc centre adopts a trigonal bipyramidal geometry with two of the anions acting as

bridging ligand In coordination chemistry, a bridging ligand is a ligand that connects two or more atoms, usually metal ions. The ligand may be atomic or polyatomic. Virtually all complex organic compounds can serve as bridging ligands, so the term is usually ...

s coordinated through the oxygen atoms in the axial positions. In solution, the dimers dissociate via scission of zinc-oxygen bonds to each bridging ligand. Further dissociation of the monomer into its constituents can occur and is undesirable as the complex is more potent in medical applications; for this reason,

zinc carbonate Zinc carbonate is the inorganic compound with the formula ZnCO3. It is a white solid that is insoluble in water. It exists in nature as the mineral smithsonite. It is prepared by treating cold solutions of zinc sulfate with potassium bicarbonat ...

can be added to formulations as it inhibits the monomer dissociation. Zinc pyrithione has a long history of use in medicated shampoos to treat

and seborrhoeic dermatitis (dandruff can be considered a mild form of seborrheic dermatitis). It exhibits both

antifungal An antifungal medication, also known as an antimycotic medication, is a pharmaceutical fungicide or fungistatic used to treat and prevent mycosis such as athlete's foot, ringworm, candidiasis (thrush), serious systemic infections such as ...

and

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 ...

properties, inhibiting the '' Malassezia'' yeasts which promote these scalp conditions. The mechanisms by which this work are the subject of ongoing study. It can be used as an

antibacterial An antibiotic is a type of antimicrobial substance active against bacteria. It is the most important type of antibacterial agent for fighting bacterial infections, and antibiotic medications are widely used in the treatment and prevention ...

agent against ''

Staphylococcus ''Staphylococcus'' is a genus of Gram-positive bacteria in the family Staphylococcaceae from the order Bacillales. Under the microscope, they appear spherical ( cocci), and form in grape-like clusters. ''Staphylococcus'' species are faculta ...

'' and ''

Streptococcus ''Streptococcus'' is a genus of gram-positive ' (plural ) or spherical bacteria that belongs to the family Streptococcaceae, within the order Lactobacillales (lactic acid bacteria), in the phylum Bacillota. Cell division in streptococci occurs ...

'' infections for conditions such as athlete's foot, eczema, psoriasis, and ringworm. It is known to be

cytotoxic Cytotoxicity is the quality of being toxic to cells. Examples of toxic agents are an immune cell or some types of venom, e.g. from the puff adder (''Bitis arietans'') or brown recluse spider (''Loxosceles reclusa''). Cell physiology Treating c ...

against '' Pityrosporum ovale'', especially in combination with

ketoconazole Ketoconazole, sold under the brand name Nizoral among others, is an antiandrogen and antifungal medication used to treat a number of fungal infections. Applied to the skin it is used for fungal skin infections such as tinea, cutaneous ca ...

, which is the preferred formulation for seborrheic dermatitis. Pyrithione itself inhibits membrane transport processes in fungi. Paints used in external environments sometimes include zinc pyrithione as a preventive against algae and mildew.

References

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J. Am. Chem. Soc. The ''Journal of the American Chemical Society'' is a weekly peer-reviewed scientific journal that was established in 1879 by the American Chemical Society. The journal has absorbed two other publications in its history, the ''Journal of Analytical ...

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Academic Press Academic Press (AP) is an academic book publisher founded in 1941. It was acquired by Harcourt, Brace & World in 1969. Reed Elsevier bought Harcourt in 2000, and Academic Press is now an imprint of Elsevier. Academic Press publishes refer ...

, year = 1976, isbn = 9780120206513, url-access = registration, url = https://archive.org/details/tautomerismofhet0000unse {{cite book, chapter = The Introduction of Substituents into the Pyridine Ring, first1 = K., last1 = Thomas, first2 = D., last2 = Jerchel, chapter-url = https://books.google.com/books?id=P_kD8RC_BxMC&pg=PA90, title = Newer Methods of Preparative Organic Chemistry, volume = 3, editor1-first = Wilhelm, editor1-last = Foerst, publisher =

, year = 1964, isbn = 9780323146104, pages = 53–110, translator-first = Henry , translator-last = Birnbaum {{cite journal, doi = 10.1039/JR9580003610, title = 721. Tautomeric pyridines. Part I. Pyrid-2- and −4-thione, year = 1958, last1 = Jones, first1 = R. A., last2 = Katritzky, first2 = A. R., journal = J. Chem. Soc., pages = 3610–3613 {{cite book, title = Male Alopecia: Guide to Successful Management, first1 = Ralph M., last1 = Trüeb, first2 = Won-Soo, last2 = Lee, publisher =

Springer Science & Business Media Springer Science+Business Media, commonly known as Springer, is a German multinational publishing company of books, e-books and peer-reviewed journals in science, humanities, technical and medical (STM) publishing. Originally founded in 1842 ...

, year = 2014, isbn = 9783319032337, chapter = 6.2.5 – Dandruff, pages = 247–250, chapter-url = https://books.google.com/books?id=0ue5BAAAQBAJ&pg=PA249 {{cite journal, journal = Dermatol. Res. Pract., volume = 2014, pages = 1–11, year = 2014, doi = 10.1155/2014/709152, pmid = 25120566, pmc = 4120804, title = Zinc Therapy in Dermatology: A Review, first1 = Mrinal, last1 = Gupta, first2 = Vikram K., last2 = Mahajan, first3 = Karaninder S., last3 = Mehta, first4 = Pushpinder S., last4 = Chauhan, doi-access = free {{cite patent, country = US, number = 2809971, status = granted, title = Heavy-metal derivatives of 1-hydroxy-2-pyridinethiones and method of preparing same, pubdate = 1957-10-15, fdate = 1955-11-22, pridate = 1955-11-22, invent1 = Bernstein, Jack, invent2 = Losee, Kathryn A., assign1 = Olin Mathieson, url = https://www.google.com.au/patents/US2809971 {{cite patent, country = US, number = 4396766, status = granted, title = Process for producing sodium and zinc pyrithione, pubdate = 1983-08-02, fdate = 1982-04-28, pridate = 1982-04-28, invent1 = Farmer, David A., invent2 = Katz, Lawrence E., assign1 =

Olin Corporation Olin Corporation is an American manufacturer of ammunition, chlorine, and sodium hydroxide. The company traces its roots to two companies, both founded in 1892: Franklin W. Olin's Equitable Powder Company and the Mathieson Alkali Works. Olin che ...

, url = https://www.google.com/patents/US4396766 Amine oxides Pyridines Thiols