A xanthate is a

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 ...

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 ...

of a xanthic acid. The formula of the salt of xanthic acid is (where R is

organyl In organic and organometallic chemistry, an organyl group (commonly denoted by the letter " R") is an organic substituent with one (sometimes more) free valence electron(s) at a carbon atom.. The term is often used in chemical patent literatur ...

group and M is usually Na or K). Xanthate also refers to the

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 conven ...

. The formula of a xanthic acid is , such as ethyl xanthic acid, while the formula of a xanthate ester is , where R and R' are organyl groups. The salts of xanthates are sometimes called ''O''-organyl dithioates. The esters of xanthic acid are sometimes called ''O'',''S''-diorganyl esters of dithiocarbonic acid. The name ''xanthate'' is derived from

Ancient Greek Ancient Greek (, ; ) includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Greek ...

(''xanthos'') meaning 'yellowish' or 'golden', and indeed most xanthate salts are yellow. They were discovered and named in 1823 by Danish chemist

William Christopher Zeise William Christopher Zeise (15 October 1789 – 12 November 1847) was a Danish organic chemist. He is best known for synthesising one of the first organometallic compounds, named Zeise's salt in his honour. He also performed pioneering studie ...

. These

organosulfur compound Organosulfur chemistry is the study of the properties and synthesis of organosulfur compounds, which are organic compounds that contain sulfur. They are often associated with foul odors, but many of the sweetest compounds known are organosulfur der ...

s are important in two areas: the production of

cellophane Cellophane is a thin, transparent sheet made of regenerated cellulose. Its low permeability to air, oils, greases, bacteria, and liquid water makes it useful for food packaging. Cellophane is highly permeable to water vapour, but may be coate ...

and related polymers from

cellulose Cellulose is an organic compound with the chemical formula, formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of glycosidic bond, β(1→4) linked glucose, D-glucose units. Cellulose is an important s ...

and (in mining) for extraction of certain sulphide bearing ores. They are also versatile intermediates in

organic synthesis Organic synthesis is a branch of chemical synthesis concerned with the construction of organic compounds. Organic compounds are molecules consisting of combinations of covalently-linked hydrogen, carbon, oxygen, and nitrogen atoms. Within the gen ...

Formation and structure

Xanthate salts of

alkali metals The alkali metals consist of the chemical elements lithium (Li), sodium (Na), potassium (K),The symbols Na and K for sodium and potassium are derived from their Latin names, ''natrium'' and ''kalium''; these are still the origins of the names ...

are produced by the treatment of an

alcohol Alcohol may refer to: Common uses * Alcohol (chemistry), a class of compounds * Ethanol, one of several alcohols, commonly known as alcohol in everyday life ** Alcohol (drug), intoxicant found in alcoholic beverages ** Alcoholic beverage, an alco ...

alkali In chemistry, an alkali (; from the Arabic word , ) is a basic salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as a base that dissolves in water. A solution of a soluble base has a pH greater than 7.0. The a ...

, and

carbon disulfide Carbon disulfide (also spelled as carbon disulphide) is an inorganic compound with the chemical formula and structure . It is also considered as the anhydride of thiocarbonic acid. It is a colorless, flammable, neurotoxic liquid that is used as ...

. The process is called xanthation. In chemical terminology, the alkali reacts with the alcohol to produce an

alkoxide In chemistry, an alkoxide is the conjugate base of an alcohol and therefore consists of an organic group bonded to a negatively charged oxygen atom. They are written as , where R is the organyl substituent. Alkoxides are strong bases and, whe ...

, which is the

nucleophile In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are ...

that adds to the

electrophilic 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 carr ...

carbon atom in CS₂. Often the alkoxide is generated

in situ is a Latin phrase meaning 'in place' or 'on site', derived from ' ('in') and ' ( ablative of ''situs'', ). The term typically refers to the examination or occurrence of a process within its original context, without relocation. The term is use ...

by treating the alcohol with

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

potassium hydroxide Potassium hydroxide is an inorganic compound with the formula K OH, and is commonly called caustic potash. Along with sodium hydroxide (NaOH), KOH is a prototypical strong base. It has many industrial and niche applications, most of which utili ...

: :ROH + CS₂ + KOH → ROCS₂K + H₂O For example,

sodium ethoxide Sodium ethoxide, also referred to as sodium ethanolate, is the Ionic compound, ionic, organic compound with the formula , , or NaOEt (Et = ethyl group, ethyl). It is a white solid, although impure samples appear yellow or brown. It dissolves in p ...

gives

sodium ethyl xanthate Sodium ethyl xanthate (SEX) is an organosulfur compound with the chemical formula . It is a pale yellow powder, which is usually obtained as the dihydrate. Sodium ethyl xanthate is used in the mining industry as a Froth flotation, flotation agent. ...

. Many alcohols can be used in this reaction. Technical grade xanthate salts are usually of 90–95% purity. Impurities include

alkali metal The alkali metals consist of the chemical elements lithium (Li), sodium (Na), potassium (K),The symbols Na and K for sodium and potassium are derived from their Latin names, ''natrium'' and ''kalium''; these are still the origins of the names ...

sulfide Sulfide (also sulphide in British English) is an inorganic anion of sulfur with the chemical formula S2− or a compound containing one or more S2− ions. Solutions of sulfide salts are corrosive. ''Sulfide'' also refers to large families o ...

sulfates The sulfate or sulphate ion is a Polyatomic ion, 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 salt (chemistry), ...

trithiocarbonate Thiocarbonate describes a family of anions with the general chemical formula (''x'' = 0, 1, or 2): *for ''x'' = 2 it is monothiocarbonate ion *for ''x'' = 1 it is dithiocarbonate ion *for ''x'' = 0 it is trithiocarbonate ion Like the carbonate d ...

thiosulfate Thiosulfate ( IUPAC-recommended spelling; sometimes thiosulphate in British English) is an oxyanion of sulfur with the chemical formula . Thiosulfate also refers to the compounds containing this anion, which are the salts of thiosulfuric acid, ...

sulfites Sulfites or sulphites are chemical compound, compounds that contain the sulfite ion (IUPAC nomenclature of inorganic chemistry, systematic name: sulfate(IV) ion), . The sulfite ion is the conjugate base of bisulfite. Although its acid (sulfurous ...

, or carbonates as well as residual raw material such as alcohol and alkali hydroxide. These salts are available commercially as powder, granules, flakes, sticks, and solutions are available. Some commercially or otherwise useful xanthate salts include: *

CH₃CH₂OCS₂Na * potassium ethyl xanthate, CH₃CH₂OCS₂K * potassium isopropyl xanthate, (CH₃)₂CHOCS₂K * sodium isobutyl xanthate, (CH₃)₂CHCH₂OCS₂Na *

potassium amyl xanthate Potassium amyl xanthate (/pəˈtæsiəm ˌæmɪl ˈzænθeɪt/) is an organosulfur compound with the chemical formula CH3(CH2)4OCS2K. It is a pale yellow powder with a pungent odor that is soluble in water. It is widely used in the mining industr ...

, CH₃(CH₂)₄OCS₂K The OCS₂ core of xanthate salts and esters, like that of the

carbonate A carbonate is a salt of carbonic acid, (), 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 group ...

s and the

s has trigonal planar molecular geometry. The central

carbon Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...

atom is sp²-hybridized. The potassium salt of the amyl xanthate () has been characterized by

X-ray crystallography X-ray crystallography is the experimental science of determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to Diffraction, diffract in specific directions. By measuring th ...

. The COCS₂ portion of the anion is planar. The C-S bond lengths are both 1.65 Å, and the C-O distance is 1.38 Å.

Reactions

Acid-base properties

Xanthic acids, with the formula ROC(S)SH, can be prepared by treating alkali metal xanthates, e.g. potassium ethyl xanthate, with

hydrochloric acid Hydrochloric acid, also known as muriatic acid or spirits of salt, is an aqueous solution of hydrogen chloride (HCl). It is a colorless solution with a distinctive pungency, pungent smell. It is classified as a acid strength, strong acid. It is ...

at low temperatures. The methyl and ethyl xanthic acids are oils that are soluble in organic solvents. Benzyl xanthic acid is a solid. They have pKas near 2. These compounds thermally decompose in the presence of base to the alcohol and carbon disulfide. Xanthic acids characteristically decompose: :ROCS₂K + HCl → ROH + CS₂ + KCl This reaction is the reverse of the method for the preparation of the xanthate salts. The intermediate in the decomposition is the xanthic acid, ROC(S)SH, which can be isolated in certain cases.

Cleavage of C-O bonds

The C-O bond in xanthate esters can be cleaved in various ways, providing a means for deoxygenation of alcohols. In Barton–McCombie deoxygenation, tributyltin hydride is the source of H atom. Several variations of this deoxygenation are known, for example using AIBN and hydrosilanes. Xanthates are intermediates in the Chugaev elimination process. They can be used to control radical polymerisation under the

RAFT A raft is any flat structure for support or transportation over water. It is usually of basic design, characterized by the absence of a hull. Rafts are usually kept afloat by using any combination of buoyant materials such as wood, sealed barre ...

process, also termed MADIX (macromolecular design via interchange of xanthates).

Reactions with electrophiles

Xanthate anions undergo

alkylation Alkylation is a chemical reaction that entails transfer of an alkyl group. The alkyl group may be transferred as an alkyl carbocation, a free radical, a carbanion, or a carbene (or their equivalents). Alkylating agents are reagents for effecting al ...

to give xanthate esters, which are generally stable: :ROCS₂K + R′X → ROC(S)SR′ + KX They can be oxidized to dixanthogen disulfides: :2 ROCS₂Na + I₂ → ROC(S)S₂C(S)OR + 2 NaI Acylation of xanthates gives alkyl xanthogen esters (ROC(S)SC(O)R') and related anhydrides. Xanthates bind to transition metal cations as bidentate

ligand In coordination chemistry, a ligand is an ion or molecule with a 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 el ...

s. The charge-neutral complexes are soluble in organic solvents.

Industrial applications

Cellulose reacts with

(CS₂) in presence of

(NaOH) to produces sodium cellulose xanthate, which upon neutralization with

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, ...

(H₂SO₄) gives

viscose Rayon, also called viscose and commercialised in some countries as sabra silk or cactus silk, is a semi-synthetic fiber made from natural sources of regenerated cellulose, such as wood and related agricultural products. It has the same molecula ...

rayon or

paper ( Sellotape or Scotch Tape). Xanthate salts (e.g. sodium alkyl xanthates, dixanthogen) are widely used as flotation agents in mineral processing.

Related compounds

Rarely encountered, thioxanthates arise by the reaction of CS₂ with thiolate salts. For example, sodium ethylthioxanthate has the formula C₂H₅SCS₂Na.

Dithiocarbamate In organic chemistry, a dithiocarbamate is a chemical compound with the general formula . It contains the functional group with the Chemical structure, structure . It is the analog of a carbamate in which both oxygen atoms are replaced by sulfur ...

s are also related compounds. They arise from the reaction of a secondary amine with CS₂. For example, sodium diethyldithiocarbamate has the formula (C₂H₅)₂NCS₂Na.

Environmental impacts

While biodegradable, this class of chemicals may be toxic to life in water at concentrations of less than 1 mg/L. Water downstream of mining operations is often contaminated with xanthates.

References

{{Functional groups Salts Functional groups Thiocarbonyl compounds