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150px, Sodium salt of ethyl xanthate Xanthate usually refers to a
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 quantiti ...
with the formula (R =
alkyl In organic chemistry, an alkyl group is an alkane missing one hydrogen. The term ''alkyl'' is intentionally unspecific to include many possible substitutions. An acyclic alkyl has the general formula of . A cycloalkyl is derived from a cycloa ...
; M+ = Na+, K+), thus they are the metal-thioate/''O''-
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. Glyceride ...
s of
dithiocarbonate 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 ...
. The name ''xanthates'' is derived from Ancient Greek ''xanthos'', meaning “yellowish, 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 studies ...
. These
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—sulfu ...
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 coat ...
and related polymers from
cellulose Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell w ...
and (in mining) for extraction of certain ores. They are also versatile intermediates in
organic synthesis Organic synthesis is a special branch of chemical synthesis and is concerned with the intentional construction of organic compounds. Organic molecules are often more complex than inorganic compounds, and their synthesis has developed into one o ...
. ''Xanthate'' can also refer to the ''O'',''S''-ester of xanthic acid. These esters have the structure ROC(=S)SR′.


Formation and structure

Xanthate salts are produced by the treatment of an alcohol, alkali, and
carbon disulfide Carbon disulfide (also spelled as carbon disulphide) is a neurotoxic, colorless, volatile liquid with the formula and structure . The compound is used frequently as a building block in organic chemistry as well as an industrial and chemical n ...
. The process is called xanthation. In chemical terminology, the alkali reacts with the alcohol to produce an alkoxide, which is the nucleophile that adds to the electrophilic carbon center in CS2. Often the alkoxide is generated in situ by treating the alcohol with sodium or
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 exploi ...
: :ROH + CS2 + KOH → ROCS2K + H2O For example,
sodium ethoxide Sodium ethoxide, also referred to as sodium ethylate, is the ionic, organic compound with the formula , or NaOEt (Et = ethane). It is a white solid, although impure samples appear yellow or brown. It dissolves in polar solvents such as ethanol ...
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 flotation agent. A closely rel ...
. Many alcohols can be used in this reaction. Technical grade xanthate salts are usually of 90–95% purity. Impurities include alkali-metal sulfides, sulfates, trithiocarbonates,
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, ...
s, sulfites, or carbonates as well as residual raw material such as alcohol and
alkali hydroxide The alkali hydroxides are a class of chemical compounds which are composed of an alkali metal cation and the hydroxide anion (OH−). The alkali hydroxides are: *Lithium hydroxide (LiOH) *Sodium hydroxide (NaOH) *Potassium hydroxide (KOH) *Rubidiu ...
. These salts are available commercially as powder, granules, flakes, sticks, and solutions are available. Some commercially or otherwise useful xanthate salts include: *
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 flotation agent. A closely rel ...
CH3CH2OCS2Na *
potassium ethyl xanthate Potassium ethyl xanthate (KEX) is an organosulfur compound with the chemical formula CH3CH2OCS2K. It is a pale yellow powder that is used in the mining industry for the separation of ores. Unlike the related sodium ethyl xanthate, the potassium s ...
, 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 industry ...
, CH3(CH2)4OCS2K The OCS2 core of xanthate salts, like that of the organic carbonates and the
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. Glyceride ...
s is characteristically planar. The central carbon is sp2-hybridized.


Reactions

The pKa of ethyl xanthic acid is 1.6. Xanthates characteristically decompose in acid: :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. Xanthate anions also undergo
alkylation Alkylation is the transfer of an alkyl group from one molecule to another. 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 The C-O bond in these compounds are susceptible to cleavage by the
Barton–McCombie deoxygenation The Barton–McCombie deoxygenation is an organic reaction in which a hydroxy functional group in an organic compound is replaced by a hydrogen to give an alkyl group. It is named after British chemists Sir Derek Harold Richard Barton and Stuart ...
, which provides a means for deoxygenation of alcohols. They can be oxidized to
dixanthogen disulfide : Structure of diethyl dixanthogen disulfide. Dixanthogen disulfides are a class of organosulfur compounds with the formula . Usually yellow solids, they are the product of the oxidation of xanthate salts. A common derivative is diethyl dixant ...
s: :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 ( 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 ele ...
s. The charge-neutral complexes are soluble in organic solvents. Xanthates are intermediates in the
Chugaev elimination The Chugaev elimination is a chemical reaction that involves the elimination of water from alcohols to produce alkenes. The intermediate is a xanthate. It is named for its discoverer, the Russian chemist Lev Aleksandrovich Chugaev (1873-1922), wh ...
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 barrel ...
process, also termed MADIX (macromolecular design via interchange of xanthates).


Industrial applications

Cellulose reacts with
carbon disulfide Carbon disulfide (also spelled as carbon disulphide) is a neurotoxic, colorless, volatile liquid with the formula and structure . The compound is used frequently as a building block in organic chemistry as well as an industrial and chemical n ...
(CS2) in presence of
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 a ...
(NaOH) to produces sodium cellulose xanthate, which upon neutralization with sulfuric acid (H2SO4) gives
viscose Rayon is a semi-synthetic fiber, made from natural sources of regenerated cellulose, such as wood and related agricultural products. It has the same molecular structure as cellulose. It is also called viscose. Many types and grades of viscose ...
rayon or
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 coat ...
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 functional group with the general formula and structure . It is the analog of a carbamate in which both oxygen atoms are replaced by sulfur atoms (when only 1 oxygen is replaced the result is th ...
s are also related compounds. They arise from the reaction of a secondary amine with CS2. For example,
sodium diethyldithiocarbamate Sodium diethyldithiocarbamate is the organosulfur compound with the formula NaS2CN(C2H5)2. It is a pale yellow, water soluble salt. Preparation This salt is obtained by treating carbon disulfide with diethylamine in the presence of sodium hydrox ...
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