Xanthate

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Sodium salt of ethyl xanthate (sodium ethylxanthate or sodium O-ethyl dithiocarbonate) Sodium-ethylxanthate-2D-skeletal.png
Sodium salt of ethyl xanthate (sodium ethylxanthate or sodium O-ethyl dithiocarbonate)
Structure of a xanthate ester Xanthate-ester-2D-general.png
Structure of a xanthate ester
Cellulose xanthate (orange) Musee de la viscose (Echirolles) abc29.JPG
Cellulose xanthate (orange)

A xanthate is a salt or ester of a xanthic acid. The formula of the salt of xanthic acid is [R−O−CS2]−M+ (where R is organyl group and M is usually Na or K). [1] Xanthate also refers to the anion [R−O−CS2]−. The formula of a xanthic acid is R−O−C(=S)−S−H, such as ethyl xanthic acid, while the formula of an ester of a xanthic acid is R−O−C(=S)−S−R', where R and R' are organyl groups. The salts of xanthates are also called O-organyl dithioates. The esters of xanthic acid are also called O,S-diorganyl esters of dithiocarbonic acid. The name xanthate is derived from Ancient Greek ξανθός (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. These organosulfur compounds are important in two areas: the production of cellophane and related polymers from cellulose and (in mining) for extraction of certain sulphide bearing ores. [2] They are also versatile intermediates in organic synthesis.

Contents

Formation and structure

Xanthate salts of alkali metals are produced by the treatment of an alcohol, alkali, and carbon disulfide. The process is called xanthation. [2] In chemical terminology, the alkali reacts with the alcohol to produce an alkoxide, which is the nucleophile that adds to the electrophilic carbon atom in CS2. [3] Often the alkoxide is generated in situ by treating the alcohol with sodium hydroxide or potassium hydroxide:

ROH + CS2 + KOH → ROCS2K + H2O

For example, sodium ethoxide gives sodium ethyl xanthate. 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, thiosulfates, sulfites, 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:

The OCS2 core of xanthate salts, like that of the carbonates and the esters has trigonal planar molecular geometry. The central carbon atom is sp2-hybridized.

Reactions

Acid-base properties

Xanthatic acids, with the formula ROC(S)SH, can be prepared by treating alkali metal xanthates, e.g. potassium ethyl xanthate, with hydrochloric acid 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. [5] These compounds thermally decompose in the presence of base to the alcohol and carbon disulfide. [6]

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.

Other reactions

Xanthate anions also undergo alkylation to give xanthate esters, which are generally stable: [7]

ROCS2K + R′X → ROC(S)SR′ + KX

The C-O bond in these compounds are susceptible to cleavage by the Barton–McCombie deoxygenation, which provides a means for deoxygenation of alcohols.

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. [2]

Xanthates bind to transition metal cations as bidentate ligands. The charge-neutral complexes are soluble in organic solvents. [8]

Structure of typical metal tris(ethylxanthate) complex. M(S2COEt)3.png
Structure of typical metal tris(ethylxanthate) complex.

Xanthates are intermediates in the Chugaev elimination process. They can be used to control radical polymerisation under the RAFT process, also termed MADIX (macromolecular design via interchange of xanthates).

Industrial applications

Simplified image of xanthation of cellulose. Xanthation.png
Simplified image of xanthation of cellulose.

Cellulose reacts with carbon disulfide (CS2) in presence of sodium hydroxide (NaOH) to produces sodium cellulose xanthate, which upon neutralization with sulfuric acid (H2SO4) gives viscose rayon or cellophane paper (Sellotape or Scotch Tape).

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

Rarely encountered, thioxanthates arise by the reaction of CS2 with thiolate salts. For example, sodium ethylthioxanthate has the formula C2H5SCS2Na. Dithiocarbamates 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. [11] Water downstream of mining operations is often contaminated with xanthates. [12]

Related Research Articles

<span class="mw-page-title-main">Alcohol (chemistry)</span> Organic compound with at least one hydroxyl (–OH) group

In chemistry, an alcohol is a type of organic compound that carries at least one hydroxyl functional group bound to carbon. Alcohols range from the simple, like methanol and ethanol, to complex, like sugars and cholesterol. The presence of an OH group strongly modifies the properties of hydrocarbons, conferring hydrophilic (water-loving) properties. The OH group provides a site at which many reactions can occur.

<span class="mw-page-title-main">Carboxylic acid</span> Organic compound containing a –C(=O)OH group

In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group attached to an R-group. The general formula of a carboxylic acid is often written as R−COOH or R−CO2H, sometimes as R−C(O)OH with R referring to an organyl group, or hydrogen, or other groups. Carboxylic acids occur widely. Important examples include the amino acids and fatty acids. Deprotonation of a carboxylic acid gives a carboxylate anion.

<span class="mw-page-title-main">Ester</span> Compound derived from an acid

In chemistry, an ester is a compound derived from an acid in which the hydrogen atom (H) of at least one acidic hydroxyl group of that acid is replaced by an organyl group. Analogues derived from oxygen replaced by other chalcogens belong to the ester category as well. According to some authors, organyl derivatives of acidic hydrogen of other acids are esters as well, but not according to the IUPAC.

<span class="mw-page-title-main">Ketone</span> Organic compounds of the form >C=O

In organic chemistry, a ketone is an organic compound with the structure R−C(=O)−R', where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group −C(=O)−. The simplest ketone is acetone, with the formula (CH3)2CO. Many ketones are of great importance in biology and in industry. Examples include many sugars (ketoses), many steroids, and the solvent acetone.

<span class="mw-page-title-main">Thiol</span> Any organic compound having a sulfanyl group (–SH)

In organic chemistry, a thiol, or thiol derivative, is any organosulfur compound of the form R−SH, where R represents an alkyl or other organic substituent. The −SH functional group itself is referred to as either a thiol group or a sulfhydryl group, or a sulfanyl group. Thiols are the sulfur analogue of alcohols, and the word is a blend of "thio-" with "alcohol".

<span class="mw-page-title-main">Tosyl group</span> Chemical group (–SO₂–C₆H₄–CH₃)

In organic chemistry, a toluenesulfonyl group (tosyl group, abbreviated Ts or Tos) is a univalent functional group with the chemical formula −SO2−C6H4−CH3. It consists of a tolyl group, −C6H4−CH3, joined to a sulfonyl group, −SO2−, with the open valence on sulfur. This group is usually derived from the compound tosyl chloride, CH3C6H4SO2Cl (abbreviated TsCl), which forms esters and amides of toluenesulfonic acid, CH3C6H4SO2OH (abbreviated TsOH). The para orientation illustrated (p-toluenesulfonyl) is most common, and by convention tosyl without a prefix refers to the p-toluenesulfonyl group.

<span class="mw-page-title-main">Diethyl malonate</span> Chemical compound

Diethyl malonate, also known as DEM, is the diethyl ester of malonic acid. It occurs naturally in grapes and strawberries as a colourless liquid with an apple-like odour, and is used in perfumes. It is also used to synthesize other compounds such as barbiturates, artificial flavourings, vitamin B1, and vitamin B6.

<span class="mw-page-title-main">Sodium ethoxide</span> Ionic compound made of a C2H5–O anion and a sodium cation

Sodium ethoxide, also referred to as sodium ethanolate, is the ionic, organic compound with the formula CH3CH2ONa, C2H5ONa, or NaOEt. It is a white solid, although impure samples appear yellow or brown. It dissolves in polar solvents such as ethanol. It is commonly used as a strong base.

<span class="mw-page-title-main">Sodium methoxide</span> Ionic organic compound (CH3ONa)

Sodium methoxide is the simplest sodium alkoxide. With the formula CH3ONa, it is a white solid, which is formed by the deprotonation of methanol. It is a widely used reagent in industry and the laboratory. It is also a dangerously caustic base.

<span class="mw-page-title-main">Alkyl nitrite</span> Organic compounds of the form R–O–N=O

In organic chemistry, alkyl nitrites are a group of organic compounds based upon the molecular structure R−O−N=O, where R represents an alkyl group. Formally they are alkyl esters of nitrous acid. They are distinct from nitro compounds.

<span class="mw-page-title-main">Dithiocarbamate</span> Chemical group (>N–C(=S)–S–)

In organic chemistry, a dithiocarbamate is a functional group with the general formula R2N−C(=S)−S−R and structure >N−C(=S)−S−. It is the analog of a carbamate in which both oxygen atoms are replaced by sulfur atoms.

Thiocarbonate describes a family of anions with the general chemical formula CS
3−xO2−
x (x = 0, 1, or 2):

<span class="mw-page-title-main">Ammonium thiocyanate</span> Chemical compound

Ammonium thiocyanate is an inorganic compound with the formula [NH4]+[SCN]−. It is an ammonium salt of thiocyanic acid. It consists of ammonium cations [NH4]+ and thiocyanate anions [SCN]−.

<span class="mw-page-title-main">Sodium ethyl xanthate</span> Chemical compound

Sodium ethyl xanthate (SEX) is an organosulfur compound with the chemical formula CH3CH2OCS2Na. 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 related potassium ethyl xanthate (KEX) is obtained as the anhydrous salt.

<span class="mw-page-title-main">Potassium ethyl xanthate</span> Chemical compound

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. It is a potassium salt of ethyl xanthic acid.

<span class="mw-page-title-main">Etabonate</span> Class of chemical compounds

Etabonate or ethyl carbonate is the chemical group with formula –CO
3–C
2H
5, or H
3C–CH
2–O–C(=O)–O–. The names are also used for esters R–OCO
2C
2H
5, for the anion [C
2H
5OCO−
2], and for salts of the latter.

In chemistry, a thioxanthate is an organosulfur compound with the formula RSCS2X. When X is an alkali metal, the thioxanthate is a salt. When X is a transition metal, the thioxanthate is a ligand, and when X is an organic group, the compounds are called thioxanthate esters. They are usually yellow colored compounds that often dissolve in organic solvents. They are used as precursors to some catalysts, froth flotation agents, and additives for lubricants.

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 for the separation of ores using the flotation process.

<span class="mw-page-title-main">Ethyl xanthic acid</span> Ethyl xanthic acid

Ethyl xanthic acid is an organic compound with the chemical formula CH3CH2−O−C(=S)−SH. It can be viewed as an O-ethyl ester of dithiocarbonic O,S-acid. Ethyl xanthic acid belongs to the category of thioacids, where the prefix thio- means that an oxygen atom in the compound is replaced by a sulfur atom.

<span class="mw-page-title-main">Potassium trithiocarbonate</span> Chemical compound

Potassium trithiocarbonate is the inorganic compound with the chemical formula K2CS3. It is the potassium salt of trithiocarbonic acid. It consists of two potassium cations K+ and the trigonal planar trithiocarbonate dianion CS2−3. It is a white solid, although impure samples often appear brown. It is prepared by the reaction of potassium sulfide or potassium hydrosulfide with carbon disulfide.

References

  1. ↑ IUPAC does not recommend the use of the term xanthate, although it is in current use in the scientific literature: IUPAC , Compendium of Chemical Terminology , 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) " Xanthate ". doi : 10.1351/goldbook.X06696
  2. 1 2 3 Roy, Kathrin-Maria. "Xanthates". Ullmann's Encyclopedia of Industrial Chemistry . Weinheim: Wiley-VCH. doi:10.1002/14356007.a28_423. ISBN   978-3527306732.
  3. ↑ This report gives a detailed procedure for the potassium ethyl xanthate: Price, Charles C.; Stacy, Gardner W. (1948). "p-Nitrophenyl sulfide". Organic Syntheses. 28: 82. doi:10.15227/orgsyn.028.0082.
  4. ↑ Perumalreddy Chandrasekaran, James P. Donahue (2009). "Synthesis of 4,5-Dimethyl-1,3-Dithiol-2-One". Organic Syntheses. 86: 333. doi: 10.15227/orgsyn.086.0333 .
  5. ↑ Millican, Robert J.; Sauers, Carol K. (1979). "General acid-catalyzed decomposition of alkyl xanthates". The Journal of Organic Chemistry. 44 (10): 1664–1669. doi:10.1021/jo01324a018.
  6. ↑ Gattow, Gerhard; Behrendt, Werner (1977). Carbon Sulfides and their Inorganic and Complex Chemistry. Stuttgart: Georg Thieme. ISBN   3135262014.
  7. ↑ Gagosz, Fabien; Zard, Samir Z. (2007). "A Xanthate-Transfer Approach to α-Trifluoromethylamines". Organic Syntheses . 84: 32; Collected Volumes, vol. 11, p. 212.
  8. ↑ Haiduc, I. (2004). "1,1-Dithiolato ligands". In McClevert, J. A.; Meyer, T. J. (eds.). Comprehensive Coordination Chemistry II. Vol. 1. pp. 349–376.
  9. ↑ Galsbøl, F.; Schäffer, C. E. (1967). Tris (O-Ethyl Dithiocarbonato) Complexes of Tripositive Chromium, Indium, and Cobalt. Inorganic Syntheses. Vol. 10. pp. 42–49. doi:10.1002/9780470132418.ch6. ISBN   9780470132418.
  10. ↑ Siegfried Hauptmann: Organische Chemie, 2. durchgesehene Auflage, VEB Deutscher Verlag für Grundstoffindustrie, Leipzig, 1985, S. 652, ISBN   3-342-00280-8.
  11. ↑ Besser, J.; Brumbaugh, W.; Allert, A.; Poulton, B.; Schmitt, C.; Ingersoll, C. (2009). "Ecological impacts of lead mining on Ozark streams: toxicity of sediment and pore water". Ecotoxicology and Environmental Safety. 72 (2): 516–526. doi:10.1016/j.ecoenv.2008.05.013. PMID   18603298.
  12. ↑ Xu, Y.; Lay, J. P.; Korte, F. (1988). "Fate and effects of xanthates in laboratory freshwater systems". Bulletin of Environmental Contamination and Toxicology. 41 (5): 683–689. doi:10.1007/BF02021019. PMID   3233367. S2CID   2696850.
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