Xanthate

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.

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:

  • sodium ethyl xanthate CH3CH2OCS2Na
  • potassium ethyl xanthate, CH3CH2OCS2K
  • potassium isopropyl xanthate, (CH3)2CHOCS2K[4]
  • sodium isobutyl xanthate, (CH3)2CHCH2OCS2Na
  • potassium amyl xanthate, CH3(CH2)4OCS2K

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]

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

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]

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. 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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