Xanthate

Xanthate usually refers to a salt with the formula ROCS₂⁻M⁺ (R = alkyl; M+ = Na⁺, K⁺).^[1] The name xanthates is derived from Greek ξανθός [ksantʰós], meaning “yellowish, golden”, and indeed most xanthate salts are yellow. These organosulfur compounds are important in two areas, the production of cellophane and related polymers from cellulose and secondly in mining for the extraction of certain ores.^[2] They are also versatile intermediates in organic synthesis. Xanthates also refer to esters of xanthic acid. These esters have the structure ROC(=S)SR'.

Formation and structure

Xanthate salts are produced by the reaction of an alcohol with sodium or potassium hydroxide and carbon disulfide:^[2][3]

ROH + CS₂ + KOH → ROCS₂K + H₂O

The reaction involves the attack of the alkoxide salt. For example, sodium ethoxide gives sodium ethyl xanthate. Virtually any alcohol can be used in this reaction. Technical grade xanthate salts are usually of 90–95% purity. Impurities include alkali-metal sulfide, sulfate, trithiocarbonates, thiosulfate, sulfite, or carbonate 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. China is a major exporter of granules.

Some commercially important xanthate salts include:

• sodium ethyl xanthate CH₃CH₂OCS₂Na,
• potassium ethyl xanthate, CH₃CH₂OCS₂K,
• sodium isopropyl xanthate
• sodium isobutyl xanthate
• potassium amyl xanthate

The OCS₂ core of xanthate salts and esters is characteristically planar. The central carbon is sp²-hybridized.

Reactions

Xanthate salts 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 to give xanthate esters, which are generally stable:^[4]

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.

Analogous to their S-alkylation, xanthates bind to transition metal cations as bidentate ligands. The neutral complexes are soluble in organic solvents.^[5]

They can be oxidized to the so-called dixanthogens:

2 ROCS₂Na + Cl₂ → ROC(S)S₂C(S)OR + 2 NaCl

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 are used as flotation agents in mineral processing. They are intermediates in the Chugaev elimination process and are used to control radical polymerisation under the RAFT process, also termed MADIX (macromolecular design via interchange of xanthates).

Related compounds

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, and are more useful. They arise from the reaction of the amine with CS₂. For example, sodium diethyldithiocarbamate has the formula (C₂H₅)₂NCS₂Na.

Environmental impacts

Xanthates may be toxic to aquatic biota at concentrations of less than 1 mg/L.^[6] Water downstream of mining operations can be contaminated.^[7]

References