Xanthate
Xanthate usually refers to a salt with the formula (R = alkyl; M = Na, K), thus they are the metal-thioate/"O"-esters of dithiocarbonate. 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. These organosulfur compounds are important in two areas: the production of cellophane and related polymers from cellulose and (in mining) for extraction of certain ores. They are also versatile intermediates in organic synthesis. "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. 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 CS. Often the alkoxide is generated in situ by treating the alcohol with sodium or potassium hydroxide: 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 important xanthate salts include: The OCS core of xanthate salts, like that of the organic carbonates and the esters is characteristically planar. The central carbon is sp-hybridized. Reactions. Xanthate salts characteristically decompose in acid: 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: 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 dixanthogens: Xanthates bind to transition metal cations as bidentate ligands. The charge-neutral complexes are soluble in organic solvents. Industrial applications. Cellulose reacts with carbon disulfide (CS) in presence of sodium hydroxide (NaOH) to produces sodium cellulose xanthate, which upon neutralization with sulfuric acid (HSO) gives viscose rayon or cellophane paper (Sellotape or Scotch Tape). Certain xanthate  salts and bisxanthates (e.g. Dixanthogen) 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 CS with thiolate salts. For example, sodium ethylthioxanthate has the formula CHSCSNa. Dithiocarbamates are also related compounds. They arise from the reaction of a secondary amine with CS. For example, sodium diethyldithiocarbamate has the formula (CH)NCSNa. 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.