Xanthate usually refers to a salt with the formula , thus O-esters of dithiocarbonate. The name xanthates is derived from Greek ξανθός 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 for extraction of certain ores. They are also versatile intermediates in organic synthesis. Xanthates also refer to esters of xanthic acid. These esters have the structure ROCSR′.
Xanthate salts are produced by the reaction of an alkoxide salt with carbon disulfide. The reaction involves the attack of the alkoxide nucleophile on the electrophile CS2. 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 residualraw 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 OCS2 core of xanthate salts, like that of the all-oxygen carbonates and other simple esters is characteristically planar. The central carbon is sp2-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, ROCSH, 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 the so-called dixanthogens: Xanthates bind to transition metal cations as bidentate ligands. The charge-neutral complexes are soluble in organic solvents.
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 2NCS2Na.
Environmental impacts
Xanthates may be toxic to aquatic life at concentrations of less than 1 mg/L. Water downstream of mining operations is often contaminated.
