150px, Sodium salt of ethyl xanthate Musée de la viscose (Échirolles) abc29

Xanthate usually refers to a

salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quanti ...

with the formula (R =

alkyl In organic chemistry, an alkyl group is an alkane missing one hydrogen. The term ''alkyl'' is intentionally unspecific to include many possible substitutions. An acyclic alkyl has the general formula of . A cycloalkyl is derived from a cycloal ...

; M⁺ = Na⁺, K⁺), thus they are the metal-thioate/''O''-

ester In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glycerides ...

s of dithiocarbonate. The name ''xanthates'' is derived from

Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Dark Ages (), the Archaic p ...

''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 compound Organosulfur compounds are organic compounds that contain sulfur. They are often associated with foul odors, but many of the sweetest compounds known are organosulfur derivatives, e.g., saccharin. Nature abounds with organosulfur compounds—sulf ...

s are important in two areas: the production of

cellophane Cellophane is a thin, transparent sheet made of regenerated cellulose. Its low permeability to air, oils, greases, bacteria, and liquid water makes it useful for food packaging. Cellophane is highly permeable to water vapour, but may be coate ...

and related polymers from

cellulose Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wall ...

and (in mining) for extraction of certain ores. They are also versatile intermediates in

organic synthesis Organic synthesis is a special branch of chemical synthesis and is concerned with the intentional construction of organic compounds. Organic molecules are often more complex than inorganic compounds, and their synthesis has developed into one o ...

. ''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 Carbon disulfide (also spelled as carbon disulphide) is a neurotoxic, colorless, volatile liquid with the formula and structure . The compound is used frequently as a building block in organic chemistry as well as an industrial and chemical ...

. 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 CS₂. Often the alkoxide is generated in situ by treating the alcohol with sodium or

potassium hydroxide Potassium hydroxide is an inorganic compound with the formula K OH, and is commonly called caustic potash. Along with sodium hydroxide (NaOH), KOH is a prototypical strong base. It has many industrial and niche applications, most of which expl ...

: :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 The alkali hydroxides are a class of chemical compounds which are composed of an alkali metal cation and the hydroxide anion (OH−). The alkali hydroxides are: * Lithium hydroxide (LiOH) * Sodium hydroxide (NaOH) * Potassium hydroxide (KOH) * Ru ...

. 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 * sodium isobutyl xanthate, (CH₃)₂CHCH₂OCS₂Na * potassium amyl xanthate, CH₃(CH₂)₄OCS₂K The OCS₂ core of xanthate salts, like that of the

organic carbonate In organic chemistry, a carbonate ester (organic carbonate or organocarbonate) is an ester of carbonic acid. This functional group consists of a carbonyl group flanked by two alkoxy groups. The general structure of these carbonates is and ...

s and the

s is characteristically planar. The central carbon is sp²-hybridized.

Reactions

The pK_a of ethyl xanthic acid is 1.6. Xanthates 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 Alkylation is the transfer of an alkyl group from one molecule to another. The alkyl group may be transferred as an alkyl carbocation, a free radical, a carbanion, or a carbene (or their equivalents). Alkylating agents are reagents for effectin ...

to give xanthate esters, which are generally stable: :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. Xanthates bind to transition metal cations as bidentate

ligand In coordination chemistry, a ligand is an ion or molecule ( functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's ele ...

s. The charge-neutral complexes are soluble in organic solvents. M(S2COEt)3

Xanthates are intermediates in the Chugaev elimination process. They can be used to control

radical polymerisation In polymer chemistry, free-radical polymerization (FRP) is a method of polymerization by which a polymer forms by the successive addition of free-radical building blocks (repeat units). Free radicals can be formed by a number of different mechanis ...

under the

RAFT A raft is any flat structure for support or transportation over water. It is usually of basic design, characterized by the absence of a hull. Rafts are usually kept afloat by using any combination of buoyant materials such as wood, sealed barrel ...

process, also termed MADIX (macromolecular design via interchange of xanthates).

Industrial applications

Cellulose reacts with

(CS₂) in presence of

sodium hydroxide Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations and hydroxide anions . Sodium hydroxide is a highly caustic base and alkal ...

(NaOH) to produces sodium cellulose xanthate, which upon neutralization with sulfuric acid (H₂SO₄) gives viscose rayon or

paper ( Sellotape or Scotch Tape). Xanthate salts (e.g. sodium alkyl xanthates,

Dixanthogen : Structure of diethyl dixanthogen disulfide. Dixanthogen disulfides are a class of organosulfur compounds with the formula . Usually yellow solids, they are the product of the oxidation of xanthate salts. A common derivative is diethyl dixanth ...

) are widely used as flotation agents in mineral processing.

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. 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. Water downstream of mining operations is often contaminated with xanthates.

References

{{Functional groups Salts Functional groups Thiocarbonyl compounds