Xanthates, decomposition

In the pulp and paper industry, UV-vis spectroscopy proves helpful in answering many related questions (e.g. around yellowing of pulp), Kappa number monitoring,alkyl xanthate decomposition, etc. 

The kinetic isotope effects in the gas phase which could distinguish between mechanisms 1 and II have not been measured. However, solution xanthate decompositions, when performed with labeled sulfur and carbon atoms, support mechanism I. These decompositions, which give mercaptan, olefin and COS, are completely analogous to carbonate decomposition. 

Xanthation is an equilibrium reaction, and as the CS2 is used to form by-products, it is replaced by decomposition of the xanthate. There is an induction period in the formation of trithiocarbonate. At elevated temperatures, xanthation is accelerated but by-product formation and xanthate decomposition are also affected in the same way. Also, the reaction is exothermic. For these reasons, the temperature is usually maintained below about 32°C by the use of cooling water, but excessive xanthation times are avoided. 

The xanthate crumb is dissolved in a dilute aqueous solution of sodium hydroxide of about 5-8% concentration. During this step, reactions of xanthation and by-product formation continue and, in some processes, more CS2 can be added to the mix to obtain improved viscose quality. Modifiers, additives, and delustrants can be included at this stage but can also be added later by injection into the viscose when it is pumped to the spinning machines. A low mixing temperature is preferred because this will minimize xanthate decomposition and byproduct formation. The viscous, orange-colored solution obtained, viscose, will have the desired composition, i.e., cellulose and caustic content depending on the intended end use. 

Nitrogen-containing modifiers such as DMA, discussed by Deshmukh [196], are generally ineffective without zinc in the spin-bath. It is known that DMA reacts with carbon disulfide in viscose to form dimethyldithiocarbamate (Equation 10.16), which is an effective agent in modifying viscose. The mechanism by which cellulose xanthate decomposition is retarded is believed to involve association of the thiocarbamate with the xanthate group by a bridging zinc atom (Equation 10.17). 

In some cases, the presence of peroxide impurities may introduce an alternative radical mechanism for xanthate decomposition. Nace, H. R. Manly, D. G. Fusco, S. /. Org. Chem. 1958,23, 687. Radical processes were also proposed for the formation of minor products in the pyrolysis of alkyl acetates. Shi, B. Ji, Y. Dabbagh, H. A. Davis, B. H.. Org. Chem. 1994, 59, 845. 

AEROPHINE 3418A promoter is widely used ia North and South America, AustraHa, Europe, and Asia for the recovery of copper, lead, and ziac sulfide minerals (see Elotatton). Advantages ia comparison to other collectors (15) are said to be improved selectivity and recoveries ia the treatment of complex ores, higher recoveries of associated precious metals, and a stable grade—recovery relationship which is particularly important to the efficient operation of automated circuits. Additionally, AEROPHINE 3418A is stable and, unlike xanthates (qv), does not form hazardous decomposition products such as carbon disulfide. It is also available blended with other collectors to enhance performance characteristics. 

Table 3. Decomposition Temperatures of Alkali Metal Xanthates ...

The heavy metal salts, ia contrast to the alkah metal salts, have lower melting points and are more soluble ia organic solvents, eg, methylene chloride, chloroform, tetrahydrofiiran, and benzene. They are slightly soluble ia water, alcohol, ahphatic hydrocarbons, and ethyl ether (18). Their thermal decompositions have been extensively studied by dta and tga (thermal gravimetric analysis) methods. They decompose to the metal sulfides and gaseous products, which are primarily carbonyl sulfide and carbon disulfide ia varying ratios. In some cases, the dialkyl xanthate forms. Solvent extraction studies of a large number of elements as their xanthate salts have been reported (19). 

Fig. 2. Decomposition of commercial sodium isopropyl xanthate solutions. Courtesy of Cytec Industries Inc.

It was found (32) that in the acid range (pH 4—6) the alkyl group does not influence the rate of decomposition, which is similar for all xanthates. In the alkaline range the rates are markedly influenced by the substitutional group, and the rates could be correlated with the Taft polar substituent constants estabhshed for the various groups. 

Reactions. The chemistry of the xanthates is essentially that of the dithio acids. The free xanthic acids readily decompose in polar solvents, the rate being 10 times greater in methanol than in hexane. The acids decompose at room temperature to carbon disulfide and the corresponding alcohol the resulting alcohol autocatalyticaHy faciUtates the decomposition. 

Further hydrolysis of the carbon disulfide and the trithiocarbonate produces hydrogen sulfide, etc (33). In another study of the decomposition of sodium ethyl xanthate [140-90-9] in flotation solutions, eleven components of breakdown were studied. The dependence of concentration of those components vs time was examined by solving a set of differential equations (34). 

The Chugaev reaction, or thermal decomposition of the substituted esters of the xanthates, gives olefins without rearrangement (35,36). For example ... 

Esters derived from the primary alcohols are the most stable and those derived from the tertiary alcohols are the least stable. The decomposition temperature is lower in polar solvents, eg, dimethyl sulfoxide (DMSO), with decomposition occurring at 20°C for esters derived from the tertiary alcohols (38). Esters of benzyl xanthic acid yield stilbenes on heating, and those from neopentyl alcohols thermally rearrange to the corresponding dithiol esters (39,40). The dialkyl xanthate esters catalytically rearrange to the dithiol esters with conventional Lewis acids or trifluoroacetic acid (41,42). The esters are also catalytically rearranged to the dithiolesters by pyridine Ai-oxide catalysts (43) ... 

Because of hydrate formation, the sodium salts tend to be difficult to dry. Excess water over that of hydration is beheved to accelerate the decomposition of the xanthate salts. The effect of heat on the dryiag of sodium ethyl xanthate at 50°C has been studied (84) ... 

Sodium azide does not react with carbonyl sulfide to form 5-hydroxy-1,2,3,4-thiatriazole, nor with carboxymethyl xanthates, RO-CS SCH2COOH, to form 5-alkoxy-l,2,3,4-thiatriazoles. The latter, however, could be prepared from xanthogenhydrazides (RO-CS NHNH2) and nitrous acid. They are very unstable and may decompose explosively at room temperature only the ethoxy compound (6) has been examined in detail. This is a solid which decomposes rapidly at room temperature and even at 0°C is transformed after some months into a mixture of sulfur and triethyl isocyanurate. In ethereal solution at 20° C the decomposition takes place according to Eq. (16)... 

Upon thermolysis of xanthates (xanthogenates) 1 olefins 2 can be obtained, together with gaseous carbon oxysulfide COS 3 and a thiol RSH 4. This decomposition process is called the Chugaev reactionanother common transcription for the name of its discoverer is Tschugaejf. 

Diazonium xanthates (ArN—NSCSOC2H5) can detonate, and this procedure should be followed carefully to ensure decomposition of the xanthate as it is formed. Under no circumstances should the diazonium solution and the potassium ethyl xanthate be mixed cold and the mixture subsequently heated. A severe detonation has been reported when such a procedure was employed during the preparation of thiocresol. 

It has been observed2 that the dropwise addition of an aqueous solution of potassium ethyl xanthate to a cold (0°) aqueous solution of diazotized orthanilic acid results in the immediate loss of nitrogen when a trace of nickel ion is present in the stirred diazonium solution.3 The catalyst can be added as nickelous chloride or simply by using a nichrome wire stirrer. When no nickel ion is added and a glass stirrer is employed, the diazonium xanthate precipitates and requires heat (32°) to effect decomposition. 

Nickel catalyst, Raney, in preparation of 2,2 bipvndine, 46, 5 VV7 J, preparation of, 46, 5 Nickel ion, as catalyst for decomposition of diazomum xanthates, 47,... 

Dining preparation of thiophenol by addition of a cold solution of potassium O-methyldithiocarbonate to a cold solution of benzenediazonium chloride, a violent explosion accompanied by an orange flash occurred [1], This was attributed to the formation and decomposition of bis(benzenediazo) disulfide. A preparation in which the diazonium solution was added to the xanthate solution proceeded smoothly [2],... 

Dining interaction of the diazonium sulfide and the O-ethyl dithiocarbonate ( xanthate ) solutions, care must be taken to ensure that the intermediate diazonium dithiocarbonate decomposes to 2-thiocresol as fast as it is formed [1]. This can be assured by presence of a trace of nickel in the solution to effect immediate catalytic decomposition [2], When the 2 solutions were mixed cold and then heated to effect decomposition, a violent explosion occurred. 

A few diazonium salts are unstable in solution, and many are in the solid state. Of these, the azides, chromates, nitrates, perchlorates (outstandingly), picrates, sulfides, triiodides and xanthates are noted as being explosive, and sensitive to friction, shock, heat and radiation. In view of their technical importance, diazonium salts are often isolated as their zinc chloride (or other) double salts, and although these are considerably more stable, some incidents involving explosive decomposition have been recorded. 

The mixed-potential model demonstrated the importance of electrode potential in flotation systems. The mixed potential or rest potential of an electrode provides information to determine the identity of the reactions that take place at the mineral surface and the rates of these processes. One approach is to compare the measured rest potential with equilibrium potential for various processes derived from thermodynamic data. Allison et al. (1971,1972) considered that a necessary condition for the electrochemical formation of dithiolate at the mineral surface is that the measmed mixed potential arising from the reduction of oxygen and the oxidation of this collector at the surface must be anodic to the equilibrium potential for the thio ion/dithiolate couple. They correlated the rest potential of a range of sulphide minerals in different thio-collector solutions with the products extracted from the surface as shown in Table 1.2 and 1.3. It can be seen from these Tables that only those minerals exhibiting rest potential in excess of the thio ion/disulphide couple formed dithiolate as a major reaction product. Those minerals which had a rest potential below this value formed the metal collector compoimds, except covellite on which dixanthogen was formed even though the measured rest potential was below the reversible potential. Allison et al. (1972) attributed the behavior to the decomposition of cupric xanthate. 

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