Xanthate formation

The cellulose molecule contains three hydroxyl groups which can react and leave the chain backbone intact. These alcohol groups can be esterified with acetic anhydride to form cellulose acetate. This polymer is spun into the fiber acetate rayon. Similarly, the alcohol groups in cellulose react with CS2 in the presence of strong base to produce cellulose xanthates. When extruded into fibers, this material is called viscose rayon, and when extruded into sheets, cellophane. In both the acetate and xanthate formation, some chain degradation also occurs, so the resulting polymer chains are shorter than those in the starting cellulose. 

First to be explored was xanthate formation. However, this thrust was doomed to failure because of the sensitivity of 107 to base, a likely result of its vinylogous aldol character. Subsequently, it was determined that the desired dehydration could be accomplished simply by heating 107 with thiocarbonyldiimidazole in 1,2-dichlorobenzene.60 This process gave rise to the desired 108 (37% isolated) along with the A2,3 isomer (10%), from which it could be readily separated chromatographically. Hydrolytic removal of the cyclic carbonate in 108 required 3 h to proceed to completion when... 

These facts all appear to support the mechanism of Ettlinger since, in the absence of alkali, no xanthate formation can occur even with primary alcohols. Although these observations make it possible to control the reaction to a certain extent, a number of special methods have been worked out for the unequivocal preparation of oxazolidine-2-thiones. These may be subdivided as follows ... 

E. Treiber, G. Porod, and W. Lang, University Graz, Austria, Osterr. Chem. Ztg., 53, 162 (1952). On the existence of orthoxanthates. Spectroscopic, ebullioscopic, and analytical data on isopropylxanthate indicate that there exists no ortho form of the xanthates of this alcohol. It is concluded that the less stable xanthates of cellulose cannot be explained by ortho-xanthate formation. 

In an approach to the total synthesis of milbemycin /) , the attempted xanthate formation of 3 occurred with [3,3] sigmatropic rearrangement yielding dithiocarbonate 4 bearing exclusively the (i -alkene geometry368. 

Modern methods of producing regenerated cellulose can be traced to the discovery in 1892 by Cross, Bevan, and beadle that cellulose can be rendered soluble by xanthate formation by treatment with sodium hydroxide and carbon disulfide and regenerated by acidification of the xanthate solution. This process is known as the viscose process. The reactions can be indicated schematically as... 

A sequence of ATR spectra for a chalcocite (CU2S) electrode in the presence of 10 " M KEX at increasing potentials is shown in Fig. 1.31a [514]. The spectra observed from —0.25 V were attributed to Cu(I)EX (Fig. 7.23). The potential dependence of the VasCOC band, the flotation recovery, and the current are shown in Fig. 1.34b. After correcting for the difference in the EX concentration in the flotation test (a concentration of 1.9 x 10 M implies a cathodic shift of the flotation curve by 0.043 V), the onset of the flotation is seen to coincide with the appearance of the IR signal and the first two peaks in the voltammogram. Thus the spectra suggest that the maximum flotation may be a result of the formation of multilayers of Cu(I)EX, and the current peaks can be attributed to different mechanisms of copper xanthate formation. However, a closer inspection of the spectra shown in Fig. 1.31a reveals a number of differences from the spectrum of bulk Cu(I)EX, which were ignored by the authors. At potentials below -1-0.05 V,... 

Xanthate formation (barattering, sulfitation) is used to produce a water-soluble cellulose derivative. At X = 300, the product is soluble, with 0.4 xanthate group per glucose unit. Commercial xanthates (or viscoses) contain 27% cellulose, 14% NaOH, 8% CS2, and 51% water. Pure sodium xanthates are colorless. The orange-red color of the commercial product is due to sodium trithiocarbonate, which results from the reaction of CS2 with free caustic soda ... 

The adsorption in this case was a precursor to lead xanthate formation by the reaction... 

The XPS study of the galena/butyl xanthate system included investigations of surfaces treated under potential control. This approach more precisely defines the chemisorbed and metal xanthate states. At underpotentials to lead xanthate formation, the S(2p) spectrum displayed an additional doublet shifted by 1.5 eV from that of the substrate (Fig. 22) and no shifted Pb(4/) component. After the potential was held above the reversible value of the PbS/Pb(C4H90CS2)2 couple, two additional doublets were evident in the S(2p) spectrum, shifted by 1.35 and 1.8 eV (Fig. 22), as well as a Pb(4f) component shifted by -1 eV. The two additional S(2p) doublets were assigned to chemisorbed xanthate and lead xanthate, respectively. 

The examples in the preceding section, of the flotation of lead and copper ores by xanthates, was one in which chemical forces predominated in the adsorption of the collector. Flotation processes have been applied to a number of other minerals that are either ionic in type, such as potassium chloride, or are insoluble oxides such as quartz and iron oxide, or ink pigments [needed to be removed in waste paper processing [92]]. In the case of quartz, surfactants such as alkyl amines are used, and the situation is complicated by micelle formation (see next section), which can also occur in the adsorbed layer [93, 94]. 

In the initial formation of the cupric xanthates, soluble xanthate complexes form prior to the precipitation of the cuprous xanthate with the concurrent formation of the dixanthogen (51). The dixanthogen can be separated by virtue of its solubiUty in ether. Older samples of alkah metal xanthates contain some dixanthogen, which is thought to form by the following reaction (33) ... 

The alkyl chloroformates react with cold ethereal dispersions of the xanthates to give the fairly stable xanthogen formates. 

The alkah metal xanthates are generally prepared from the reaction of sodium or potassium hydroxide with an alcohol and carbon disulfide. The initial reaction is the formation of the alkoxide, which reacts with carbon disulfide to give the xanthate ... 

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) ... 

The reaction of chloroformates with sodium xanthates results in the formation of alkyl xanthogen formates that are useful as flotation agents in extraction of metals from their ores (48). 

LEUCKART Thiophenot Synthesis Formation of Ihiophenols from diazonium salts and xanthates... 

Olefin formation (preferentially less substituted) from alcohols via xanthate pyrolysis. 

The thermolysis of xanthates derived from primary alcohols yields one olefin only. With xanthates from secondary alcohols (acyclic or alicyclic) regioisomeric products as well as fi/Z-isomers may be obtained see below. While acyclic substrates may give rise to a mixture of olefins, the formation of products from alicyclic substrates often is determined by the stereochemical requirements the /3-hydrogen and the xanthate moiety must be syn to each other in order to eliminate via a cyclic transition state. 

The majority of the work on xanthates of divalent nickel has, in recent years, been centered on the formation of base adducts with [Ni(Rxant)2]. Carlin and Siegel (348) and Daktenieks and Graddon (349) reported the formation of paramagnetic [Ni(Etxant)2B2] or [Ni(Etaxant)2B], where B = pyridine, 4-methylpyridine, bipyridyl, or... 

For a review of the formation and reactions of xanthates, see Dunn, A.D. Rudorf, W. Carbon Disulphide in Organic Chemistry Ellis Horwood Chichester, 1989, p. 316. 

Scheme 5.9 illustrates some of the conditions that have been developed for the reductive deoxygenation of alcohols. Entries 1 to 4 illustrate the most commonly used methods for generation of thiono esters and their reduction by tri-M-butylstannane. These include formation of thiono carbonates (Entry 1), xanthates (Entry 2), and thiono imidazolides (Entries 3 and 4). Entry 5 is an example of use of dimethyl phosphite as the hydrogen donor. Entry 6 uses r .s-(trimethylsilyl)silane as the hydrogen atom donor. 

The electrochemical mechanism can be well explained with the mineral pyrite. The collector ion is xanthate ion (CT), a member in the anodic sulfydryl collectors group. Two electrochemical reactions occur on the surface of the pyrite. There is the formation of dixanthogen (C2) by anodic oxidation of xanthate ion (CT) on the surface of pyrite coupled with cathodic reduction of adsorbed oxygen. These reactions are shown below ... 

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