Xanthates special

There is an enormous literature on thiocarbonyl compounds, due in part to the technical and industrial importance of many of them, including thioamides, thioureas, xanthates, dithiocarbamates and so forth. An excellent, and recent, general review is available.107 There are also specialized reviews germane to the present chapter Griffin, Woods, and Klayman2 discussed the use of thioureas in the synthesis of heterocycles the preparation of thiazoles from thioamides is included in a three-part volume on Thiazoles 108 the use of carbon disulfide in the synthesis of trithiones and related heterocycles has been reviewed by Mayer109 and Huisgen110 has reported numerous examples of 1,3-dipolar cycloadditions in which carbon disulfide was used. 

Native gold and its alloys, which are free from surface contaminants, are readily floatable with xanthate collectors. Very often however, gold surfaces are contaminated or covered with varieties of impurities [4], The impurities present on gold surfaces may be argentite, iron oxides, galena, arsenopyrite or copper oxides. The thickness of the layer may be of the order of 1-5 pm. Because of this, the flotation properties of native gold and its alloys vary widely. Gold covered with iron oxides or oxide copper is very difficult to float and requires special treatment to remove the contaminants. 

Geometric effects coupled with diffusion and nucleation usually control the rates of all solids deposition phenomena. Such effects can be put to good use in the production of special products such as cellulose yarn (rayon), by the precipitation of cellulose in filament form as it emerges as sodium cellulose xanthate liquid from the spinnerets into a bath containing sulphuric acid, which extracts the sodium as sodium sulphate, and the carbon disulphide. In a similar manner, the fabrication of aromatic polyimide fibres is performed by dissolving the polymer in concentrated sulphuric acid and forcing the solution through spinnerets into water. 

The rest of the chapter has been devoted to special topics and in materials science there are many possibilities. Those selected include the mechanism of the flotation of minerals in which the addition of a certain organic to the solution causes a specific mineral to become hydrophobic so that it is exposed to air bubbles, the bubbles stick to it and buoy the mineral up to the surface, leaving unwanted minerals on the bottom of the tank. It turns out that the mechanism of this phenomenon involves a mixed-potential concept in which the anodic oxidation of the organic collector, often a xanthate, allows it to form a hydrophobic film upon a semiconducting sulfide or oxide, but only if there is a partner reaction of oxygen reduction. This continues until there is almost full coverage with the dixanthate, and the surface is thereby made water-repelling. 

A number of sulfur-centered radical scavengers have been employed for Meerwein type carbothiolation reactions [109, 110]. The most prominent of those are certainly xanthates [111-113] and thiocyanates, among which the latter have received special attention recently. As shown in Scheme 21, thiocyanates are well-suited for the functionalization of activated and non-activated alkenes [114, 115]. Remarkably, the reaction of 56 with 2-methallyl chloride to give 57 is not significantly impeded by the possible (3-fragmentation of a chlorine radical, which would lead to allylation products [116]. With an activated and a non-activated alkene present in a substrate... 

In this book, he emphasized the importance of the microscopic and the submicroscopic structure of fibrous high polymers. The reactions of cellulose with water, aqueous alkalis, organic bases, ammonia, and strong salt solutions were all stressed. Special attention was given to various types of cellulose esters, to cellulose xanthate, and to the cellulose ethers. The oxidation of cellulose under a variety of conditions was described, as were the hydrolysis reactions. The latter included discussions on reversion and on the kinetics of acid hydrolysis. It is interesting to note that Heuser, who earlier had criticized the terms hydrocellulose and oxycellulose, and had... 

The use of sulfones as relays can be extended to the introduction of a large variety of appendages. Vinylation is especially important because of the general difficulty of performing vinylations on sp centres using transition metal chemistry. The examples assembled in Scheme 14 thus acquire a special significance [31]. It is also noteworthy that both the allylation and vinylation reactions, as well as some of the more exotic transformations discussed later in this section, all of which employ sulfone-based reagents, can be applied not just to aliphatic xanthates but also to iodides and tellurides [29,37-44]. 

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 ... 

Certain esters (Chapter 9) are specifically designed to produce alcohol derivatives that are more labile than the alcohols themselves. The special case of the xanthate ester (the Chugaev reaction, Scheme 8.69) has often been used to effect elimination when temperatures for simple ester pyrolysis are too high and other processes intrude. 

Sulfide minerals are relatively easier to separate by chemisorption because they can use the major collectors such as xanthates and dithiosphophates. Certain special additives with high surface energy capabilities can also be added to separate different grades of sulfides (e.g., to sink pyrite while floating chalcopyrite). 

Mielczarski, Suoninen, and their co-workers utilized XPS to provide information on the structure of adsorbed xanthates and dithiophos-phate on metals and metal sulfides. They interpreted their results in terms of the initial distribution of the adsorbed thiol without any special orientation, followed by island formation as the coverage increased and, eventually, a well-ordered monolayer. In subsequent multilayer formation by the metal thiol compound, the orientation of the thiol was considered to become more random. Interestingly, it was noted that other ions preadsorbed on the surface, such as hydroxyl and carbonate, were gradually removed as the xanthate monolayer developed. Only one sulfur environment was observed for surface xanthate species, supporting the conclusion that xanthate is bonded to metal atoms in the surface layer through both sulfur atoms (see Section VII.2). No differences were reported between the binding energies of the thiol in the initial monolayer and those of the bulk thiol compound. 

The resultant alkali cellulose is pressed to remove most of the excess NaOH solution and is shredded to increase the surface area and make the material easier to process. The shredded alkali cellulose is pre-aged with the presence of oxygen in the ambient air to reduce and control the cellulose molecular weight. After reaching the desired molecular weight, carbon disulfide (CS ) is added to the shredded alkali cellulose in a specially designed vessel called a Barette at 20-30°C for 1-3 hours to cany out the xanthation step, which leads to the formation of sodium cellulose xanthate. The following is the main reaction in the xanthation step ... 

---