Elimination sulfide

Purification of refinery gases by elimination of hydrogen sulfide as well as Claus units for sulfur recovery began to make their appearance. 

Two efficient syntheses of strained cyclophanes indicate the synthetic potential of allyl or benzyl sulfide intermediates, in which the combined nucleophilicity and redox activity of the sulfur atom can be used. The dibenzylic sulfides from xylylene dihalides and -dithiols can be methylated with dimethoxycarbenium tetrafiuoroborate (H. Meerwein, 1960 R.F. Borch, 1968, 1969 from trimethyl orthoformate and BFj, 3 4). The sulfonium salts are deprotonated and rearrange to methyl sulfides (Stevens rearrangement). Repeated methylation and Hofmann elimination yields double bonds (R.H. Mitchell, 1974). 

Various S-nucleophiles are allylated. Allylic acetates or carbonates react with thiols or trimethylsilyl sulfide (353) to give the allylic sulfide 354[222], Allyl sulfides are prepared by Pd-catalyzed allylic rearrangement of the dithio-carbonate 355 with elimination of COS under mild conditions. The benzyl alkyl sulfide 357 can be prepared from the dithiocarbonate 356 at 65 C[223,224], The allyl aryl sufide 359 is prepared by the reaction of an allylic carbonate with the aromatic thiol 358 by use of dppb under neutral condi-tions[225]. The O-allyl phosphoro- or phosphonothionate 360 undergoes the thiono thiolo allylic rearrangement (from 0-allyl to S -allyl rearrangement) to afford 361 and 362 at 130 C[226],... 

Electron-deficient alkenes add stereospecifically to 4-hydroxy-THISs with formation of endo-cycloadducts. Only with methylvinyl-ketone considerable amounts of the exo isomer are produced (Scheme 8) (16). The adducts (6) may extrude hydrogen sulfide on heating with methoxide producing 2-pyridones. The base is unnecessary with fumaronitrile adducts. The alternative elimination of isocyanate Or sulfur may be controlled using 7 as the dipolarenOphile. The cycloaddition produces two products, 8a (R = H, R = COOMe) and 8b (R = COOMe, R =H) (Scheme 9) (17). Pyrolysis of 8b leads to extrusion of furan and isocyanate to give a thiophene. The alternative S-elimi-nation can be effected by oxidation of the adduct and subsequent pyrolysis. 

In contrast to the 4-hydroxy isomers, the thermally stable 5-hydroxy-THISs add to the C=C bond of cyclopropenylidenes (4. 18, 27. 28). The adducts eliminate carbonyl sulfide, and the strained bond breaks resulting in ring-expansion with formation of pyridin-4-ones. -thiones, or -imines. or 4-alkylidenedihydropvridines (20, X = 0. S.NR. or CRR ) (Scheme 19). 

HydrOxy-THISs react with diethoxycarbonylazine producing a 1,2,4-triazole via addition, elimination of carbonyl sulfide (29). and subsequent loss of the ester groups (Scheme 20) (30). 

Hydroxy-THISs add regioselectively to the C=N bonds of isocyanates or isothiocyanates. The initially formed cycloadducts eliminate carbonyl sulfide with formation of 4-hydroxy- or 4-mercaptoimidazolium hydroxide inner salts (21) (Scheme 21). 4-Hydroxyimidazolium hydroxide... 

Thioketenes can be prepared in several ways, from carboxyHc acid chlorides by thionation with phosphoms pentasulfide [1314-80-3] 2 5 ketene dithioacetals by -elimination, from l,2,3-thiadia2oles with flash pyrolysis, and from alkynyl sulfides (thioacetylenes). The dimeri2ation of thioketenes to 2,4-bis(alkyHdene)-l,3-dithietane compounds occurs quickly. They can be cleaved back pyrolyticaHy (63). For a review see Reference 18. 

The goal of lubrication is elimination of this wear and minimizing friction otherwise encountered in dry sliding. This is accompHshed ideally with complete separation of the mbbing surfaces with a full film of lubricant. When complete hill-film separation is impossible, surface chemical effects of a lubricating oil and its additives, or solid-film lubricants such as graphite and molybdenum sulfide, can assist. 

Pyrometa.llurgica.1 Processes. Nickel oxide ores are processed by pyrometaHurgical or hydrometaHurgical methods. In the former, oxide ores are smelted with a sulfiding material, eg, gypsum, to produce an iron—nickel matte that can be treated similarly to the matte obtained from sulfide ores. The iron—nickel matte may be processed in a converter to eliminate iron. The nickel matte then can be cast into anodes and refined electrolyticaHy. 

Solvent extraction—purification of wet-process phosphoric acid is based on preferential extraction of H PO by an organic solvent vs the cationic impurities present in the acid. Because selectivity of acid over anionic impurities is usually not sufficient, precipitation or evaporation steps are included in the purification process for removal. Cmde wet-process acid is typically concentrated and clarified prior to extraction to remove post-precipitated sludge and improve partition of the acid into the solvent. Concentration also partially eliminates fluoride by evaporation of HF and/or SiF. Chemical precipitation of sulfate (as Ba or Ca salts), fluorosiUcates (as Na salt), and arsenic (as sulfides) may also be used as a prepurification step preceding solvent extraction. 

In the presence of sulfide or sulfhydryl anions, the quinonemethide is attacked and a benzyl thiol formed. The P-aryl ether linkage to the next phenylpropane unit is broken down as a result of neighboring-group attack by the sulfur, eliminating the aryloxy group which becomes reactive phenolate ion (eq. 2). If sulfide is not present, a principal reaction is the formation of the stable aryl enol ether, ArCH=CHOAr. A smaller amount of this product also forms in the presence of sulfhydryl anion. 

Unlike other stabilizers, tin mercaptide, or the mercaptan that is formed after the HCl reacts with the mercaptide, can react with the allyhc chlorine to produce a sulfide (47), thus eliminating the labile chlorine groups and stopping the unzipping. 

Calcium and magnesium can be titrated readily with disodium ethylenediaminetetraacetate, with Eriochrome Black T as the indicator. The solution is buffered at pH 10.0. Certain metal ions interfere with this procedure by causing fading or indistinct end points. Cyanide, sulfide, or hydroxjiamine can be used to eliminate or minimise the interferences. 

The ziac concentrate is first roasted ia a fluid-bed roaster to convert the ziac sulfide to the oxide and a small amount of sulfate. Normally, roasting is carried out with an excess of oxygen below 1000°C so that comparatively Htfle cadmium is eliminated from the calciaed material ia this operation (3). Siace the advent of the Imperial Smelting Ziac Furnace, the preliminary roasting processes for ziac and ziac-lead concentrates result ia cadmium recovery as precipitates from solution or as cadmium—lead fume, respectively, as shown ia Figure 1. 

Zn is determined by direct titration with EDTA with xelenol indicator after iron elimination with acetate ions and copper - with sulfide ions. 

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