Sensitizers xanthates

From the data generated, higher dosages of NaHS are required to achieve activation of malachite. From plant and laboratory experience [13], the sulphidization method using xanthate collector is sensitive to the following, major factors ... 

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. 

A thermographic material with only one heat-sensitive component is prepared by reaction of an organometallic compound such as phenylmercury(II) nitrate with a thiourea derivative.230 The product is ball milled and coated in a binder such as poly(vinyl acetate). Other organometallic compounds disclosed are those of bismuth, tin, gallium and germanium combined with a variety of sulfur-containing compounds such as thioamides and xanthates. 

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... 

Other C=S-containing esters derived from alcohols can also be defunctionalized in a similar fashion. Imidazolylthiocarbonic esters, which in contrast to xanthates can be synthesized under neutral conditions, are one example. This is important for the reduction of base-sensitive alcohols. Figure 1.41 shows a reaction of this type. If the corresponding xanthate was pre-... 

Copper. Copper is the most easily detected element by AAS. Although copper has been determined as the dithizone complex or ethyl-xanthate (6,13), air-acetylene AAS analysis by P CAM 173 or S-186 (12) is superior. Copper lamps are used to test AAS instrumentation because the Cu HC1 sensitivity is nearly independent of lamp current, and a large linear range is observed for the analytical curve. 

The spot-test technique, for which the reaction is particularly well adapted, is as follows. Place a drop of the nearly neutral or faintly acid test solution on a spot plate, introduce a minute crystal of potassium xanthate, followed by 2 drops of 2m hydrochloric acid. An intense red-violet colouration is obtained. Sensitivity 0-04 pg Mo. Concentration limit 1 in 250,000. 

In their comparative studies, Rogovin and coworkers confirmed the sensitivity of 0-(methoxycarbonyl)celluloses toward aqueous alkali, noted earlier by Heuser and Schneider, but found an 0-[(methylthio)thiocar-bonyl] derivative to possess enhanced stability, similar to that of cellulose acetate. Thus, whereas the action of N sodium hydroxide at 25° causes almost complete de-esterification of O-(methoxycarbonyl) cellulose in 5 minutes, the methyl xanthate suffers only 20 % hydrolysis during 1 hour. No quantitative data of this nature are available for the corresponding mono-thiocarbonate. The product from its iodine oxidation was far more stable than that (see p. 147) derived from cellulose xanthate. The 0-[(methyl-thio)thiocarbonyl] derivative of methyl a-D-glucopyranoside was seemingly more alkali-sensitive than the cellulose analog described above, possibly on account of its higher solubility. 

From the first-order rate constants obtained in different solvents (in sealed ampoules), it is apparent that this isomerization is not very sensitive to the polarity of the medium, in accordance with an isopolar, six-membered activated complex [156]. A similar small solvent effect has been observed for the [3,3]sigmatropic rearrangement of allyl S-methyl xanthate to allyl methyl dithiol carbonate [559]. 

The reduction of the p-acyloxy sulfone is most often carried out with sodium amalgam, as the examples below indicate. The reductive elimination can be buffered with disodium hydrogenphosphate for sensitive substrates. In certain applications it has proven advantageous to utilize lithium or sodium in ammonia. For example, Keck s synthesis of pseudomonic acid C made use of the lithium/ammonia reductive elimination to simultaneously form an alkene and deprotect a benzyl ether.In studies directed toward the same target, Williams made use of a reductive elimination procedure developed by Lythgoe, involving the formation of the xanthate ester followed by reduction with tri-n-butyltin hydride. ... 

The loss of C(0)S and MeSH occur at lower temperatures than the carboxylic ester pyrolysis and thus is more suitable for the preparation of sensitive alkenes. For example, heating the xanthate derived by sequential treatment of c -2-benzylcyclopen-tanol with sodium in toluene followed by addition of excess carbon disulfide and methyl iodide furnishes, via a 5yn-elimination, 3-benzylcyclopentene in 90% yield... 

The dehydration of cyclic systems is equally possible. As mentioned previously, the elimination is sensitive to steric effects. For example, in the case of cyclohexanol derivative 32a, Chugaev elimination yielded 39% of 33a and 10% 34a. For 32b and 32c, 46% of 33b and 6% of 34b, and 36% of 33c and 10% 34c were received, respectively.23 These results indicate the elimination proceeded as to avoid the formation of a double bond exo to the ring. Interestingly, the temperature required to affect the transformation was 200 °C for 32a and 32b, whereas it dropped to 100 °C for 32c. This is due to the increasing bulk adjacent to the xanthate, decreasing the stability of the xanthate with elimination providing relief of the steric crowding. 

For thallium, determinations at the 276.78 nm line with an acetylene/air flame are used throughout. Matrix problems are very low, but the sensitivity with regard to the low level of occurrence is poor. The sensitivity can be increased by mounting a slotted quartz tube on the burner head STAT = "slotted tube atom trap") (Milner, 1983), which leads to a detection limit of about 20 mg/kg in the solid sample, which is insufficient for the analysis of biological matrices. In MIBK extracts, determination of thallium is much more sensitive in flame AAS than in aqueous solutions (till about 7-fold). This can be used for solvent extraction of thallium from 0.1M HBr (Hubert and Chao, 1985), as xanthate at pH 8 (Aihara and Kiboku, 1980), or as iodide with tri-n-octylphosphinoxide into MIBK, and direct aspiration of the organic phase into the flame. 

The physical properties of these aryl xanthates resemble closely those of their alkyl counterparts. The thallium compounds are heat- and light-sensitive, and they decompose in acetone or THF to form TI2S, TP,... 

The majority of orientational studies refer to the pyrolysis of acetates although in cases in which corresponding xanthates have been decomposed the product distribution is found to be almost identical. Of course, bearing in mind the considerable difference in reaction temperature, the two pyrolyses may not always lead to the same product proportions and the more reactant-like nature of the xanthate reactions may cause them to be less sensitive to thermodynamic effects than the pyrolyses of the acetates. 

As with the other syn thermal eliminations, there are no intermediates prone to undergo skeletal rearrangement. The principal application of the xanthate method has been in situations in which acid-catalyzed alcohol dehydration would be accompanied by skeletal rearrangement, or in which a very sensitive olefin is being produced. 

The same sensitivity can be achieved in the in situ ATR spectra of the Vfl CH2 bands of surfactants with the chains 12-16 carbon lengths adsorbed on colloid oxide films [174, 178], For coarser particles of a higher refractive index, the sensitivity decreased. For example, for xanthate adsorbed on chalcopyrite (CuFeS2) particles <30 p,m size (Fig. 7.26), surface sensitivity is about 0.3 ML [179]. For such substrates, the SNR is comparable to that from the in situ ATR spectra... 

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