Replacement sulfur

Any radioactive nucUde or isotope of an element can be used as a radioactive tracer, eg, chromium-51 [14392-02-0] cobalt-60 [10198-40-0] tin-110 [15700-33-1] and mercury-203 [13982-78-0],hut the preponderance ofuse has been for carbon-14 [14762-75-5],hydj ogen-3 [10028-17-8] (tritium), sulfur-35 [15117-53-0], phosphoms-32, and iodine-125 [14158-31 -7]. More recendy phosphoms-33 has become available and is used to replace sulfur-35 and phosphoms-32 in many appUcations. By far the greater number of radioactive tracers produced are based on carbon-14 and hydrogen-3 because carbon and hydrogen exist in a large majority of the known natural and synthetic chemical compounds. 

The weight of the superphosphate can be reduced by replacing sulfuric acid with phosphoric acid (obtained by sulfuric acid acting on phosphate rock followed by product separation, or by the... 

Some attempts have been made to transform the conventional lead accumulator into a dissolution accumulator by replacing sulfuric acid with tetrafluoro-boric acid (HBF4) but the highly corrosive and toxic acid was not finally accepted [16]. 

The reaction of vegetable oils, especially castor oil, with sulfuric acid to obtain wetting agents for the so-called Turkish red oil dying process has long been known. Therefore attempts soon were made to extend this procedure to other OH-containing structures and to replace sulfuric acid with phosphoric acid [ ] ... 

All selenium-containing proteins and enzymes in animals, microorganisms and plants incorporate selenocysteine non-specifically105 or as part of Se-dependent antioxidant enzymes such as glutathione peroxidase, (EC 1.11.1.9) which has a Se-cysteine residue in its active site.116 120 An active form of Se, selenophosphate, is produced by selenophosphate synthetase in several bacteria. This active form is required for the production of Secys-tRNA, a precursor for Se-cysteine.121 In a similar vein, a Se-containing modified-tRNA nucleoside, 5-methylaminomethyl-2-selenouridine, encodes a selenouridine synthase which replaces sulfur in tRNA with selenium.122... 

A straightforward preparation of [Fe4Se4Br4] uses Na2Se, with DMF as solvent.Selenium can replace sulfur in a few high-nuclearity iron-sulfur clusters, e.g., [FegSeg(PEt3)4Cl2] " and... 

Cyclization (Scheme 8) was accomplished using either phosphoryl chloride or aluminum chloride. A range of substituents were introduced into the benzene ring (alkyl, aryl, or CN) or the 3- or 4-position (alkyl or aryl) and selenium successfully replaced sulfur in these reactions. 

In 1957, Schwartz and associates showed that the toxic element selenium was also a nutritional factor essential for prevention of the death of liver cells in rats.527 Liver necrosis would be prevented by as little as 0.1 ppm of selenium in the diet. Similar amounts of selenium were shown to prevent a muscular dystrophy called "white muscle disease" in cattle and sheep grazing on selenium-deficient soil. Sodium selenite and other inorganic selenium compounds were more effective than organic compounds in which Se had replaced sulfur. Keshan disease, an often fatal heart condition that is prevalent among childen in Se-deficient regions of China, can be prevented by supplementation of the diet with NaSe03.528 Even the little crustacean "water flea" Daphnia needs 0.1 part per billion of Se in its water.529... 

Desulfo enzymes can be regenerated by anaerobic treatment with sulfide plus dithionite. Cyanide-inactivated turkey liver xanthine dehydrogenase is also reactivated by selenide, suggesting that selenium may replace sulfur in the active centre. 

EXTENSIONS AND COMMENTARY With an entirely new hetero atom in the molecule (the selenium), and with clear indications that large dosages would be needed (100 milligrams, or more), some discretion was felt desirable. There was certainly an odd taste and an odd smell. I remember some early biochemical work where selenium replaced sulfur in some amino acid chemistry, and things got pretty toxic. It might be appropriate to get some general animal toxicity data before... 

Hydrogen fluoride is also used as a catalyst in the production of aviation gasoline. More recently, it has replaced sulfuric acid in the alkylation process for the production of automobile gasoline. 

Hydrochloric acid is replacing sulfuric acid in some applications such as metal pickling, which is the cleaning of metal surfaces by acid etching. It leaves a cleaner surface than sulfuric acid, reacts more slowly, and can be recycled more easily. It is used in chemical manufacture especially for phenol and certain dyes and plastics. In oil well drilling, it increases the permeability of limestone by acidifying the drilling process. 

This direct, oxidative condensation of methane to acetic acid in one-pot could be competitive with the current three-step, capital intensive process for the production of acetic acid based on methane reforming to CO, methanol synthesis from CO, and generation of acetic acid by carbonylation of methanol. Key improvements required with the PdS04/H2S04 system, however, will be to develop more stable, faster, and more selective catalysts. Although it is possible sulfuric acid could be utilized industrially as a solvent and oxidant for this reaction, it would be desirable to replace sulfuric acid with a less corrosive material. This chemistry has recently been revisited, verified, and extended by Bell et al., who used Cu(II)/02 as the oxidizing system [22],... 

Clavulanic acid (80), isolated from Strepto-myces clavuligeruSy is similar in structure to the penicillins, except oxygen replaces sulfur in the five-membered ring (123). Clavulanic acid has weak antibacterial activity, but is a potent inhibitor of p-lactamases (124). A mixture of clavulanic acid and the j8-lactamase-sensitive amoxycillin was introduced in 1981 as Augmentin and has proved to be an effective combination to combat jS-lactamase-pro-ducing bacteria (125). In 2001,20 years after its launch, Augmentin is the best-selling antibacterial worldwide. 

Chemistry related to that outlined in Schemes 38 and 39 led to equivalent species with selenium and tellurium replacing sulfur.372-375 In addition, a selection of the molecular variants (179)-(182) have been characterized. 

Selected for development by Midgley and his coworkers in the late 1920s, CFC-12 (CF2CI2) was considered to be the ideal refrigerant to replace sulfur dioxide and ammonia.The outstanding properties of CFC-12 quickly led to the search for other CFCs. Kinetic Chemicals, a joint venture between Du Pont and General Motors, was formed to manufacture the new materials, and later it became part of the Freon Division of Du Pont. ... 

An ingenious procedure for replacing sulfur in a thiolactam by a -CH COR group involves alkylation of sulfur with an a-haloketone, followed by sulfur extrusion either with triphenyl phosphine or by heating in DMF (with or without added sodium ethoxide). Th reaction has been successfully applied thus far to isoqu oline-l(2H)-thione, quinazoline-4(3H)-thlone, and 6-mercaptopurine. A closely related intramolecular sulfur displacement reaction with intriguing synthetic potential is Illustrated by the conversion of the S-alkylation product from 6-chloropurine ribonucleosld gand cysteine to the isomeric N-alkylated product by treatment with alkali. 

These unusual heterocycles and their analogues, such as with selenium and tellurium replacing sulfur, have been intensively studied since the discovery that single crystals of TTF.TCNQ (tetrathiafulvalene.tetracya-noquinodimethane) show electrical conductivity. The bis(ethylenedithio) analogue of TTF, usually known as BEDT-TTF, is particularly useful. Incidentally, the electron-donating ability of TTF allows its use as a radical initiator for diazonium salts. " ... 

The 0/1 couple in Au(tfd)21 (tfd = bis(trifluoromethyl)ethylenedithiolate, lb) occurs at a slightly lower potential (+1.32 V vs SCE in CH2CI2) than the mnt counterpart911. This is in accord with observations that in other transition metal 1,2-dithiolene complexes the ease of oxidation is dependent on X with X = Ph > CF3 > CN9. Replacing sulfur with the less electronegative atom, selenium, results in a lower oxidation potential for Aul lds)2 (tds = bis(trifluoromethyl)ethylenediselenolate, lc) relative to [Au( ltd) 12. More electronegative substituents make a complex easier to reduce, as expected. Thus, the 1/2 couple for Au(mnt)2 occurs approximately 500 mV more positive than the same couple for [Au(tds)2]. ... 

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