Arsenic reagents

Since carbomethoxymethylene triphenylarsorane and benzoyl-methylene triphenylarsorane are more active toward ketones than corresponding phosphorus reagents, we used these arsenic reagents to react with acrylic, crotonic, and a-methylacrylic esters, respectively, and obtained satisfactory results (43). The reactions were stereospecific and the yields of products (39) were moderate to excellent depending upon the esters used. Table VI shows the products and the yields. 

Optically Active Phosphorus or Arsenic Reagents Used in Asymmetric Carbonyl Olefination... 

Tab. 7.1. Classification of chiral phosphorus or arsenic reagents used in asymmetric olefination. 

Iodine has the lowest standard electrode potential of any of the common halogens (E = +0.54 V) and is consequently the least powerful oxidising agent. Indeed, the iodide ion can be oxidised to iodine by many reagents including air which will oxidise an acidified solution of iodide ions. However, iodine will oxidise arsenate(lll) to arsenate(V) in alkaline solution (the presence of sodium carbonate makes the solution sufficiently alkaline) but the reaction is reversible, for example by removal of iodine. 

It is usually preferable to oxidise the compound directly as follows. Intimately mix 0 02-0 05 g. of the eompound with 3 g. of sodium peroxide and 2 g. of anhydrous sodium carbonate in a niekel erucible. Heat the crueible and its eontents with a small flame, gently at first, afterwards more strongly until the eontents are fused, and eontinue heating for a further 10 minutes. Allow to stand, extract the contents of the crucible with water, and filter. Add exeess of eoneentrated nitrie acid to the filtrate and test with ammonium molybdate reagent as above. A yellow preeipitate indicates the presenee of phosphorus. It must be borne in mind that the above treatment 1 eonvert any arsenie present into arsenate. 

The pyrometaHurgical processes, ie, furnace-kettle refining, are based on (/) the higher oxidation potentials of the impurities such as antimony, arsenic, and tin, ia comparison to that of lead and (2) the formation of iasoluble iatermetaUic compounds by reaction of metallic reagents such as 2iac with the impurities, gold, silver and copper, and calcium and magnesium with bismuth (Fig. 12). 

Silver diethyldithiocarbamate [1470-61-7] is a reagent commonly used for the spectrophotometric measurement of arsenic in aqueous samples (51) and for the analysis of antimony (52). Silver iodate is used in the determination of chloride in biological samples such as blood (53). 

Arsenic trioxide may be made by burning arsenic in air or by the hydrolysis of an arsenic trihaUde. Commercially, it is obtained by roasting arsenopyrite [1303-18-0] FeAsS. It dissolves in water to a slight extent (1.7 g/100 g water at 25°C) to form a weaMy acidic solution which probably contains the species H AsO, orthoarsenous acid [36465-76-6]. The oxide is amphoteric and hence soluble in acids and bases. It is frequendy used as a primary analytical standard in oxidimetry because it is readily attainable in a high state of purity and is quantitatively oxidized by many reagents commonly used in volumetric analysis, eg, dichromate, nitric acid, hypochlorite, and inon(III). 

Heteropolyacids (HPA) are the unique class of inorganic complexes. They are widely used in different areas of science in biochemistry for the precipitation of albumens and alkaloids, in medicine as anticarcinogenic agents, in industry as catalysts. HPA are well known analytical reagents for determination of phosphoms, silica and arsenic, nitrogen-containing organic compounds, oxidants and reductants in solution etc. 

Inorganic ar senic normally occurs in two oxidation states As(V) and As(III). Arsenic (V) gives a significantly lower response than ar senic (III). For pre-reduction As(V) to the As(III) concentrated hydrochloric acid and potassium iodide/ascorbic acid reagents were used. As organoarsenic compounds do not react with sodium tetrahydi oborate, they were decomposed with a mixture of HNO and on a hot plate. 

Arsenic III oxide (arsenic trioxide, arsenious oxide) [1327-53-3] M 197.8, three forms m 200°(amorphous glass), m 275°(sealed tube, octahedral, common form, sublimes > 125° without fusion but melts under pressure), m 312°, pKj 9.27, pK 13.54, pK 13.99 (for H3ASO3). Crystd in octahedral form from H2O or from dil HCl (1 2), washed, dried and sublimed (193°/760mm). Analytical reagent grade material is suitable for use as an analytical standard after it has been dried by heating at 105° for l-2h or has been left in a desiccator for several hours over cone H2SO4. POISONOUS (particulary the vapour, handle in a ventilated fume cupboard). 

The salt K2[SN2] is an important reagent for the preparation of other sulfur diimide derivatives when MesSiNSNSiMes is not sufficiently reactive. For example, both acyclic and cyclic arsenic(Iir) compounds, Bu2AsNSNAs Bu2 and BuAs(NSN)2As Bu, respectively, have been obtained in this way." ... 

In recent years, biochemists have developed an arsenal of reactions that are relatively specific to the side chains of particular amino acids. These reactions can be used to identify functional amino acids at the active sites of enzymes or to label proteins with appropriate reagents for further study. Cysteine residues in proteins, for example, react with one another to form disulfide species and also react with a number of reagents, including maleimides (typically A ethylmaleimide), as shown in Figure 4.11. Cysteines also react effectively... 

Many people view the Skraup/Doebner-von Miller reaction as the worst witch s brew of all the heterocyclic syntheses. The reaction can be violently exothermic. A variety of oxidizing reagents and additives have been added in an effort to improve yields, including iron (III) and tin (IV) salts, nitrobenzenes, iodine and various acids such as boric and arsenic. Cohn s conditions for the Skraup reaction using an iron salt and boric acid in concentrated sulfuric acid are frequently employed. ... 

Arsenic Pantoxide (Arsenic Acid Anhydride, Arsenic Oxide). As2Os, mw 229.84, white amorph powd, mp 315° (decompn), d 4.32g/cc. Sol in ale, acids, alkalies and w. Prepn is by heating a rnixt of As2 3 with coned HN03 (d 1.38g/cc) until the evoln of nitrogen oxides ceases. The soln of H3As04 is then evapd to dryness, redissolved in w and reevapd until a temp of over 300° is reached. It is used as a chemical reagent... 

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