Preparation of pure

From most solvents Ss crystallizes as orthorhombic a-Ss. Monoclinic pSg is stable above 96 °C and is usually obtained by cooling liquid sulfur slowly below the triple-point temperature of 115 °C. At 25 crystals of pSg convert to polycrystalline a-Ss in less than 1 h but are stable for several weeks at temperatures below -20 C [27]. y-S is metastable at all temperatmes and occasionally crystallizes by chance, for example from ethanolic solutions of ammonium polysulfide [28], by decomposition of copper ethybcanthate [29] or in the preparation of bis(dialkylthiophosphoryl)disulfane [30]. Smpris-ingly, y-Ss occurs also naturally as the minerd rosickyite. Fmthermore, 7-8 is a component of stretched plastic sulfur which is obtained by quenching liquid sulfur from 350 C to 20 °C (in cold water) and stretching the fibers obtained in the direction of their axes. According to an X-ray diffraction study, this fibrous sulfur consists of helical polymeric sulfur chains (S j see below) which form pockets filled with Ss molecules as the monoclinic y-allotrope [31]. 

In principle, there is only one method to prepare Sg, and that is the reaction of titanocene pentasulfide with either S4CI2 [32] or S4(SCN)2 [33]. The needed dichlorotetrasulfane S4CI2 can be most conveniently prepared by carefully controlled chlorination of cyclo-Sf, in CCI4 at 20 °C [33]  

The solvent and the S2CI2 are distilled off from the mixture and the residue is used for the preparation of Sg  

However, since S4CI2 is an oily liquid which owing to its instability cannot be purified by distillation and consequently always contains small amounts of other dichlorosulfanes, it is recommended to convert it to S4(SCN)2 [identical to S6(CN)2]. Dicyanohexasulfane consists of chain-like molecules which form an odorless solid (m.p. 35 °C) that can be easily recrystallized for purification although it fairly rapidly polymerizes at room temperature [33]  

This reaction takes place at 0 °C in CS2 based on the starting material Se the yield of S4(SCN)2 is 27%. This product reacts in CS2 solution at 20 °C with titanocene pentasulfide to Sg in 18% isolated yield  

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The preparation of pure primary amines by the interaction of alkyl halides and ammonia is very difficult, because the primary amine which is formed reacts with unchanged alkyl halide to give the secondary amine the latter... 

Another difficulty in this reaction lies in the preparation of pure chloroacetaldehyde. The low yield observed is due to simultaneous formation of by-products (polyhalogenation). So vinylchloride was used as a starting material for this synthesis (449). A simpler method is to react chlorine with vinylchloride in aqueous solution and then to dehydrate the semihydrated chloroacetaldehyde by distillation through a column of calcium chloride heated to 70 to 90 C (451). 

There was a tendency to use these resins mixed with urea—formaldehyde or melamine-type resins. Preparation of pure tria2ones or uron resins is difficult and expensive (61,62). Furthermore, the basic nature of the amine nitrogen in tria2one permits the use of mixtures of tria2ones with other agents to yield finishes that retain strength in hypochlorite bleaching. 

Titanium tetraiodide can be prepared by direct combination of the elements at 150—200°C it can be made by reaction of gaseous hydrogen iodide with a solution of titanium tetrachloride in a suitable solvent and it can be purified by vacuum sublimation at 200°C. In the van Arkel method for the preparation of pure titanium metal, the sublimed tetraiodide is decomposed on a tungsten or titanium filament held at ca 1300°C (152). There are frequent hterature references to its use as a catalyst, eg, for the production of ethylene glycol from acetylene (153). 

Preparation of pure K2O2, Rb202 and CS2O2 by this route is difficult because of the ease with which they oxidize further to the superoxides MO2. Oxidation of the metals with NO has been used but the best method is the quantitative oxidation of the metals in liquid ammonia solution (p. 78). The peroxides can be regarded as salts of the dibasic acid H2O2. Thus reaction with acids or water quantitatively liberates H2O2 ... 

The mixture is allowed to stand at room temperature overnight, the crystals filtered off and washed on a filter with acetone. The product Is obtained as colorless needles, which melt at 111° to 112.5°C. The methylsulfate is not stable indefinitely. For preparation of pure chloride salt it is desirable to use methylsulfate which gives no titratable acidity with sodium hydroxide using bromophenol blue as indicator. 

V.A. Bessonova, V.I. Konstantinov, L.A. Stalyarova, E.G. Polyakov, Abstracts of reports on confer. Preparation of pure powders of refractive metals by method of chlorine metallurgy. IMet AN SSSR, Moscow, (1974) 22. 

Isobacteriochlorins, since they are tetrahydroporphyrins, can be obtained by tetrahydrogena-tion of porphyrins and dihydrogenation of chlorins. However, alkali-metal reduction of porphyrins and metalloporphyrins always gives a mixture of chlorins, bacteriochlorins or isobacteriochlorins.14 The method of choice for the preparation of pure isobacteriochlorins, e.g. 2, is the diimide reduction of zinc(II) chlorins, e.g. l.15a,b... 

Under certain condition, however, reactions are still preferably conducted in solution. This is the case e.g., for heterogeneous reactions and for conversions, which deliver complex product mixtures. In the latter case, further conversion of this mixture on the solid support is not desirable. In these instances, the combination of solution chemistry with polymer-assisted conversions can be an advantageous solution. Polymer-assisted synthesis in solution employs the polymer matrix either as a scavenger or for polymeric reagents. In both cases the virtues of solution phase and solid supported chemistry are ideally combined allowing for the preparation of pure products by filtration of the reactive resin. If several reactive polymers are used sequentially, multi-step syntheses can be conducted in a polymer-supported manner in solution as well. As a further advantage, many reactive polymers can be recycled for multiple use. 

Few kinetic studies of the decompositions of higher oxides have been reported one probable reason is that the preparation of pure samples of these highly reactive compounds is difficult. Accordingly, interest has been largely restricted to the most readily available substances which are the alkali and alkaline earth peroxides (02-), superoxides (02) and ozonides (03). Some of these may be hydrated. E values reported [656] for the dehydrations of M02 8 H20 (288—313 K) were 96, 163 and 63 kJ mole-1 for the Ca, Sr and Ba compounds, respectively. 

Bordwell and Boutan (BB)81 carried out extensive work on the methylsulfmyl group in 1957. It must be emphasized that they found that the preparation of pure arylmethyl sulfoxides from arylmethyl sulfides by oxidation was not a trivial matter. The frequently recommended reagent, hydrogen peroxide in acetic acid, tended to give sulfoxides contaminated with appreciable quantities of sulfones, which could not be removed by fractional crystallization. Oxidation by nitric acid was found to be more satisfactory. 

The real Willamson synthesis with metallic sodium and sodium chloroacetate is only used for the preparation of pure ether carboxylates for analysis purposes or to obtain physicochemical measurements [229]. 

A laboratory preparation of pure Li uses a finely powdered mixture of Li,0 and a noble metal (Pt, Pd or Ir), which is heated > 1100°C in a stream of Hj. The pure intermetallic phase so produced is then heated under vacuum and the Li can be isolated by condensation from the vapor phase. ... 

C21-0076. Describe the industrial preparation of pure Si, starting from impure Si02. Include balanced chemical reactions. 

In the case of the preparation of pure [CeMIMJCl (purity > 99%), the evaluation of the actual experimental results and the outranking of the different solution candidates resulted in the following optimal synthesis prescription T = 100°C, t = 30 h, mim/ C6Hi3C1 = 1 1, cg[ i,ase = 4.6 molL , solvent free. This parameter configuration has been found to represent the best trade-off between EF, EHP and CF. 

Ryan, V. A., and J.W. Pringle Preparation of Pure Americium. Report... 

Several reports have appeared concerning the addition of elemental phosphorus to rc-systems, either involving addition-elimination or direct addition across a ji-linkage. These proceed in reasonable yield for the preparation of pure organophosphorus materials. 

This is a general method for making N-alkylallenimines, and the following ones have been made in this way N-methyl-,6 N-propyl-,6 N-isopropyl-,4 N-butyl-,4 N-hexyl-,e and N-(3,5,5-tri-methylhexyl)-.4 N-Z-Butylallenimine6 and l-(l-allenimino)-2-hydroxy-3-butene 7 have also been prepared by this method, but with sodium amide/2-bromoallylamine mole ratios of 1.75 and 2.1, respectively. This method has been used for the preparation of pure N-alkylpropargylamines from 2-chloroallylaminesA7 The optimum sodium amide/2-chloroallylamine ratio for the preparation of N-alkylpropargylamines is 2.1. 

Examples of the preparation of pure allylstannanes by this route are given in Equations (95) and (96). 

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