Extractants oxygen atoms, presence

Poloxamers are used primarily in aqueous solution and may be quantified in the aqueous phase by the use of compleximetric methods. However, a major limitation is that these techniques are essentially only capable of quantifying alkylene oxide groups and are by no means selective for poloxamers. The basis of these methods is the formation of a complex between a metal ion and the oxygen atoms that form the ether linkages. Reaction of this complex with an anion leads to the formation of a salt that, after precipitation or extraction, may be used for quantitation. A method reported to be rapid, simple, and consistently reproducible [18] involves a two-phase titration, which eliminates interferences from anionic surfactants. The poloxamer is complexed with potassium ions in an alkaline aqueous solution and extracted into dichloromethane as an ion pair with the titrant, tet-rakis (4-fluorophenyl) borate. The end point is defined by a color change resulting from the complexation of the indicator, Victoria Blue B, with excess titrant. The Wickbold [19] method, widely used to determine nonionic surfactants, has been applied to poloxamer type surfactants 120]. Essentially the method involves the formation in the presence of barium ions of a complex be-... 

Discussion. Molybdenum(VI) in acid solution when treated with tin(II) chloride [best in the presence of a little iron(II) ion] is converted largely into molybdenum(V) this forms a complex with thiocyanate ion, probably largely Mo(SCN)5, which is red in colour. The latter may be extracted with solvents possessing donor oxygen atoms (3-methylbutanol is preferred). The colour depends upon the acid concentration (optimum concentration 1M) and the concentration of the thiocyanate ion (1 per cent, but colour intensity is constant in the range 2-10 per cent) it is little influenced by excess of tin(II) chloride. The molybdenum complex has maximum absorption at 465 nm. 

The above-mentioned results indicate the additive effect of protons. Actually, a catalytic process is formed by protonation of the metal-oxygen bond instead of silylation. 2,6-Lutidine hydrochloride or 2,4,6-collidine hydrochloride serves as a proton source in the Cp2TiCl2-catalyzed pinacol coupling of aromatic aldehydes in the presence of Mn as the stoichiometric reduc-tant [30]. Considering the pKa values, pyridinium hydrochlorides are likely to be an appropriate proton source. Protonation of the titanium-bound oxygen atom permits regeneration of the active catalyst. High diastereoselectivity is attained by this fast protonation. Furthermore, pyridine derivatives can be recovered simply by acid-base extraction or distillation. 

Specific solvents extract 20%-40% of the coal at temperatures below 200°C (390°F) and the nature of the extract is believed to be similar to, or even represent, the original coal. Specific solvents extract coal by a process of physical dissolution and the nature of the coal extract and the parent coal is believed to be similar. Hence, such solvents can be considered nonse-lective in their action on coal and usually contain a nitrogen atom and an oxygen atom with unshared electrons as a lone pair (Dryden, 1951a). Pyridine, N-methylpyrrolidone, dimeth-ylformamide, and dimethylacetamide are examples of this type of solvent. They are mostly nucleophilic in nature due to the presence of a lone pair of electrons on the nitrogen atom. 

Insofar as the ability of solvents to extract material from coal can be correlated with the presence of an unshared pair of electrons on a nitrogen atom or an oxygen atom in a solvent molecule, it is difficult to fully rationalize such a concept. Eor example, some nonsolvents (of which methanol may be cited as an example) have the ability to swell coal almost as much as the more specific solvents (such as pyridine) (Franz, 1979 Szeliga, 1987). Furthermore, the approximate linear relationship between extract yields and the internal pressures of the solvents no longer holds when the solvents are used at temperatures below their normal boiling points. 

The inorganic lanthanide triflate complexes Ln(OTf)3 (made in aqueous solution) have been shown by Kobayashi to be efficient Lewis-acid catalysts for hydroxy-methylation (using commercial aqueous formaldehyde solutions) of silicon enolates in aqueous medium (water -i- THF) or even in water alone in the presence of a surfactant. In these reactions, activation proceeds by coordination of the aldehyde oxygen atom by the Ln center that is a strong Lewis acid due to its hard character. Among the lanthanide triflates, ytterbium triflate was found to be the most active catalyst, but scandium triflate can sometimes also be efficiently used. Enantio-selective versions are also known in the presence of chiral macrocyclic ligands. The water-soluble catalyst is recovered in water after extraction of the organic products. 

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