Hydroxamic acids solvent extraction

Taylor, R.J., Sinkov, S.I., Choppin, G.R., May, I. 2008. Solvent extraction behavior of neptunium(rV) ions between nitric acid and diluted 30 % tri-butyl phosphate in the presence of simple hydroxamic acids. Solvent Extr. Ion Exch. 26 (1) 41-61. 

Hydroxamic acid exists in two tautomeric forms, (1) and (2), and such keto-enol tautomerism provides a number of sites for coordination and chelation. The keto form (1) predominates in acid media and the enol form (2) in alkaline media 25 this has been corroborated by the extraction of vanadium benzohydroxamic acid complexes in organic solvents.26... 

If the desired hydroxamic acids are sufficiently hydrophobic, workup is easily carried out using extraction and flash chromatography. However, many hydroxamic acids are soluble in polar solvents, which causes problems during isolation and purification. The structure of hydroxamic acid thermolysin inhibitors can be made more potent by the introduction of a malonyl moiety to match the specificity of thermolysin (Scheme 3). For example, in the synthesis of HONHCOCH(Bzl)CO-L-Ala-Gly-NH2 (4), O-benzylhydroxylamine was employed in the synthetic scheme to facilitate the isolation and purification of the intermediate 2.[101 The final precursor is a benzyl-protected hydroxamate 3 that can be deprotected by hydrogenolysis without byproducts contaminating the desired hydroxamic acid 4. 

TBP is a sufficiently powerful extractant for actinides that it may be used in diluted form. Dilution improves the hydrodynamic properties of the solvent, allowing more complete and rapid phase disengagement. Typically concentrations of 20-30 v/o TBP in OK are used in process flowsheets. Although TBP is relatively stable as an extractant, radiolysis does lead to the formation of some acidic phosphate esters, HDBP and H2MBP, which can impair process performance.289 An aqueous alkali wash of the recycled solvent is usually carried out to remove those by products. Radiolytic degradation of the diluent in the presence of nitric acid can result in the formation of hydroxamic acids,290 which can lead to fission product retention by the organic phase. Again the solvent wash is used to prevent the accumulation of such species. A comprehensive account of the industrial utilization of TBP has recently been published.291... 

The yield of radiolysis products depends strongly on the presence of an aqueous phase in the system, and on its composition. The presence of water and nitric acid in the solvent produces additional free radicals by radiolysis (14, 302, 303), leading to functionalized compounds of extractants and diluents (304). In the case of alkanes, specific compounds like nitroparaffins, alcohols, hydroxamic acids, and nitronic acids have been identified (21, 43, 51). Taharaoui and Morris have summarized the results published in this held (79). 

Healy, T.V., Pilbeam, A. 1974. Metal retention by hydroxamic acids in irradiated TBP solutions. Proceedings of the International Solvent Extraction Conference ISEC 74, 8-14 September, Lyon, France, Vol. 1, 459 -67. 

Other solvent degradation products derived from the diluent as well as from TBP may arise. Butyl lauryl phosphoric acid (HELP) has been used as a model for diluent derived diesters of phosphoric acid and at concentrations above 2xlO M enhances Zr extraction by TBP. HELP is not removed by alkali washing of the solvent but BLP is transferred into water at low ionic strength. Dialkylphosphorane complexes of Zr may also arise from irradiation and can stabilize emulsions. Diluent derived hydroxamic acids are present at too low a concentration (10 -10 M) to completely account for the extent of zirconium retention which occurs in... 

Impure uranyl nitrate from step 5 is purified in step 8 by one or more additional cycles of solvent extraction and, in some plants, by treatment with silica gel or hydroxamic acid. 

Formation constants of 3d metal ions with A-m-tolyl-p-substituted benzohydroxamic acids and of rare earths with thenoylhydroxamic acid have been determined. Formation constants of proton and metal complexes of iV-phenyl-2-thenoyl- and A-p-tolyl-2-thenoyl-hydroxamic acids have also been determined. In addition, study has been made of the mixed ligand complexes involving nicotine- and isonicotino-hydroxamic acids. A method of extraction and spectrophotometric determination of vanadium with chlorophenylmethylbenzohydroxamic acid has also been published. It may be mentioned that hydroxamic acids (in particular, the A-phenylbenzohydroxamic acid) have been widely used as analytical reagents for metal ions. Solvent extraction of titanium by benzo- or salicyl-hydroxamic acid in the presence of trioctylamine in the form of coloured complexes has been reported. A-w-Tolyl-p-methoxybenzohydroxamic acid has been used for extraction and spectrophotometric determination of Mo and W from hydrochloric acid media containing thiocyanate. 

Hydroxamic acids reagents for the solvent extraction and spectrophotometric determination of metals. Y. K. Agrawal and S. A. Patel, Rev. Anal. Chem., 1980, 4, 237-278 (167). 

Solvent extraction by a crown hydroxamic acid in chloroform for the determination of La in monazite sand has been reported by Agrawal and Shrivastav (1997). La can be determined spectrophotometrically in the oiganic phase between 1.2 and 20 ppm, or by ICP/AES with a detection limit of O.lSppb. A two-fold excess of Y, Ce, Pr, or Nd did not interfere with the spectrophotometric determination, and higher concentrations could be tolerated when fluoride or oxalate were present in the aqueous phase. 

A neutral, ninhydrin-positive, non-extractable (from aqueous media), viscous syrup. Rf in butanol-acetic acid-water, 0.32 (Rf values known in four other solvents). Hydroxamic linkage cleaved in 2 hrs at 50° in 0.5N HC1, yielding N8-hydroxyornithine. Reductive hydrolysis in 48%... 

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