Extractants steric properties

The importance of the acidic and steric properties of mono-acidic phosphorus-based extractants in the extraction of metals has been well established. The work of Mason, Peppard, et al. (2, 3) has shown that combined acidic and steric effects which result from altering the structure of the acidic extractants may be varied to give a wide range of extraction constants (Ks) for specific metals. Ks = K[H+]a/F, where Ks is a constant characteristic of the system, K is the distribution ratio, H is the hydrogen ion concentration in the equilibrated aqueous phase, F is the concentration of extractant in the equilibrated organic phase, a and b are the respective hydrogen ion and extractant dependencies. Recent studies, to be published, involving the... 

Although the separation factors for the extraction of U(VI) and Th(IV) into DA[AP] and D(4-MPe-2)[BP] differ greatly (a factor of 28) due to the difference in steric properties of the extractants, the parameters of extractant dependencies, Figures 2 and 3, and HNO3 dependencies, Figure 4, show a parallel relationship for the two extractants. The apparent independence of these parameters from the steric properties of the extractant greatly simplifies the selection of an extractant and the specific concentrations of extractant and HNO3 required for a desired separation. 

The molecular descriptors for a CoMFA analysis number in the hundreds or thousands, even for datasets of twenty or so compounds. A multiple regression equation cannot be fitted for such a dataset. In such cases. Partial Least Squares (PLS) is the appropriate method. PLS unravels the relationship between log (1/C) and molecular properties by extracting from the data matrix linear combinations (latent variables) of molecular properties that best explain log (1/C). Because the individual properties are correlated (for example, steric properties at adjacent lattice points), more than one contributes to each latent variable. The first latent variable extracted explains most of the variance in log (1/C) the second the next greatest degree of variance, etc. At each step iP and s are calculated to help one decide when enough variables have been extracted—the maximum number of extracted variables is found when extracting another does not decrease x substantially. Cross-validation, discussed in Section 3.5.3, is commonly used to decide how many latent variables are significant. For example, Table 3.5 summarizes the CoMFA PLS analysis... 

Extractants are often used with modifiers (e.g., hydrophobic alcohols, alkylphenols, sterically hindered esters of carboxylic acids, and tributyl phosphate). Modifiers are used for two different reasons first, to increase the solubility of extractants and their complexes and to avoid the formation of the third phase, and second, to modify the extraction properties, i.e., the extraction and stripping abilities. The first option is usually exploited in systems that contain various amines. The second option is associated with hydro-xyoximes and the formation of tailored blends, which optimize the extraction-stripping properties and adjust them to the aqueous feed, i.e., to the acidity and concentration of copper(II). Modifiers that form hydrogen bonds with hydroxyoxime molecules decrease their extraction ability but facilitate back-extraction with acids. Weak hydroxyoxime or j -diketone extractants (e.g., 2-hydroxy-5-alkylbenzo-phenone oxime) can also act as modifiers of strong hydroxyoxime extractants (e.g., salicylaldoxime and its alkyl derivatives). 

Relationships connecting stmcture and properties of primary alkylamines of normal stmcture C, -C gin chloroform and other solvents with their ability to extract Rh(III) and Ru(III) HCA from chloride solutions have been studied. The out-sphere mechanism of extraction and composition of extracted associates has been ascertained by UV-VIS-, IR-, and H-NMR spectroscopy, saturation method, and analysis of organic phase. Tertiary alkylamines i.e. tri-n-octylamine, tribenzylamine do not extract Ru(III) and Rh(III) HCA. The decrease of radical volume of tertiary alkylamines by changing of two alkyl radicals to methyl make it possible to diminish steric effects and to use tertiary alkylamines with different radicals such as dimethyl-n-dodecylamine which has not been used previously for the extraction of Rh(III), Ru(III) HCA with localized charge. 

We have studied the extractant behavior of a series of compounds containing the carbamoylmethylphosphoryl (CMP) moiety in which the basicity of the phosphoryl group and the steric bulk of the substituent group are varied (10,LL). These studies have led to the development of extractants which have combinations of substituent groups that impart to the resultant molecule improved ability to extract Am(III) from nitric acid and to withstand hydrolytic degradation. At the same time good selectivity of actinides over most fission products and favorable solubility properties on actinide loading are maintained (11). 

Examples of the application of correlation analysis to diene and polyene data sets are considered below. Both data sets in which the diene or polyene is directly substituted and those in which a phenylene lies between the substituent and diene or polyene group have been considered. In that best of all possible worlds known only to Voltaire s Dr. Pangloss, all data sets have a sufficient number of substituents and cover a wide enough range of substituent electronic demand, steric effect and intermolecular forces to provide a clear, reliable description of structural effects on the property of interest. In the real world this is not often the case. We will therefore try to demonstrate how the maximum amount of information can be extracted from small data sets. 

All these methods have found applications in theoretical considerations of numerous problems more or less directly related to solvent extraction. The MM calculated structures and strain energies of cobalt(III) amino acid complexes have been related to the experimental distribution of isomers, their thermodynamic stability, and some kinetic data connected with transition state energies [15]. The influence of steric strain upon chelate stability, the preference of metal ions for ligands forming five- and six-membered chelate rings, the conformational isomerism of macrocyclic ligands, and the size-match selectivity were analyzed [16] as well as the relation between ligand structures, coordination stereochemistry, and the thermodynamic properties of TM complexes [17]. 

The influence of steric effects on the thermodynamic parameters of extraction has been discussed in numerous publications (see the review in Ref. 12 and references therein). For a small series of selected extractants, a decrease of log Kex as a function of the length of alkyl substituents has been discovered.6970 However, in other cases, this dependence passes through a maximum or is not regular at all.71 The point is that the increase in the volume and branching of the substituents leads to some changes of molecular properties decrease of solubility in the aqueous phase, increase of steric hindrance upon the complexation, decrease of the extractant s aggregation, etc.12 Some of these factors strengthen extraction, but others weaken it. 

Polyfunctional phosphinopyridine VV,P-dioxides, (phosphinomethyl)pyridine A,P-dioxides and bis(phosphinomethyl)pyridine VV,P,P-trioxides have been prepared, and selected coordination chemistry with actinide ions has been explored. The phosphinopyridine A,P-dioxides form bidentate chelates with uranyl and Th , and in the solid state these complexes display six-membered chelate rings that appear to be relatively sterically congested." " The solvent extraction properties of these ligands are not unique since they resemble the performance of trialkylphosphine oxides." ... 

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