Extractants steric influences

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]. 

In a recent work (48), the influence of the variation of some factors, concerning the polymerization, on the steric composition of the polymer has been studied subjecting the obtained raw polymer to subsequent extractions with the following series of solvents employed at their boiling point ether, n-heptane and n-octane. 

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. 

Steinberg et al. (1987) studied the persistence of 1,2-dibromoethane (EDB) in soils and found that low amounts of the organic were released with time, particularly if EDB had not been freshly added to the soil (Fig. 6.3). They suggested that the slow release rate was due to EDB being trapped in soil micropores where release is influenced by extreme tortuosity and/or steric restrictions. It was estimated that based on a radial diffusion model, 23 and 31 years would be required for a 50% equilibrium in EDB release to occur from two Connecticut soils. The previous studies point out that while sorption of pesticides is usually rapid and often reversible in the laboratory, extraction from field soils is extremely slow and often requires multiple extractions or even chemical dissolution of the soil matrix. 

In certain applications, the quaternary ammonium salts have advantages over tertiary amines for actinide extractions. Quantitative extraction of the transplutonium elements from nitrate media by quaternary ammonium salts can be achieved with a lower aqueous-phase nitrate concentration than is required for tertiary amines thus, aluminum nitrate may be used instead of lithium nitrate. The separation factor between Am and Cm can be as high as three in a quaternary ammonium nitrate system (50). The effect of the length of the alkyl chain on Cm, Am, Bk, Cf, and Es extraction by alkyldioctylammonium nitrates suggests that steric factors substantially influence the extraction selectivity (51). A comparison of the extraction of tetravalent and hexavalent actinides by tetraheptyl ammonium nitrate shows that tetravalent ions are more easily extracted than hexavalent ions e.g.,... 

The influence of the acid pretreatment of BEA on ifs acfivity in the model reaction between 2-MN and AAN was analyzed in depth/ The contribution of the inner and outer surfaces of the catalyst was examined by considering the selectivity with respect to the bulky product 28 and the linear product 29, which are assumed to be formed on fhe outer and on both the inner and outer surfaces of fhe catalyst, respectively/ " It is shown that the production of EFAL species located in the micropores of BEA subjected to high heating rate calcination provokes the increase in the selectivity of the less hindered 29 because the formation of the bulky 28 is sterically hampered. Indeed, when the external surface of zeolite BEA is passivated by coating with amorphous silica, a significant increase in the selectivity of 29 is observed, and fhis resulf is a clear example of shape-selective acylation with zeolite catalyst. On the contrary, acid treatment increases the catalytic activity of fhe oufer surface due to the extraction of the catalytically active EFAL species out of the micropores, leading to the preferential formation of fhe bulky 28. 

The desorption kinetics can be divided into the fast and slow reactions. For the fast reaction, the rate of Cd desorption is generally in accord with the stability constants of Cd-extractant ligand complexes chloride > citrate > acetate > nitrate. For the slow reaction, the rate of Cd desorption is apparently influenced by the size of extractant molecules citrate > acetate > chloride > nitrate. A longer induction period for an extractant with a larger molecule is required for Cd desorption from the micropore surface. Therefore, stability constants of Cd-extractant ligand complexes, and steric factor of the molecular size of the extractants in relation to the pore size of the oxides merit attention in understanding the kinetics of Cd desorption. 

Steric hindrance in a porous structure may strongly influence ion selectivity. Moreover, the bulk physicochemical properties of the ions in water may play an important role in the ion extraction/separation process. To give an example, ion hydration, which is characterized by hydration entropy and enthalpy, is known to be involved in many specific ion effects in solution. 

Many investigators have shown that nonspecific reagents as diverse as calcium phosphate gel, EDTA, histidine, and nonspecific proteins activate succinoxidase preparations in otherwise unfavorable environments. The mechanism of the activation is not established, but it has been repeatedly suggested that the activators in some manner influence the steric orientation of components of the particulate succinoxidase. Another component of electron-transport systems has been implicated by Nason and Lehman. DPNH oxidation by a particulate fraction of rat muscle was found to be decreased by extraction of 10 per cent of the lipid with isooctane the activity was restored by addition of a-tocopherol (vitamin E) or the lipid extracted from muscle or bovine serum albumin. These lipids are able to reverse the inhibition of cytochrome c reduction caused by antimycin A. It has not been determined whether the tocoph-... 

The starting material for assembly of asar[ ]arene macrocycles is tetramethoxybenzene (12.31), which can be prepared from commercial dihydrojybenzoquinone (12.33) in abundant quantities. We subjected (Scheme 12.11) a mixture of tetramethojybenzene and paraformaldehyde to Friedel-Crafts alleviation conditions at 80 °C in chlorinated solvents, with BF3 OEt2 as the Lewis acid. Influenced by the prior work on the synthesis of pillar[n]arenes - where pillar[5]arene is formed primarily under similar reaction conditions - we were expecting this reaction mixture to form asar[5]ar-ene as the major product. To our surprise, we found, however, that the reaction mixture produced only asar[6]arene and not even a trace of asar[5]arene. It is most likely that the increased steric demand imposed on the macrocyclic framework by the two additional methojyl groups not present in pillar[ ]arenes is responsible for this striking difference in reactivity between the asar[ ]arene and pillar[ ]arene families of macrocycles. Soxhlet extraction of the crude reaction mixture with acetonitrile as the solvent was then used as a scalable method of purification to access pure asar[6]arene (12.32a) in bulk quantities. 

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