Site selection, factors affecting

Delle Site, A., Factors affecting sorption of organic compounds in natural sorbent/water systems and sorption coefficients for selected pollutants. A review, J. Phys. Chem. Ref. Data, 30, 187-439, 2001. 

Product distributions obtained on esterification of nucleosides and nucleotides under basic conditions throw further light on factors affecting selective reactivity. p-Toluenesulfonylation of adenosine 5 -monophosphate in aqueous alkali yielded exclusively (in 54-61% yield) the 2 -p-toluenesulfonate.107 Lack of reaction at HO-3 was attributed either to formation of a phosphoric p-toluenesulfonic anhydride, which sterically protected this hydroxyl group, or to the higher acidity of HO-2. It has been shown that the acidic site (with pKa 12.5) in adenosine is associated with the presence of both HO-2 and HO-3, as replacement of either of these by hydrogen, or of HO-2 by methoxyl, results in loss of this acidity.108 Inductive effects, or the sta-... 

Carbohydrates and glycosides recently emerged as a novel class of nucleic acid binding compounds. A detailed study of the factors affecting the site-selectivity of some recently discovered antitumor antibiotics has shed new light on the role that oligosaccharides may play in nucleic acid recognition. 

Figure 8.27 Approximate competitive range of current membrane nitrogen production systems. Many site-specific factors can affect the actual system selection...

Site selection is probably the activity with the single greatest potential cost impact. The site factors affecting cost can go well beyond those affecting capital cost some, like those listed below, would also impact production and marketing costs. 

The attainable enrichment and clean-up in SPE depend primarily on the selectivity and affinity of the sorbent for the selected target analyte or analytes, the sample load capacity for the analytes and the rate of mass transfer to and from the binding sites, the latter affecting the minimum desorption volume and thus the enrichment that can be obtained. Other factors of importance are the reproducibility of the recovery yields and the stability and reusability of the sorbent when online procedures are desired. For hydrophobic analytes satisfactory results are usually obtained using standard reversed phase sorbents. Thus hydrophobised silica (C8, Cl8), styrene-divinylbenzene copolymers (PS-DVB) and graphitised carbon black (GCB) are the conventional sorbent materials used in SPE (Fig. 15.2)... 

FIGURE 4.27 Illustration of the approximate competitive range of current membrane nitrogen production systems. Many site-specific factors can affect the actual system selection. (From Baker R., Membrane Technology and Applications, McGraw-HiU, New York, 2000. With permission.)... 

Typical reforming catalyst are bifunctional, with a metal function (Pt, Pt-Re, Pt-Ir, etc] and an acid function (chlorided alumina). During operation, along with the desired reactions, the deposition of carbonaceous material occurs over the metal and the acid sites (1). This coke is the most important factor affecting the lifetime of the catalyst. As a consequence of this deposition, the reactor temperature must be raised to maintain the same octane number In the reformate. The partially deactivated catalyst has different selectivity than the fresh one. This suggests that the coke is deposited to a different extent on the metallic and on the acid function, and that the more demanding reactions are preferentially deactivated (2). 

There are many problems still to be solved for this quite simple reaction. For the reaction to be reversible and fast the alkyls must be unstable, but the criteria for stability of different types of alkyls are not fully established. It must be noted that the alkyl group occupies only one coordination site, whereas in the transition state and in the hydridoalkene complex, two sites must be available, so that clearly a prime factor is coordinative unsaturation which allows decomposition of the alkyl by elimination of alkene. The factors affecting the direction of addition, or the selectivity in elimination from secondary alkyls to give cis- or inm -isomers, are similarly not well understood. For the first, it appears that the greater the polarity of the M—H bond in the direction Ma —H[Pg.787]

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