Selectivity effects on

Chaudhary et al. found that the pressure of fluoroform had an effect on the substrate specificity of subtilisin Carlsberg protease in transesterifications. Specificity towards methanol over propanol increased considerably with a pressure increase from 75 to 200 bar. This is clear evidence that nucleophilic specificity can be altered by changing the SCF conditions [49]. 

Michor et al. investigated four lipases and an esterase in the stereospecific transesterification of racemic ( )-menthol and ( )-citronellol in SCCO2. The transesterification of racemic menthol was reasonably fast and gave high 

Kamat et al. found a pressure effect on the stereoselectivity of transesterifications in supercritical fluoroform [64]. The stereoselectivity of subtilisin Carlsberg and Aspergillus proteases increased, while the activity decreased with increasing fluoroform pressure from 66 to 352 bar. They conclude that the stereoselectivity of these proteases increases because fluoroform becomes more hydrophilic as the pressure rises. This finding is in accordance with a previously presented hypothesis for substrate binding in enzymes. 

stereoselectivities from 0% to 100% have been reported for enzymatic reactions in SCFs. These results show that enzymatic stereoselectivity can be altered by adjusting the properties of the SCF. However, only very qualitative rules exist at present with which to predict how reaction selectivities might change with conditions for a new substrate/enzyme/SCF system. 

A few examples of increased selectivity can be found in the literature [85-90] where the steric course, and the chemo- or regioselectivity of reactions can be altered under the action of microwave irradiation compared with conventional heating. They have been listed and discussed in a recent review by De la Hoz et al. [91] and are discussed in Chapter 5 of this book. The main problem when trying to attribute accurately any MW effect lies in the fact that the kinetic control of reactions is not ensured. Of course, the only serious conclusions need to eliminate the thermodynamic control of reactions which is, unfortunately, highly probable when high temperatures are concerned. 

Langa et al. [48, 94, 95], while performing cycloaddition of N-methylazomethine ylide with C70 fullerene, proposed a rather similar approach. Theoretical calculations predict an asynchronous mechanism, suggesting that this phenomenon can be explained by considering that, under kinetic control, microwave irradiation will favor the more polar path corresponding to the hardest transition state . 

Two representative examples from our laboratory will be described below. The first concerns the aromatic nucleophilic substitution (SnAc) of potassium methox-ide with either activated (p-nitrophenyl) or nonactivated (a-naphthyl) aromatic 

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Each olefin is more soluble than the paraffin of the same chain length, but the solubiHty of both species declines as chain length increases. Thus, in a broa d-boiling mixture, solubiHties of paraffins and olefins overlap and separation becomes impossible. In contrast, the relative adsorption of olefins and paraffins from the Hquid phase on the adsorbent used commercially for this operation is shown in Figure 2. Not only is there selectivity between an olefin and paraffin of the same chain length, but also chain length has Httie effect on selectivity. Consequentiy, the complete separation of olefins from paraffins becomes possible. 

For rubbeiy membranes (hydrophobic), the degree of swelling has less effect on selectivity. Thus the permeate pressure is less critical to the separation, but it is critical to the driving force, thus flux, since the vapor pressure of the organic will be high compared to that of water. 

The gas approximates plug flow except in wide columns, but the liqiiid undergoes considerable oa mixiug. The latter effect can be reduced with packing or perforated plates. The effect on selectivity may become important. In the oxidation of hquid /i-butane, for instance, the ratio of methyl ethyl ketone to acetic acid is much higher in plug flow than in mixed. Similarly, in the air oxidation of isobutane to tei t-huty hydroperoxide, where te/ t-butanol also is obtained, plug flow is more desirable. 

The initial investigation focused on the use of threitol-derived auxiliaries with various substituent groups on the dioxolane ring (Table 3.3). However, it became evident that the oxygen atoms in the substituents had a detrimental effect on selectivity. Comparison of the diastereoselectivities for the ketals 69-71, which contain Lewis basic sites in the substituents at the 1 and 2 positions, with those from simpler diol derived ketals 72-74 demonstrates the conflicting effects of numerous coordination sites. The simpler, diol-derived ketals provide superior results compared to the threitol derived ketals. The highest diastereoselectivity is observed in the case of the 1,2-diphenyl ethane-1,2-diol derived ketal 74. 

This is because the increased turbulence from higher flow rates decreases the possibility for inclusion complexation, a necessary event for chiral recognition in reversed phase. Some effect has also been observed in the new polar organic mode when (capacity factor) is small (< 1). Flow rate has no effect on selectivity in the typic normal-phase system, even at flow rates up to 3 inL miir (see Fig. 2-11). 

Calo et al. (ref. 5) studied solvent effects on selective bromination of phenol with NBS and found the selectivity of bromination depended on the polarity of the solvents. But thereafter no investigation concerning the solvent effects was reported. We report the effects systematically. 

Selected clay stabilizers are shown in Table 1-10. Thermal-treated carbohydrates are suitable as shale stabilizers [1609-1611]. They may be formed by heating an alkaline solution of the carbohydrate, and the browning reaction product may be reacted with a cationic base. The inversion of nonreducing sugars may be first effected on selected carbohydrates, with the inversion catalyzing the browning reaction. 

Not surprisingly, PSD of the support had a significant impact on the hyam activity of the catalyst. The most active catalyst, A, had the slowest filtration rate. Also, by adding a small amount of Pt an improvement in activity with minimal effect on selectivity could be achieved. Lastly, by a slight modification in the preparation method, and/or by the addition of Fe the already very active catalysts could be even further improved. 

Li, J. and Carr, P.W., Evaluation of temperature effects on selectivity in RPLC separations using polybutadiene-coated zirconia, Anal. Chem., 69 (11), 2202,... 

Changing the organic component of the mobile phase can sometimes have a profound effect on selectivity. Fig. 3.1g shows a case where the order of elution is reversed when the mobile phase is changed from methanol/water 50 50 to tetrahydrofuran/water 25 75 ... 

Nickel effects on selected aquatic plants and animals... 

Atrazine effects on selected species of terrestrial plants... 

Diflubenzuron effects on selected aquatic organisms laboratory studies... 

Benzo[a]pyrene effects on selected aquatic vertebrates... 

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