The Significance of Selectivity

Certain limitations were noted however. First, the plot did not predict the zero value of selectivity for highly unstable ions, expected from the reactivity- selectivity principle. Secondly, as discussed on p. 77, solvolytic rate constants are only a very approximate measure of substrate reactivity. Furthermore, the possibility exists of solvent sorting, in which the concentration of potential nucleophiles around the reaction intermediate differs from that in the bulk solution. Such a phenomenon would, of course, seriously diminish the significance of selectivity as a measure of reactivity, and, while such a possibility... 

When the carbonization process is divided into its distinct physical and chemical parts and both are considered according to their contributions to the overall process, only then is a description of the mechanism possible. Carbon precursors and the products of their carbonization are characterized by various test methods whose objectives can be the control of coking, a description of the carbon or the determination of its suitability for further application. This paper considers the significance of selected common characterization procedures. 

We here touch upon a subject of difficulty and violent controversy the significance of selective advantage. Selection will favour the increasing perfection of Batesian mimicry since, other things being equal, those individuals which best resemble their model will contribute most to posterity but it will also ensure that the quality selected for will cease to be an asset. For any increase in the relative numbers of a mimic compared with its model, whether due to more perfect deception or other cause, can only be carried up to the point of equipoise, that at which the value of the resemblance is balanced by making a species conspicuous when it is in fact, relatively palatable and defenseless. ... 

Studies on solvent effects on the endo-exo selectivity of Diels-Alder reactions have revealed the importance of hydrogen bonding interactions besides the already mentioned solvophobic interactions and polarity effects. Further evidence of the significance of the former interactions comes from computer simulations" and the analogy with Lewis-acid catalysis which is known to enhance dramatically the endo-exo selectivity (Section 1.2.4). 

The scientific method, as mentioned, involves observation and experimentation (research) to discover or establish facts. These are followed by deduction or hypothesis, establishing theories or principles. This sequence, however, may be reversed. The noted twentieth-century philosopher Karl Popper, who also dealt with science, expressed the view that the scientist s work starts not with collection of data (observation) but with selection of a suitable problem (theory). In fact, both of these paths can be involved. vSignificant and sometimes accidental observations can be made without any preconceived idea of a problem or theory and vice versa. The scientist, however, must have a well-prepared, open mind to be able to recognize the significance of such observations and must be able to follow them through. Science always demands rigorous standards of procedure, reproducibility, and open discussion that set reason over irrational belief. 

With the more concentrated solution the results, as regards loss of intermolecular selectivity, were similar to those obtained with nitronium salts (table 4.1, column a), whilst with the more dilute solution a more usual situation was revealed. The significance of the former observations is again open to doubt because of the likelihood that mixing was relatively slow, and also because reaction upon encounter is here a serious probability. 

The advantages of selective removal of fines from a batch crystallizer have been demonstrated (66,67). These experimental programs showed narrowiag of crystal size distributions and suggest significant reductions ia the fraction of a product that would consist of fines or undersize material. 

The relative propensity of radicals to abstract hydrogen or add to double bonds is extremely important. In radical polymerization, this factor determines the significance of transfer to monomer, solvent, etc. and hence the molecular weight and end group functionality (Chapter 6). It also provides one basis for initiator selection (Section 3.2.1). 

Discuss the significance of the selectivity coefficient of an ISE. How would you determine its value ... 

COX-2 in asthma pathogenesis and its significance in the use of selective inhibitors. Clin Exp Allergy 2002 32 339-342. 

In the case of selective oxidation catalysis, the use of spectroscopy has provided critical Information about surface and solid state mechanisms. As Is well known( ), some of the most effective catalysts for selective oxidation of olefins are those based on bismuth molybdates. The Industrial significance of these catalysts stems from their unique ability to oxidize propylene and ammonia to acrylonitrile at high selectivity. Several key features of the surface mechanism of this catalytic process have recently been descrlbed(3-A). However, an understanding of the solid state transformations which occur on the catalyst surface or within the catalyst bulk under reaction conditions can only be deduced Indirectly by traditional probe molecule approaches. Direct Insights Into catalyst dynamics require the use of techniques which can probe the solid directly, preferably under reaction conditions. We have, therefore, examined several catalytlcally Important surface and solid state processes of bismuth molybdate based catalysts using multiple spectroscopic techniques Including Raman and Infrared spectroscopies, x-ray and neutron diffraction, and photoelectron spectroscopy. 

This agrees with industrial data reported by Roprs (1989) the amount of heat exchanged in a 50 litres reactor was estimated to be 0.2 W kg k while that for a 22.5 m reactor is only 0.04-0.08 W kg K. The significance of the influence of the reactor size on heat transfer, and in turn on the yield and selectivity, is illustrated by Example 53.1.4. 

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