Selectivity relations

Most of these results have been obtained in methanol but some of them can be extrapolated to other solvents, if the following solvent effects are considered. Bromine bridging has been shown to be hardly solvent-dependent.2 Therefore, the selectivities related to this feature of bromination intermediates do not significantly depend on the solvent. When the intermediates are carbocations, the stereoselectivity can vary (ref. 23) widely with the solvent (ref. 24), insofar as the conformational equilibrium of these cations is solvent-dependent. Nevertheless, this equilibration can be locked in a nucleophilic solvent when it nucleophilically assists the formation of the intermediate. Therefore, as exemplified in methylstyrene bromination, a carbocation can react 100 % stereoselectivity. 

Figure 2. Comparison of initial rates and selectivities (related to olefin and peroxide) for the epoxidation of a-isophorone with t-butyl hydrojjeroxide at 60 C over (a) low-temperature aerogel (LT) (20 wt% Ti02), (b) titania-on-silica, (c) TS-1 and (d) xerogel.

X Selectivity should not be merged with specificity selectivity relates to multicomponent analysis and specificity to single component analysis... 

If two independent variables are involved in the model, plots such as those shown in Figure 2.5 can be of assistance in this case the second independent variable becomes a parameter that is held constant at various levels. Figure 2.6 shows a variety of nonlinear functions and their associated plots. These plots can assist in selecting relations for nonlinear functions of y versus x. Empirical functions of more than two variables must be built up (or pruned) step by step to avoid including an excessive number of irrelevant variables or missing an important one. Refer to Section 2.4 for suitable procedures. 

In the 1940s and 1950s, a considerable amount of research was funded to find and develop the chemists impossible dream a process for the direct oxidation of ethylene to EO, without any by-products. Finally, Union Carbide found the silver bullet that did the joE)—a catalyst made of silver oxide. Silver oxide is the only substance found having sufficient activity and selectivity. (Activity relates to the amount of conversion, selectivity relates to the right yield.) Moreover, ethylene is the only olefin affected in this way. The others, propylene, butylene, etc., tend to oxidize completely, forming carbon dioxide and water. But when silver oxide is used as a catalyst with ethylene, the dominant reaction is the formation of EO. Some ethylene still ends up being further oxidized, as much as 25% in some processes, as shown in Figure 10—2. 

The differences in the way octane and coke selectivity relate to the unit cell are illustrated in Figure 1. 

As in previous conferences, the section on catalysis contains the most papers. A general review of the different reactions which can be catalyzed by zeolites is presented by Kh. M. Minachev. H. W. Kouwenhoven discusses the isomerization of paraffins on zeolites. Cracking, isomerization, and electron transfer reactions are discussed in several papers. Correlations between particular activities and physicochemical properties are covered. Selectivities related to crystal size and molecular shapes are also studied. Most of the work is still done on modified Y zeolites, but mor-denite and erionite also receive attention. 

The selectivity relation holds very well for hydrogen exchange reactions of toluene with acids (Shatenshtein, 1958, 1960b). The straight line in Fig. 6 represents the equation... 

Hits from the screen were used to select related compounds for further testing from the Lilly collection of compounds [33], The HPLC assay was used to measure MurD inhibition and standard antimicrobial assays were carried out with S. pneumoniae, Haemophilus influenzae 76, Staphylococcus aureus 027, and Moraxella catarrhalis BC-1. A series of indole and phenoxypropyl amine derivatives with activity against MurD, were discovered that exhibited broad-spectrum antibacterial activity. 

Shape selectivity related to molecular concentration in zeolite micropores... 

Provides a guide to selected related literature, particularly in the area of commercial application... 

There are two distinct selectivities configurational selectivity relates to syn vs. anti addition regio (or directio) selectivity is concerned with 1-Nu, 2-E+ vs. 1-E+, 2-Nu addition. The resulting orientations will be labelled specific (0 or 100%) when one product or process is exclusive otherwise they may range from highly selective to non-selective (>0 or < 100%) - A reaction will be termed stereoconvergent if the same composition of product isomers is obtained on two or more reaction paths. 

Excel Tip. Sometimes it s difficult to select a chart element by clicking on it (for example, if two chart elements are almost superimposed). Instead of selecting with the mouse pointer, you can use the up and down arrows on the keyboard to select chart elements. This allows you to select each chart element in turn (Chart, Plot, Axis, Series 1, etc.) the name of the selected chart element is displayed in the reference area of the formula bar. By using the left and right arrows, you can select related chart elements within a group (e.g.. Series 1 Point 1, Series 1 Point 2, etc.). 

These studies have demonstrated that the homogeneously catalyzed oxychlorination of ethylene to 1,2-dichloroethane is feasible under relatively mild operating conditions without the need for catalyst circulation and that selectivities better than 96% can be achieved in sustained operations. A rational basis for reactor design has been devised, taking into account such factors as ethylene- and oxygen-mass transfer rates, chemical kinetics, water vapor pressure over the catalyst, selectivity relation-... 

Idriss H, Kim KS, Barteau MA (1991) Structure-activity and structure-selectivity relations for reactions of carboxylic adds on TiO (001) surfaces. Stud Surf Sci Catal 64 327... 

Fig, 3,53 Chemical structures of glutamic acid, MSG, and selected related substances... 

An opposite asymmetric induction (rt-facial selectivity related to the stereogenic center) to that found above for the fluoroalkene from the si-2si face (top face) was observed in the cyclo-... 

Advances in the understanding of structure-activity/selectivity relations for enzymes evolving from the use of x-ray, NMR, and other instrumental methods for characterization of enzyme structures should contribute to the development of improved immobilized enzyme systems for both analytical and industrial applications. Immobilized enzyme technology has enormous potential, but significant advances on several fronts are necessary prior to widespread industrial use of this technology. Katchalski-Katzir has discussed this problem in a review of past successes and failures in efforts to employ immobilized enzymes in the food, pharmaceutical, and chemicals industries. ... 

Several approaches have been investigated. Marcotte et al. [40] deduce from the frequency of occurrences of certain words in the abstract whether a paper discusses a certain protein-protein interaction. Shatkay et al. [41] analyze the similarity of direction of different documents by providing models for a scientific topic. They train a stochastic model describing the frequency of occurrences of certain words on a given set of documents based on the scientific topic in question. Then they select related literature based on this model. Other systems go beyond counting words. Some systems invoke rules based on the structure of simple sentences [42], More involved language parsing is performed in [43], [44],... 

Several solvents have been tested in the epoxidation of a- isophorone with t-butyl hydroperoxide (TBHP). The best performance of the aerogel was observed in low polarity solvents such as ethylbenzene or cumene (Table 1). In these solvents 99 % selectivity related to the olefin converted was obtained at 50 % peroxide conversion, independent of the temperature. Rasing temperature resulted in increasing initial rate and decreasing selectivity related to the peroxide. The low peroxide efficiency is explained by the homol5d ic peroxide decomposition. Protic polar solvents were detrimental to the reaction due to their strong coordination to the active sites. There was no epoxide formation in water. 

Preliminary experiments revealed that the selectivity of titania-silica aerogel in the epoxidation of P-isophorone was moderate. The selectivity related to the olefin converted was below 90 % at low temperature, and dropped rapidly at 80 °C or above. The most important side reactions were the formation of 3,5,5-trimethyl-2-cyclohexene-4-hydroxy-l-one (2) by ring opening of the epoxide (1), and the isomerization of P- to a-isophorone (3), as shown in Scheme 2. Epoxidation of 2 and 3, and the oxidation at the OH group of 2 to a dicarbonyl compound were slow and the amounts of these by-products were usually aroimd 1 % or less. 

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