Degree of selectivity

This highly flexible process allows the best optimization of yields in desirable products and it features a high degree of selectivity. 

Nevertheless, chemists have been planning their reactions for more than a century now, and each day they run hundreds of thousands of reactions with high degrees of selectivity and yield. The secret to success lies in the fact that chemists can build on a vast body of experience accumulated over more than a hundred years of performing millions of chemical reactions under carefully controlled conditions. Series of experiments were analyzed for the essential features determining the course of a reaction, and models were built to order the observations into a conceptual framework that could be used to make predictions by analogy. Furthermore, careful experiments were planned to analyze the individual steps of a reaction so as to elucidate its mechanism. 

Ion-selective electrodes are available for the electro analysis of most small anions, eg, haUdes, sulfide, carbonate, nitrate, etc, and cations, eg, lithium, sodium, potassium, hydrogen, magnesium, calcium, etc, but having varying degrees of selectivity. The most successful uses of these electrodes involve process monitoring, eg, for pH, where precision beyond the unstable reference electrode s abiUty to deUver is not generally required, and for clinical apphcations, eg, sodium, potassium, chloride, and carbonate in blood, urine, and semm. 

As was the case for kinetic resolution of enantiomers, enzymes typically exhibit a high degree of selectivity toward enantiotopic reaction sites. Selective reactions of enaiitiotopic groups provide enantiomerically enriched products. Thus, the treatment of an achiral material containing two enantiotopic functional groups is a means of obtaining enantiomerically enriched material. Most successful examples reported to date have involved hydrolysis. Several examples are outlined in Scheme 2.11. 

Two factors of paramount importance in understanding the chemistry of metal-ammonia reductions are the acidity of the reaction medium and the relative rates of all reactions possible with a given combination of reagents. The control or appreciation of these factors permits one to achieve a certain degree of selectivity in metal-ammonia reductions in spite of the vigor of the reducing agents. 

The procedure of choice utilizes a mixture of the orthoformate, the corresponding alcohol and p-toluenesulfonic acid, with or without a diluent. Dimethoxypropane also gives good yields of 3-methoxy-A ethers with a high degree of selectivity in the presence of 17- and 20-ketones. " ... 

The results shown for compounds (26)-(29) and (36) suggest that a rather subtle balance of various factors has to be considered for a prediction of relative yields and structures of products. Although fission of the more highly alkyl substituted bond by a-cleavage proceeds with a high degree of selectivity in the primary photochemical step, the subsequent disproportiona-... 

The vapor-phase catalytic replacement of chlorine by fluorine with hydrogen fluoride as the fluorine source has been the subject of a number of patents for the synthesis of Freons or Genetrons This topic has been carefully reviewed in the literature [2, p 97ff] One advantage of using a catalyst with hydrogen fluoride is to allow some degree of selectivity in the displacement of a specific chlorine from... 

Other advances include the construction of seven- and nine-membered rings via the analogous [4-1-3] and [6-1-3] cycloadditions with dienes and trienes respectively. Heterocycles, such as tetrahydrofurans and pyrrolidines, are accessible using carbonyl compounds and imines as substrates. The following discussion is organized around these recent discoveries. It serves to illustrate the versatility and the high degree of selectivity which are some of the distinctive features of the Pd-TMM chemistry. 

The ends of polymer chains are often not representative of the overall chain composition. This arises because the initiator and transfer agent-derived radicals can show a high degree of selectivity for reaction with a particular monomer type (Section 3.4). Similarly, there is specificity in chain tennination. Transfer agents show a marked preference for particular propagating species (Section 6.2.2 and 6.2.3). The kinetics of copolymerization are such that the probability for termination of a given chain by radical-radical reaction also has a marked dependence on the nature of the last added units (Section 7.4.3). 

In the presence of a precious metal catalyst, aryl halides can undergo dehalo-dimerization to give biaryl products, with varying degrees of selectivity. The major byproduct of this reaction is usually the dehalogenated aryl compound. This type of chemistry is currently one of the very few viable means for the large scale preparation of biaryl compounds. 

As discussed earlier, selectivity is the consequence of the interplay between toxicokinetic and toxicodynamic factors. Some examples are given in Table 2.8, which will now be briefly discussed (data from Walker and Oesch 1983, and Walker 1994a,b). These and other examples will be described in more detail under specific pollutants later in the text. In the table, comparisons are made between the median lethal doses or concentrations for different species or strains. Comparisons are made of data obtained in lethal toxicity tests where the same route of administration was used for species or strains that are compared. The degree of selectivity is expressed... 

Figure 3.33 Experimental results for degree of selectivity and conversion for ethylene when using micro-channel reactors with different thicknesses of silver coatings [42].

Weatherley (1998) has discussed all the relevant aspects of the separation of low molecular weight biologically produced molecules by solvent extraction. A high degree of selectivity can be realized by careful selection of the solvent. Problems associated with the rheology of the broth, the presence of surfactants and solid materials needs to be recognized. There is a scope to consider intensified electrostatic contact for broth dispersion and separation. Examples covered in this treatise include penicillin G and cA-dihydrodiols. 

Using these differences in reactivity it is often possible to selectively hydroalumi-nate one of several double bonds present in a substrate. Some representative examples are given in Scheme 2-7 [14, 44, 45]. A high degree of selectivity in the final example is obtained by attaching a bulky BUjAl group to the alcohol functionality and thereby deactivating the terminal aUylic C=C bond. 

Immunosorbents have also found applicability in on-line SPE analysis. An antibody is immobilized on to a silica support and used as an affinity ligand to retain targeted analytes. Components not recognized by the antibody are not retained and some degree of selectivity is attained. Recoveries of 87-103% were obtained for atrazine, simazine, DEA, propazine, and terbuthylazine at the 0.2 xgL concentration level when using immunosorbent SPE (80 mg silica and 2 mg anti-atrazine and anti-chlortoluron antibodies) on-line with LC/APcI-MS however, this method is not applicable to DIA (0% recovery). This compound may be better retained when using an... 

Although the improved extraction kinetics also increase the concentration of coextractives in the final extract, some degree of selectivity can be achieved by careful selection of the solvent or solvents used. Matrix co-extractives may be removed, or at least partially removed, by placing a suitable sorbent, such as alumina, at the exit of the extraction cell to remove lipid co-extractives. Excellent recoveries of both polar and nonpolar pesticides from a wide range of foodstuffs have been reported. Specific applications include organophosphorus and A-methylcarbamate pesticides. 

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