The concept of selectivity

Sometimes the desired reaction is the unlikely one, but we may still obtain a good selectivity by choosing a very specific catalyst that enhances the rate of the desired reaction relative to the undesired but more likely one with a sufficiently high factor. 

So in general we have to consider the kinetics of the various possible reactions, and we choose such reaction conditions that the highest selectivity is obtained. Still, even if undesired byproducts are produced in relatively small amounts, the technical consequences may be considerable. In general, each contaminant has to be removed from the main product in a separate purification step, irrespective of its relative amount. The byproduct may require additional treatment before it can be disposed of. In many instances it has to be converted by a chemical process to a compound that is not harmful. Therefore an accurate control of reaction selectivity is a very important aspect of chemical reactor development. 

We may also encounter a situation in which the desired reaction product itself 

In chemical literature four different selectivity parameters are used  

In considering the chemical kinetics of complex reaction systems the differential selectivities are directly relevant, but for reactor design the use of integral selectivities is often more practical. Note that for perfectly mixed continuous reactors differential and integral selectivities are the same. 

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It should be apparent that the principles of selective ion transport are independent of the specific models being treated here and that many of these principles are at variance with what were traditional views on the basis of selective membrane permeation by inorganic ions. Thus, the concept of selectivity among monovalent cations being based on values of hydrated radii is replaced by the... 

The key to minimizing waste is, as in other industries, to design more precision into organic synthesis. Chemists use the concept of selectivity as a measure of how efficiently a synthesis is performed. The standard definition of selectivity is the yield of product divided by the amount of substrate converted, expressed as a percentage. Organic chemists also distinguish among other forms of selectivity ... 

The concept of selectivity parameters has a physicochemical relevance, and it is proved experimentally that among solvents with similar functionality there is a great similarity with the selectivity parameters [42]. This fact is very important at the molecular level of the phenomena, and it is the best proof of the predominant role of functionality in intermolecular interactions of the solvent and solute, and the solvent and stationary phase. 

Selectivity and specificity are important performance characteristics of analytical procedures, especially in connection with validation processes. Nevertheless, both terms are used mostly verbal and a quantification is avoided, as a rule (IUPAC see Vessman et al. [2001]). Moreover, the concepts of selectivity and specificity are used interchangeably and synonymously. Occasionally, specificity is regarded as an intensification of selectivity, viz. the ultimate of selectivity (den Boef and Hulanicki [1983] Persson and Vessman [1998, 2001] Prichard et al. [2001]). 

The concept of selectivity and specificity has been applied to characterize interferences appearing in two different ICP-MS techniques (Horn [2000]). Classical ICP-MS with pneumatic nebulization and ETV-ICP-MS are compared for the determination of traces of zinc in sea-water. Whereas spectral interferences decrease using the ETV device, nonspectral interferences increase significantly (Bjorn et al. [1998]). A quantitative comparison of the both analytical procedures, here called PN (pneumatic nebulization) and ETV (electrothermal vaporization, Sturgeon and Lam [1999]) is possible by means the specificity as a function of the Zn concentration (Horn [2000]). The spectral interferences on the four zinc isotopes are listed in Table 7.4. 

As expected, the concept of selectivity and specificity is closely related to that of sensitivity. The same may be anticipated for the concept of robustness and ruggedness. 

When the target molecule bears several chiral centres, the most obvious simplification in the retrosynthetic analysis is to eliminate all the chiral or stereogenic centres and then to design the synthesis with as much stereoselectivy as possible (for the concept of "selectivity" see below 8.2). However, as pointed out by Corey... 

The concept of "selectivity" must be clearly distinguished from the term "specificity" [4][5], Specific, applied to a reaction, means that two (or more) isomeric starting materials give -under the same reaction conditions- different reaction products which are also isomers. Depending upon the isomers we may be considering, we may refer to "regiospecificity" (structural isomers) or to "stereospecificity" (either diastereospecificity or enantiospecificity). For instance, the formation of we5o-2,3-dibromobutane by addition of bromine to ( )-2-butene, in contrast with the formation of the d,l - 2,3-dibromobutene from the (Z)-2-butene, is a case of diastereospecificity. 

In contrast with the concept of "selectivity", "specificity" does not admit degrees a reaction is or is not "specific". Diels-Alder cycloadditions, for instance, are "diastereospecific" in the sense that two diastereomeric dienophiles (the Z and the E isomers) react with the same diene, under similar reaction conditions, to give two diastereomeric adducts. Although specific reactions are always selective, the reverse is not true. 

The concept of alternative reaction pathways leads to the concepts of selectivity and stereospecificity in hydrogenation catalysis. 

This form of selectivity is based on the concept of selectively blocking the access of all interfering species to the active region of the transducer. It is a form of filtration. The blockage can be achieved by size discrimination. For instance, a dialysis... 

In general, relative values of rate constants for all pathways from a single reactant are obtained by product analysis, which relates to the concept of selectivity (see Chapter 2). Selectivity in the present context may be defined as the ratio of the rate of formation of one component to the rate of formation of another and, in the simple example of Scheme 4.2, the selectivity of A for the formation of B rather than C is given by Equation 4.6 ... 

The concept of selectivity is most commonly encountered (and most useful in mechanistic investigations, see Chapter 2) when a reactant or a reactive intermediate has alternative bimolecular routes it is then also very useful in yield optimisation in chemical process development [12]. The reaction in Scheme 4.3 involves an electrophilic intermediate (X) which is captured by nucleophilic reagent C (which could be solvent). If another nucleophilic species (D) is added to the reaction mixture, the additional product D—X is formed in competition with C—X. If Ap is known (e.g. if D is known to react with electrophiles at the diffusion limit), then values for [D], [C] and the product ratio [C—X]/[D—X] allow determination of kc, i.e. quantitative information about the reactivity of X with C, and information about the selectivity of X in reactions with nucleophiles. 

In order to overcome this drawback, the concept of selective blending was exploited. Selective blending of PPE with low-viscous PS allowed one to control the microstructure, to refine the phase size, and to adjust the foaming characteristics of the individual phases of PPE/SAN blends. Appropriate blend compositions allowed simultaneous nucleation and cooperative expansion of both phases, generally leading to bimodal cell size distributions in the micron range. Due to cell nucleation and growth in both blend phases, the density could be further reduced when compared to PPE/SAN blends. Moreover, the presence of coalesced foam structure and particularly macroscopic defects could be avoided, and the matrix of the foamed structure was formed by the heat resistant PPE/PS phase. 

There are a number of reasons to try to characterize dopamine receptor in addition to the fact that it is fashionable. Two of the major reasons well described by Goldberg and Kohli are (1) the importance of dopamine receptors subtypes responsible for physiological and pharmacological actions and (2) the utilization of the concept of selective dopamine receptors in the design and characterization of new drugs for cardiovascular and renal therapies. 

Excluded from this list is sieving, to which the concept of selectivity is not applicable. For completeness, we have subdivided the FFF family into sedimentation FFF, thermal FFF, flow FFF, and steric FFF to show how the selectivity of each of these subtechniques compares to that of the other fractionation methods. The values reported here differ from S values reported elsewhere (12), which refer to mass rather than size selectivity. 

When k values have been determined, the concept of selectivity can be introduced and a separation factor calculated. The degree of separation between two band centers after elution is called the selectivity or separation factor. It is the ratio of the k values defined by equation (8-6) ... 

In industrial chemical operations, a reactant A or reactants A and B very often react to form not only desired products, but also undesirable ones. Therefore, in chemical reactor technology the concepts of selectivity and yield are often used. 

The concept of selective site occupancy is useful, and will be seen to have a fairly general applicability. Indeed, the way in which the enthalpy of dissolved H is affected by the environment of the absorption site needs to be investigated further. 

The presence in an organic structure of several functions of similar reactivity toward the same reagent led organic syn-thesists to the development of the concept of selective protection and deprotection. Numerous protecting groups have been successfully used, but all of them must obey the following criteria ... 

The concept of selective sequestration of non-product species was first demonstrated using solid-supported scavengers with electrophilic and nucleophilic character in amine acylation, amine alkylation, and reductive amination protocols [46]. Since then, a wide range of scavenger reagents has become commercially available from various suppliers. The structures and functions of these scavenger resins are shown in Table 1. 

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