Selectivity of a chemical reaction

It is not the catalytic activity itself that make zeolites particularly interesting, but the location of the active site within the well-defined geometry of a zeolite. Owing to the geometrical constraints of the zeolite, the selectivity of a chemical reaction can be increased by three mechanisms reactant selectivity, product selectivity, and transition state selectivity. In the case of reactant selectivity, bulky components in the feed do not enter the zeolite and will have no chance to react. When several products are formed within the zeolite, and only some are able to leave the zeolite, or some leave the zeolite more rapidly, we speak about product selectivity. When the geometrical constraints of the active site within the zeolite prohibit the formation of products or transition states leading to certain products, transition state selectivity applies. 

It may be mentioned that the concept of choosing a derivative with a particular detector in mind is quite frequently employed in residue analysis. And with the development of more diversified selective detectors, we are sure to see more of it. Thiophosphoryl derivatives of phenols for the flame photometric detector (59), nitrophenyl derivatives of amines and thiols (60) and brominated anilines for the EC detector (61), chloro-acetylated phenols for the microcoulometric detector (62), and many other examples (63) would be worth mentioning. The selectivity of a chemical reaction combined with the selectivity of a gas chromatographic detector can provide superior analytical eflBciency. 

As introduced in Chapter 10, Section 10.3.8.3, the selectivity of a chemical reaction may be increased above that predicted from its rate constant ratio if the reaction is run under conditions that cause the product to crystallize while the reaction is in progress. A related example is presented in Example 11-3 below. A common system that can be modified by crystallization is the classic consecutive-competitive reaction as follows ... 

Selection of a chemical reaction as a basis for the calculation and defining its stoichiometric coefficients... 

The selectivity of a chemical reaction is a very important criterion. Besides the chemo- and regioselectivity, the stereoselectivity, i.e. the favored or excluded formation of one or several stereoisomers in the course of a chemical reaction, plays an important role. If there is a formation of (S)- and (K)-enantiomers from a prochiral compound, an enantioselective reaction takes place. What are the reasons for the growing interest in enantioselective reactions and preparation of homochiral compounds Firstly, it is certainly the wish of the chemist to imitate the ability of nature by stereospecific synthesis in the laboratory. Secondly, there are some practical and economic reasons many natural products and a great number of synthetic drugs have a chiral structure and the enantiomers can differ markedly in their biological activity. Sometimes only one of the enantiomers exhibits the wanted optimal activity, while the other is less active or totally inactive, or even toxic. 

In this section we review the time-dependent wavepacket representation of photodissociation, photoabsorption, and resonance Raman scattering introduced by Heller and co-workers. Our approach to controlling the selectivity of a chemical reaction builds on concepts taken from all three of these areas, so an introductory review is appropriate before proceeding further. The reader is referred to Heller s beautiful review article1 and to the original literature2-4 for further details. 

Although information about the overall reaction can be obtained from knowledge of global parameters such as electronegativity and hardness, the reactivity of a particular site of a molecular species can be explained by local quantities such as electron density (p(rj), Fukui function (/(F)) [75], local softness [64], or local hardness [76,77], The dependence of these local quantities on reaction coordinate reflects the usefulness of these quantities in predicting the site selectivity of a chemical reaction. The most important local descriptor is the density p(F) itself, the basic variable of DFT [78], given as ... 

Effective control of chemo and steric selectivity of a chemical reaction lies in our ability to manipulate nano-environment of active sites. Due to difiGculty in manipulating the active site on the nanoscale and lack of fxmdamental nderstanding of the reaction mechanism, development of chemo- and stereoselective catalysts has relied heavily upon empirical studies. One successful xample of fine-tuning steric environment of the active site is the use of chiral diphosphite ligands to control the selectivity of styrene hydroformylation on Rh omplex catalysts (1-3). 

Recent Advances in Indirect Electrochemical Synthesis. Indirect electrochemical synthesis has tremendous potential as an environmentally benign procedure since the selectivity of a chemical reaction can be obtained without the production of toxic or hazardous byproducts. Two recent reviews (50, 57) exhaustively cover the field through the mid 1980 s. Before describing our results, we will briefly survey recent advances in the field. 

Effective stoichiometric factors in (2.8) characterize the selectivity of a chemical reaction (see Chapter 4). 

The conversion and selectivity of a chemical reaction can be improved by performing the reaction in an MR. However, according to the limitation of membrane application... 

Catalysts change reaction rates by promoting different mechanisms for the reactions. It is to be noted that even though a catalyst is not consumed in the reaction, it does take part in the chemical reaction but is not observed in the overall reaction. Catalysts activate molecules and reduce the activation energy necessary for reactions to occur. Catalysts do not change the state of equilibrium they only act to increase the rates at which the equilibrium state is attained. Catalysts can affect yield and selectivity of a chemical reaction because of their ability to change the reaction mechanism. 

However, when a bed does not contain internals of any sort, the movement of the bubbles in the bed is umestricted. As bubbles rise, they gradually increase in size and tend to move horizontally toward the center of the bed (Fig. 2). Much of the gas is short-circuited through the bubbles, which greatly limits the conversion and selectivity of a chemical reaction, especially for the Group B particles at high superficial gas velocities. 

As the previous paragraph recalls, the observation of non-monotonous behaviour with respect to time requires several conditions, in particular the selection of a chemical reaction "ad hoc". In the majority of cases, although not always( ), the reaction mechanism should include an intrinsic source of instability. In a schematic manner, we can visualize this as a feedback loop, such that the effect produced reinforces the cause provoking it. Autocalalysis is the best known and commonest example of such a situation, being of opposite nature to a law of moderation. It is not. 

The adsorption is actually the first step of a heterogeneous chemical reaction process and occurs on flat surfaces or porous solids or on smooth planes and films. Understanding this process is crucial to explain the activity and selectivity of a chemical reaction. It is a well-studied phenomenon, but initially it was differently interpreted. Berzelius [1] was one of the first to note that the adsorption is a process where the surface tension causes the condensation of gases in pores. He showed that the vapor pressure in a small drop is much larger than in the bulk fluid and proposed the following relation ... 

Selectivity (chemo-, regio-, and stereoselectivity) control is a key issue for organic synthesis. In addition to controlling the selectivity by developing appropriate catalytic systems, the selectivity of a chemical reaction could be controlled by the restriction of the reaction in a confined nanospace. For example, we discussed the enhancement of enantioselectivity by the pore confinement effect in Section 10.4.1. In this section, we will discuss the selectivity control of a chemical reaction by the isolation of the substrates and the restriction on the rotational and translational motions of the substrates in a confined nanospace. 

The diffusion of reactants and products in porous-material-based nanoreactor could be greatly affected by the surface properties, which in turn could influence the catalytic activity and even selectivity of a chemical reaction taking place in the confined nanospace. Generally, organic molecules are hydrophobic and the silica-based mesoporous nanopores are hydrophihc. The difficulty in the diffusion of reactants and products in hydrophobic nanopores may reduce the reaction conversions [123]. Thus, the surface hydrophobic modification of the nanopore may benefit fast diffusion of the substrate, which may, in turn, contribute to the improved activity. When a reaction involves incompatible substrates, such as oil and water, the amphiphilic modification of the nanopore microenvironment is a smart strategy because the amphiphilic nanopore should provide a microenvironment... 

This example illustrates a subtle control of a chemical reaction by a delicate manipulation of tire stereochemical environment around a metal centre dictated by tire selection of tire ligands. This example hints at tire subtlety of nature s catalysts, tire enzymes, which are also typically stereochemically selective. Chiral catalysis is important in biology and in tire manufacture of chemicals to regulate biological functions, i.e., phannaceuticals. 

Shape-selective zeolites can also be used to discriminate among potential products of a chemical reaction, a property called product shape selectivity. In this case, the product produced is the one capable of escaping from the zeolite pore structure. This is the basis of the selective conversion of methanol to gasoline over... 

Membranes in catalysis can be used to improve selectivity and conversion of a chemical reaction, improve stability and lifetime of the catalyst, and improve the safety of operation. The most well-known example is in situ removal of products of an equilibrium-limited reaction. However, many more ways of application of a membrane can be thought of [1-3], such as using the membrane as a reactant distributor to control the reactant concentration levels in the reactor, or performing catalysis inside the membrane and having control over reactant feed and product removal. 

This method is primarily concerned with the phenomena that occur at electrode surfaces (electrodics) in a solution from which, as an absolute method, through previous calibration a component concentration can be derived. If desirable the technique can be used to follow the progress of a chemical reaction, e.g., in kinetic analysis. Mostly, however, potentiometry is applied to reactions that go to completion (e.g. a titration) merely in order to indicate the end-point (a potentiometric titration in this instance) and so do not need calibration. The overwhelming importance of potentiometry in general and of potentiometric titration in particular is due to the selectivity of its indication, the simplicity of the technique and the ample choice of electrodes. 

The use of membranes in reaction processes can serve two different purposes (1) to enhance the productivity of a chemical reactor, i.e. by shifting the chemical equilibrium situation. (2) to influence the path of a chemical reaction, i.e. by affecting the reaction selectivity. 

It is very instructive to compare the kinetics and plausible mechanisms of reactions catalyzed by the same or related catalyst(s) in aqueous and non-aqueous systems. A catalyst which is sufficiently soluble both in aqueous and in organic solvents (a rather rare situation) can be used in both environments without chemical modifications which could alter its catalytic properties. Even then there may be important differences in the rate and selectivity of a catalytic reaction on going from an organic to an aqueous phase. TTie most important characteristics of water in this context are the following polarity, capability of hydrogen bonding, and self-ionization (amphoteric acid-base nature). 

The importance of catalyst stability is often underestimated not only in academia but also in many sectors of industry, notably in the fine chemicals industry, where high selectivities are the main objective (1). Catalyst deactivation is inevitable, but it can be retarded and some of its consequences avoided (2). Deactivation itself is a complex phenomenon. For instance, active sites might be poisoned by feed impurities, reactants, intermediates and products (3). Other causes of catalyst deactivation are particle sintering, metal and support leaching, attrition and deposition of inactive materials on the catalyst surface (4). Catalyst poisons are usually substances, whose interaction with the active surface sites is very strong and irreversible, whereas inhibitors generally weakly and reversibly adsorb on the catalyst surface. Selective poisons are sometimes used intentionally to adjust the selectivity of a particular reaction (2). 

Microscopic control of the outcome of a chemical reaction, the well-defined breaking (if not formation) of one or several selected bonds in a molecule, is a long-standing dream in chemistry. Many technological advances, above all the invention and development of lasers, have therefore been welcomed in the past as a possible decisive step toward its realization. 

We begin with a broad-stroke description of the different strategies for using quantum mechanical interference to control product selectivity in a chemical reaction. 

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