Chemical reactions selectivity

This leads to the possibiUty of state-selective chemistry (101). An excited molecule may undergo chemical reactions different from those if it were not excited. It maybe possible to drive chemical reactions selectively by excitation of reaction channels that are not normally available. Thus one long-term goal of laser chemistry has been to influence the course of chemical reactions so as to yield new products unattainable by conventional methods, or to change the relative yields of the products. 

Zeolite chemistry is an excellent example of how a three-dimensional surface can alter the course of chemical reactions, selecting for one product out of a host of potential candidates. In addition to the many commercial applications that they have found, shape-selective zeolites have provided the basis for a rich new area of catalytic science and technology, one expected to spawn yet more materials, knowledge, and applications. 

Table 12.2 Consecutive Chemical Reactions Selection of the Best Grid Point Based on the Volume Criterion and Through the Use of the Information Index...

The chemical reactions selected for the proposed synthetic pathway will obviously depend on the structure of the target compound. However, a number of general considerations need to be borne in mind when selecting these reactions. 

Active components are responsible for the principal chemical reaction. Selection of the active component is the first step in catalyst design. As knowledge of catalytic mechanisms on various materials advances, methods for selection are becoming more scientific, if perhaps still empirical. These are examined in subsequent chapters. 

Cum, G., GaUo, R., Spadaro, A., and Galli, G., Effect of static pressure on the ultrasonic activation of chemical reactions. Selective oxidation at benzylic carbon in the liquid phase, J. Chem. Soc. Perkin Trans., 2, 375-383, 1988. 

FIGURE 8.1 Molecular-level overview of a catalytic chemical reaction. Selected elementary-like steps of the CO oxidation reaction CO adsorption, surface diffusion of CO, the TS of the C0 +0 surface reaction, the COj desorption, and various adsorbed species. 

Tannor D J and Rice S A 1985 Control of selectivity of chemical reaction via control of wave packet evolution J. 

The reactivity of size-selected transition-metal cluster ions has been studied witli various types of mass spectrometric teclmiques [1 ]. Fourier-transfonn ion cyclotron resonance (FT-ICR) is a particularly powerful teclmique in which a cluster ion can be stored and cooled before experimentation. Thus, multiple reaction steps can be followed in FT-ICR, in addition to its high sensitivity and mass resolution. Many chemical reaction studies of transition-metal clusters witli simple reactants and hydrocarbons have been carried out using FT-ICR [49, 58]. 

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. 

Deposition by chemical reaction is a vast field tliat cannot be surveyed in tire limited space here. Two particular examples have been selected because tliey illustrate tire close relation between fundamental surface chemistry research... 

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. 

More elaborate scheme.s can he envisaged. Thus, a. self-organizing neural network as obtained by the classification of a set of chemical reactions as outlined in Section 3,5 can be interfaced with the EROS system to select the reaction that acmaliy occurs from among various reaction alternatives. In this way, knowledge extracted from rcaetion databases can be interfaced with a reaction prediction system,... 

Optically inactive starting materials can give optically active products only if they are treated with an optically active reagent or if the reaction is catalyzed by an optically active substance The best examples are found m biochemical processes Most bio chemical reactions are catalyzed by enzymes Enzymes are chiral and enantiomerically homogeneous they provide an asymmetric environment m which chemical reaction can take place Ordinarily enzyme catalyzed reactions occur with such a high level of stereo selectivity that one enantiomer of a substance is formed exclusively even when the sub strate is achiral The enzyme fumarase for example catalyzes hydration of the double bond of fumaric acid to malic acid m apples and other fruits Only the S enantiomer of malic acid is formed m this reaction... 

Selectivity Selectivity is rarely a problem in molecular absorption spectrophotometry. In many cases it is possible to find a wavelength at which only the analyte absorbs or to use chemical reactions in a manner such that the analyte is the only species that absorbs at the chosen wavelength. When two or more species contribute to the measured absorbance, a multicomponent analysis is still possible, as shown in Example 10.6. 

If metallic electrodes were the only useful class of indicator electrodes, potentiometry would be of limited applicability. The discovery, in 1906, that a thin glass membrane develops a potential, called a membrane potential, when opposite sides of the membrane are in contact with solutions of different pH led to the eventual development of a whole new class of indicator electrodes called ion-selective electrodes (ISEs). following the discovery of the glass pH electrode, ion-selective electrodes have been developed for a wide range of ions. Membrane electrodes also have been developed that respond to the concentration of molecular analytes by using a chemical reaction to generate an ion that can be monitored with an ion-selective electrode. The development of new membrane electrodes continues to be an active area of research. 

Boron trifluoride catalyst may be recovered by distillation, chemical reactions, or a combination of these methods. Ammonia or amines are frequently added to the spent catalyst to form stable coordination compounds that can be separated from the reaction products. Subsequent treatment with sulfuric acid releases boron trifluoride. An organic compound may be added that forms an adduct more stable than that formed by the desired product and boron trifluoride. In another procedure, a fluoride is added to the reaction products to precipitate the boron trifluoride which is then released by heating. Selective solvents may also be employed in recovery procedures (see Catalysts,regeneration). 

Water Activity. The rates of chemical reactions as well as microbial and en2yme activities related to food deterioration have been linked to the activity of water (qv) in food. Water activity, at any selected temperature, can be measured by determining the equiUbrium relative humidity surrounding the food. This water activity is different from the moisture content of the food as measured by standard moisture tests (4). 

The use of state-selective chemistry has been an experimental tool to elucidate the dynamics of chemical reactions, but its appHcation to practical chemical process control to enhance yields of specific products is ia the developmental stage. 

The examples given above represent only a few of the many demonstrated photochemical appHcations of lasers. To summarize the situation regarding laser photochemistry as of the early 1990s, it is an extremely versatile tool for research and diagnosis, providing information about reaction kinetics and the dynamics of chemical reactions. It remains difficult, however, to identify specific processes of practical economic importance in which lasers have been appHed in chemical processing. The widespread use of laser technology for chemical synthesis and the selective control of chemical reactions remains to be realized in the future. 

The process temperature affects the rate and the extent of hydrogenation as it does any chemical reaction. Practically every hydrogenation reaction can be reversed by increasing temperature. If a second functional group is present, high temperatures often lead to the loss of selectivity and, therefore, loss of desired product yield. As a practical measure, hydrogenation is carried out at as low a temperature as possible which is stiU compatible with a satisfactory reaction rate. 

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