Reactants and Reagents

Variations in the structure of the reactant and reagent may have a marked influence on the course of a reaction. 

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Not so for synthesis in the chemical industry where a compound must be prepared not only on a large scale but at low cost There is a pronounced bias toward reactants and reagents that are both abundant and inexpensive The oxidizing agent of choice for example in the chemical industry is O2 and extensive research has been devoted to develop mg catalysts for preparing various compounds by air oxidation of readily available starting materials To illustrate air and ethylene are the reactants for the industrial preparation of both acetaldehyde and ethylene oxide Which of the two products is ob tamed depends on the catalyst employed... 

All reactants and reagents were obtained from Sigma-Aldrich and used without purification. 

Thermal solid-state reactions were carried out by keeping a mixture of powdered reactant and reagent at room temperature or elevated temperature, or by mixing with pestle and mortar. In some cases, the solid-state reactions proceed much more efficiently in a water suspension medium or in the presence of a small amount of solvent. Sometimes, a mixture of solid reactant and reagent turns to liquid as the reaction proceeds. All these reactions are called solid-state reactions in this chapter. Solid-state reactions were found to be useful in the study of reaction mechanisms, since it is easy to monitor the reaction by continuous measurement of IR spectra. 

Predict the products of the following reactions on the basis of the reaction mechanism and anticipated transition structure. Be sure to consider all elements of stereochemistry. Unless otherwise specified, the reactants and reagents are racemic. 

Standard reactant and reagent loading protocols are necessary to produce samples that behave reproducibly. The level of hydration for example, if not precisely controlled, can lead to different experimental observations on otherwise identical... 

Reactants and reagents can be conveniently loaded into the dry zeolite by adsorption. This can be accomplished by intimately mixing the solid or liquid reactant and the powdered zeolite, by absorption from the gas phase, or by diffusion in a solvent slurry containing the zeolite and dissolved reactant. The choice of solvent for the slurry method is critical. It must be volatile enough to be removable at a pressure and temperature that does not result in evacuation of the reactant or its decomposition. In addition, the reactant must have a greater affinity for the interior of the zeolite than for the slurry solvent itself. The lack of affinity for the interior of the zeolite is an acute problem for non-polar hydrocarbons that lack binding sites for the intrazeolitic cations. The use of fluorocarbons such as perfluorohexane as slurry solvents takes advantage of the fluorophobicity of many hydrocarbons and has alleviated this problem to some extent.29... 

In most, if not all, of the transition metal-catalyzed organometallic reactions including the Pd- or Ni-catalyzed cross-coupling, solvents are not used merely to dissolve and dilute reactants and reagents. They often serve as cocatalysts or promoters and even reactants in a limited number of cases. More than 20 solvents have been used for the Pd- or Ni-catalyzed cross-coupling (Table 2). In the absence of any specific information, it is not unreasonable to choose first THF. Frequently, it is desirable to use one or more cosolvents typically for an increased level of solvent polarity and/or electron-donating ability. One of the most frequently used solvents for this purpose is DMF, but some others, such as NMP, pyridine and NMI (A-methylimidazole), have also been used frequently. In some extreme cases,... 

Excesses of solution-phase reactants and reagents can be used to drive reactions to completion these reagents can then be washed away from polymer-bound intermediates. 

In the third group, pericyclic reactions, there are no intermediate ions or radicals, but the reactant and reagent orientate themselves so as to form a cyclic transition state (usually, but not necessarily, six-membered in type) around which there is an electron flow leading to the formation of the new bonding arrangement [e.g. (iii)]. 

The term stereoselective is often confused with the term stereospecific, and the literature abounds with views as to the most satisfactory definition. To offer some clarification, it is perhaps timely to recall a frequently used term, introduced a decade or so ago, namely the stereoelectronic requirements of a reaction. All concerted reactions (i.e. those taking place in a synchronised process of bond breaking and bond forming) are considered to have precise spatial requirements with regard to the orientation of the reactant and reagent. Common examples are SN2 displacement reactions (e.g. Section 5.10.4, p. 659), E2 anti) elimination reactions of alkyl halides (e.g. Section 5.2.1, p.488), syn (pyrolytic) elimination reactions (Section 5.2.1, p.489), trans and cis additions to alkenes (e.g. Section 5.4.5, p. 547), and many rearrangement reactions. In the case of chiral or geometric reactants, the stereoisomeric nature of the product is entirely dependent on the unique stereoelectronic requirement of the reaction such reactions are stereospecific. 

A stereoselective reaction on the other hand is one in which the stereo-electronic requirement of the reaction mechanism is such that two equally valid alternative pathways are available for the same mechanistic interaction between reactant and reagent. However, either the free energies of activation of the alternative reactions or the thermodynamic stabilities of the products differ, so that one isomer is formed in preference to the other selection has occurred. An example is provided by the reduction of cholestan-3-one (32). Equatorial attack (i) or axial attack (ii) of the hydride ion is mechanistically equally feasible and stereoelectronically defined. However, steric interactions between the hydride ion source and the conformationally fixed steroid molecule, together with considerations as to whether the reaction was under kinetic or thermodynamic control, would determine that the reaction is proceeding in a stereoselective manner. 

A recognition of the concept of stereoselectivity has led the way to a better understanding of the selection of reactants and reagents in order to introduce chirality into a symmetrical molecule, i.e. an asymmetric synthesis. 

Enzymology has its own terminology. Reactants and reagents are generally referred to as substrates. Some enzymes only function when bound to additional, non-protein species. These helpers are called cofactors. The combination enzyme + cofactor is called a holoenzyme, while an enzyme that is missing its cofactor is called an apoenzyme. Cofactors can be either inorganic (e.g., metal ions) or organic. If... 

The use of molecular switches as components in the circuits of the future is attractive since one kg of molecules could contain 1023 or more components, depending on the molecular weight of the molecule. This is an incredibly large number of components, more than the number of solid-state transistors ever produced in the history of the semiconductor industry. The chemistry used to produce these molecules can be carried out in standard chemical plant reactors using well-characterized reactants and reagents. 

The process was performed for many months yielding 700 g of monofluorinated product with a nine-channel microstructured reactor [60]. A continuous 150 h operation was performed without decline of yield or conversion. Even in the scale-out to a 30-channel reador (see Figure 5.27), no loss in performance was noticed. A single feed system distributed the reactants and reagents to the various microchannels. 

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