Consequences selection

In general, the optimum conditions cannot be precisely attained in real reactors. Therefore, the selection of the reactor type is made to approximate the optimum conditions as closely as possible. For this purpose, mathematical models of the process in several different types of reactors are derived. The optimum condition for selected parameters (e.g., temperature profile) is then compared with those obtained from the mathematical expressions for different reactors. Consequently, selection is based on the reactor type that most closely approaches the optimum. 

Double-bond migration often passes unnoticed, for unless tracers are employed, there may be no direct evidence remaining that migration has occurred. Nonetheless, the fact that it does occur can have a number of important consequences. Selective removal of cis homoconjugated dienes and trienes in natural oils, used to make edible hydrogenated fats, depends mainly on prior isomerization of multiple unsaturation into conjugation under hydrogenation conditions (J9). 

Fire detection and suppression form the basis of fire control, with emergency back-up procedures to mitigate the consequences. Selected key tactics for working with flammable chemicals are summarized in Table 5.11. 

Catalysis opens reaction pathways that are not accessible to uncatalysed reactions. It should be self-evident that thermodynamics predict whether a reaction can occur. So, catalysis influences reaction rates (and as a consequence selectivities), but the thermodynamic equilibrium still is the boundary. Catalysis plays a key role in chemical conversions, although it is fair to state that it is not applied to the same degree in all sectors of the chemical industry. While in bulk chemicals production catalytic processes constitute over 80 % of the industrially applied processes, in fine chemicals and specialty chemicals production catalysis plays a relatively modest role. In the pharmaceutical industry its role is even smaller. It is the opinion of the authors that catalysis has a large potential in these areas and that its role will increase drastically in the coming years. However, catalysis is a multidisciplinary subject that has a lot of aspects unfamiliar to synthetic chemists. Therefore, it was decided to treat catalysis in a separate chapter. 

In addition, the composition of the electrolyte solution can strongly influence sample solubility and detection, native conformation of biopolymers, molecular aggregation, electrophoretic mobility, and EOF, which can be altered as a consequence of the adsorption of the components of the BGE onto the capillary wall. Consequently, selecting the proper composition of the electrolyte solution... 

In the case of olefin epoxidation, the formation of the diol is prevented since MTO and the epoxide are topologically separated (MTO is absorbed in the zeolite whereas the epoxide remains in solution). As a consequence, selectivities are very high (95%, Table 11) except with 1-methylcyclohexene (entry 2, Table 11). 

Since all the sink terms, initial distribution and one-reactant densities, n, are of the same form for all reactants, the double summation can be replaced by N N — 1)12. To preserve the independence of the reactants, the sink term. )k must be strongly localised to minimise the chance of simultaneous reaction paths being available and the form 5( r - — th = R) is suitable. Consequently, selecting the reactants 1 and 2 as representative of the reaction, the double integral is... 

Chemicals and the containment materials for chemical reaction do not interact equally with the commonly used microwave frequencies for dielectric heating and consequently selective heating may be achieved. Specifically, it is possible to cool the outside of the vessel with a coolant that is transparent to microwaves (solid C02 or liquid N2) and thereby have cold walls that still allowthe microwave energy to penetrate and heat the reactants, which are microwave active, in the vessel. Also for solid-state reactions contamination from the crucible walls may be minimised. 

Since the inception ofthe superelectrophilic concept in the 1970s1 and 1980s 2 first formulated as protosolvation of cationic intermediates, superelectrophiles as highly reactive dicationic and tricationic intermediates have been successfully observed and characterized.3-5 Consequently, selected examples of superelectrophiles are also covered in this chapter where appropriate, whereas various organic transformations, in which the involvement of superelectrophilic intermediates is invoked or superelectrophiles are observed, are treated in Chapter 5. 

Frequently, several reactions proceed simultaneously, and consequently selectivity and yield in networks of parallel and series reactions with respect to a certain desired target component D are essential quantities. 

Ammonia is sometimes used as a reducing agent for the selective reduction of NOE in emissions from industrial installations, but unreacted ammonia creates a secondary air pollution problem because it is itself hazardous. Consequently, selective catalytic oxidation (SCO) is required to convert traces of ammonia to nitrogen downstream of the reactor 134... 

In this section we will confine ourselves to the oxidation of hydrocarbons. The addition of oxygen to a hydrocarbon is nearly always thermodynamically favoured. When selective oxidation is aimed at, the reaction has to be kinetically controlled because the most stable situation is the complete combustion into water and carbon dioxide. Usually, the reactivity of intermediate and end products (alcohols, aldehydes, etc.) is higher than that of the starting material. As a consequence, selectivity is difficult to achieve and the catalyst has to be designed carefully promoters and modifiers are often essential for the success of the... 

The order of ease of reductive dehalogenation of organic halides in the same type of structural environment is I > Br > Cl F. This order is parallel with the dissociation energy of carbon-halogen bonds (HsC—I 234 kJ mol- H3C—Br 293 kJ mol" H3C—Cl 351 kJ mol- H3C—F 452 kJ moL ) and is generally observed in the reduction of alkyl halides. Consequently, selective reduction of di- or polyhalides containing different halogen atoms is possible. Fluorides are often removed only with difficulty and examples of such reductions are comparatively limited. 

Macrolide antibiotics are administered orally, but many of them exhibit low and/or variable degrees of oral absorption, low serum concentrations, and short half-lives. Consequently, selection of semi-synthetic derivatives for development has often been guided by greater oral bioavailability, longer half-life, and higher and more prolonged concentrations in serum and tissues. The clinical pharmacokinetic parameters of several 16-membered macrolides have recently been reviewed [58, 233, 234],... 

TBDPS ethers are generally cleaved under the same acidic conditions as those used for TBS ethers but longer reaction times are necessary and consequently selective removal of TBS groups in the presence of TBDPS groups is very common [331,332]. The electron-withdrawing effect of the phenyl substituents enhances the electrophilicity of the silicon atom and therefore is more susceptible towards nucleophiles. For this reason it is possible to reverse the tendency of TBS ethers to cleave more easily than TBDPS ethers using ion fluoride or basic hydrolysis [333]. Some examples in glycal derivatives are shown in Scheme 54. 

In various brain areas neuronal histamine release and synthesis is regulated presynaptically by histamine H3 receptors [1]. Recent studies revestl that H3 receptors not only act as autoreceptors to regulate the release euid synthesis of histamine [2, 3], but also modulate the release of other neurotransmitters, like acetylcholine [4], serotonin [5], dopamine [6] and noradrenaline [7]. In view of these widespread modulatory activities of the histamine H3 receptors, important roles for this receptor subtype in the various mammalian brain functions have been indicated [8, 9]. Consequently, selective ligands for the H3 receptor have been suggested to be benificial in the treatment of e.g. epilepsia, Alzheimer disease, sleeping and attention-deficit disorders [10, 11,12, 13, 14, 15,16],... 

The two double bonds of dicyclopentadiene are very different in reactivity and, consequently, selective reactions are easy to achieve. 

In all of these studies, the presence of the Group IB metal, copper or gold, with the Group VIII metal, nickel or palladium, led to inhibition of hydrogenol-ysis on the Group VIII metal. As a consequence, selectivities for isomerization of the n -alkanes to branched alkanes, and for aromatization of methylcy-dopentane to benzene, were improved. The studies of the Netherlands workers on nickel-copper alloys, like those conducted in our laboratories on the same system, eliminated the possibility of complications caused by the presence of a carrier and were therefore useful for clarifying the selectivity phenomenon. 

The polarity of the substrate is therefore not less important than the polarity of the free radical in determining the sensitivity to polar effects and the consequent selectivity and synthetic interest. 

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