Occurrences of Side Reactions

Besides large differences in activity, large differences in selectivity also occur. Usually the reduction of selectivity is caused by the occurrence of side reactions, such as isomerization and alkylation of the solvent. The extent to which these side reactions occur depends upon the kind of reactant,... 

The results from the literature are not easy to interpret due to the limited temperature ranges, small variations in particle sizes and the occurrence of side reactions with chain branching. 

The current efficiency may range from 25-30% to as high as 90-100%. The loss in efficiency may be due to several factors which influence the chemical and the electrochemical reactions at the electrodes. For example, factors such as the occurrence of side reactions, the passage of current by electronic conduction, and the dislodging of the deposited product from the electrode may substantially reduce the value of T. In general, the current... 

Inhibitors slow down oxidation by breaking chains or breaking down hydroperoxide. The inhibitory action of an antioxidant ceases when it is completely consumed. The duration of inhibitory effect depends on the mechanism of action of the antioxidant, the nature of inhibitory reactions, and the occurrence of side reactions in which the inhibitor is uselessly consumed. The action of the antioxidant in a given system can expediently be characterized in terms of inhibitory capacity. The capacity of a chain-breaking inhibitor can be characterized by the inhibition stoichiometric coefficient f[ 18]. 

Analogous techniques facilitating sequencing from a polypeptide s C-terminus remain to be satisfactorily developed. The enzyme carboxypeptidase C sequentially removes amino acids from the C-terminus, but often only removes the first few such amino acids. Furthermore, the rate at which it hydrolyses bonds can vary, depending on what amino acids have contributed to bond formation. Chemical approaches based on principles similar to the Edman procedure have been attempted. However, poor yields of derivatized product and the occurrence of side reactions have prevented widespread acceptance of this method. 

If complexity is suspected from the kinetic behavior, the effect of products and of possible impurities and the occurrence of side reactions should be considered. Later we shall see that medium composition (Sec. 2.9), temperature and pressure (Sec. 2.3) are other important parameters that affect rate. The rate law incorporating these effects is obtained by further experiments of the type indicated in step 3. 

However, the reaction products were not, apparently, identified20f. Trifluoro-acetate and trichloroacetate were extracted and precipitated from the acidified solution as their silver and mercurous salts, respectively, and the carbon dioxide produced by pyrolysis of these was analyzed in the mass spectrometer. So the occurrence of side-reactions leading to other carboxylate salts cannot be completely ruled out. Just one possibility is the halodecarboxylation of the anion, followed by hydrolysis of the dihalocarbene or haloform produced, to formate, viz. 

The possible occurrence of side reactions due to epoxide isomerisation has been considered by Tanaka and Kakiuchi48) and by Luston et al.49,50). 

The accurate determination of rate constants for the reactions of 19F atoms is often hampered by the presence of reactive F2 and by the occurrence of side reactions. The measurement of the absolute concentration of F atoms is sometimes a further problem. The use of thermal-ized 18F atoms is not subject to these handicaps, and reliable and accurate results for abstraction and addition reactions are obtained. The studies of the reactions of 18F atoms with organometallic compounds are unique, inasmuch as such experiments have not been performed with 19F atoms. In the case of addition reactions, the fate of the excited intermediate radical can be studied by pressure-dependent measurements. The non-RRKM behavior of tetraallyltin and -germanium compounds is very interesting inasmuch as not many other examples are known. The next phase in the 18F experiment should be the determination of Arrhenius parameters for selected reactions, i.e., those occurring in the earth s atmosphere, since it is expected that the results will be more precise than those obtained with 19F atoms. 

Another variable is the relative amounts of nucleotide component and condensing agent in joining-reactions. The occurrence of side reactions catalyzed by DCC is minor, so a large (5-10-fold) excess of the reagent is routinely used to optimize yields and minimize reaction times. However, after prolonged reaction in pyridine, byproducts... 

After removal of the solvent, the silyl tag was cleaved (step e) and the final triphasic extraction (step b, Fig. 8.23) gave the array L8 in the organic phase, removing TBAF-related impurities and the fluorous tag by extraction into the aqueous and fluorous phases, respectively. Yields and purities are reported together with the structures of the library individuals L8a-j in Table 8.3. A few compounds were obtained as mixtures, probably due to lower reactivity of corresponding monomers and occurrence of side reactions. 

The Friedel-Crafts acetylation of toluene (equation 3, Scheme 2.3) takes place smoothly at room temperature, but aromatic systems containing electron-withdrawing groups, such as dichlorobenzene or nitrobenzene, react very sluggishly under these conditions. The use of more forcing conditions might be undesirable owing to the occurrence of side reactions. 

Thus, each method has its own advantages and disadvantages, depending on the starting material, the availability of reagents, the cost, and the occurrence of side reactions. 

Thereby it must be emphasized that the application of methods from organic chemistry in polymer synthesis was only possible for chain structures, that are sufficiently soluble (see section 1.3). Here again, careful consideration of the mechanistic details of the reaction and the occurrence of side reactions are important ingredients of both oligomer and polymer synthesis. Thus, the Knoevenagel eondensation is feasible for eyano-substituted precursors 71... 

An attempt to carry out routine capping procedures with the NH3/CH2O and CH3NO2/CH2O systems under basic conditions failed because of the occurrence of side reactions. Therefore, a stepwise... 

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