Competition methods limitations

Davies and Warren have investigated the nitration of naphthalene, ace-naphthene and eight dimethylnaphthalenes in acetic anhydride at o °C. Rates relative to naphthalene were determined by the competition method, and the nitro-isomers formed were separated by chromatographic and identified by spectrophotometric means. The results, which are summarised in the table, were discussed in terms of various steric effects, and the applicability of the additivity rule was examined. For the latter purpose use was made of the data of Alcorn and Wells (table 10.2) relating to the nitration of monomethyl-naphthalenes at 25 °C. The additivity rule was found to have only limited utility, and it was suggested that the discrepancies might be due in part to the... 

This non-competitive method has several practical limitations. Since the ordinary precision of determination of rate constants, (8kL/kL) or (Ske/kn), is on the order of a few percent, the method is limited as a practical matter to large, primary kinetic isotope effects, generally of hydrogen. This, because deuterium, the common heavy isotopomer for hydrogen, is available at 100% abundance at reasonable cost, and for hydrogen KIE s are usually large enough to constrain the relative error, 8(kL/kH)/(kL/kH), to acceptable values. 

Inasmuch as the fractional amounts of conversion can be determined with an accuracy of better than a half per cent only in a limited number of favorable cases, and because of the requirement of samples of high isotopic enrichment, the chemical competitive method is essentially limited to studies of the isotopes of hydrogen. 

It will be noted from Table 13 that single temperature rate coefficients are listed and not Arrhenius parameters. The latter are not readily derived from the original type of experiments, which are not suitable for the measurement of temperature coefficients because of the difficulty of assessing the variations of quantities such as the diffusion coefficient and the limit of detectability of sodium atoms. An interesting competitive method has been developed to study Na and K atom reactions which overcomes these difficulties [146]. The alkali metal atoms were reacted in diffusion flame experiments with pairs of organic halides. By labelling one of the halides with Cl and by analysing for the total concentration and... 

Olah and Lin studied the boron trlfluoride-catalyzed reaction of methyl nitrate with aromatic compounds in nltromethane.2 Using competition methods they found that the toluene to benzene rate ratio was 25 and that the rate ratios for the methylben-zenes. Table IX, reached a limiting value, about 1000. Unfortunately, kinetic rate data were not obtained. However, the leveled reactivity of the polymethylbenzenes suggests that these compounds react at the encounter controlled diffusion limit. 

When evaluating the product and methods for its manufacture, possibly including the design of a process to produce the desired chemicals, the methods of capital-cost estimation, profitability analysis, and optimization, discussed previously in Chapters 16-18, are often utilized. However, for products that can be sold at a high price, due to large demand, lack of competition, and limited production, detailed profitability analysis and optimization are less important. For these products, it is most important to reduce the design and manufacturing time to capture the market before competitive products are developed. 

The analytical parameters, minimum detection limits, accuracy, and speed, which determine the applicability of the PIXE method and are important from the point of view of its comparison with other competitive methods of instrumental analysis, can be deduced from the... 

The detection limit is usually lower in noncompetitive than in competitive methods, although similar sensitivities have been achieved for both kinds of methods in some instances [58,59]. The two antibodies involved in sandwich methods give these methods higher specificity than that achieved with methods in which only one antibody is used. The main drawback of sandwich methods does not apply to proteins, the target of this chapter. For small molecules, steric hindrance and the nonavailability of two separate epitopes prevents the use of sandwich procedures for their analysis. 

An alternate approach to the study of fast reactions is by means of competition methods. Here the rate of a process is measured separately in systems where chemical reaction can and cannot occur. If the process is perturbed by the reaction, the difference in the rates is a measure of the rate of reaction. For example, an unstable species may be formed photochemically by irradiation and its lifetime determined. The experiment is then altered by adding a substance which can react with the unstable species it will thus have a shorter lifetime from which the rate of the chemical reaction can be deduced. To be a useful technique the chemical and physical processes must proceed at roughly similar rates which limits the applicability of this approach, as is emphasized in Fig. 4.1. Other competition experiments (and the corresponding observables) have been made using polarography (current), nuclear magnetic resonance (line width), electron spin resonance (line width), and diffusion (diffusion coefficient). 

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