Mechanistic interpretation

2 Mechanistic Interpretation - Very little work has been done to probe the mechanism of degradation of polymers on solid bases. However, the catalytic 

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The first example of enantioselective catalysis of a Diels-Alder reaction was reported in 1979 . Since then, an extensive set of successful chiral Lewis-acid catalysts has been prepared. Some selected examples will be presented here together with their mechanistic interpretation. For a more complete... 

The mechanism of chemiluminescence is still being studied and most mechanistic interpretations should be regarded as tentative. Nevertheless, most chemiluminescent reactions can be classified into (/) peroxide decomposition, including biolurninescence and peroxyoxalate chemiluminescence (2) singlet oxygen chemiluminescence and (J) ion radical or electron-transfer chemiluminescence, which includes electrochemiluminescence. 

In such a process, the water molecule fonned in the elimination step is captured primarily fiom the fixmt side, leading to net retention of configuration for the alcohol. For the ester, the extent of retention and inversion is more balanced, although it vari among individual systems. It is clear om die data in Table 5.18 that the two pairs of stereoisomeric amines do not form the same intermediate, even though a simple mechanistic interpretation would sugg that both would fmm the 2-decalyl cation. The coUap of the ions to product is pvidoitly so rapid that diere is not time for relaxation of the initially formed intermediates to reach a common stnicture. 

Offer a mechanistic interpretation of each of the following phenomena ... 

Mention should be made of issues that can complicate the mechanistic interpretation of photochemical reactions ... 

These mechanistic interpretations can also be applied to the hydrogenation of cyclohexanones. In acid, the carbonium ion (19) is formed and adsorbed on the catalyst from the least hindered side. Hydride ion transfer from the catalyst gives the axial alcohol (20). " In base, the enolate anion (21) is also adsorbed from the least hindered side. Hydride ion transfer from the catalyst followed by protonation from the solution gives the equatorial alcohol (22). 

Most recent publications by Hartshorn and his collaborators introduce a note of caution with regard to the mechanistic interpretation of results obtained in BF3-catalyzed epoxide cleavage reactions. They point out the marked dependence of the course of the reaction on the solvent used, either... 

The values of A and E provide a full description of the kinetic data, but it may be desirable, for mechanistic interpretation, to express the results in terms of the activation parameters A// and A5. We developed equations in Section 5.2 for this purpose for convenience these are repeated here ... 

These apply to a bimolecular reaction in which two reactant molecules become a single particle in the transition state. It is evident from Eqs. (6-20) and (6-21) that a change in concentration scale will result in a change in the magnitude of AG. An Arrhenius plot is, in effect, a plot of AG against 1/T. Because a change in concentration scale alters the intercept but not the slope of an Arrhenius plot, we conclude that the values of AG and A, but not of A//, depend upon the concentration scale employed for the expression of reactant concentrations. We, therefore, wish to know which concentration scale is the preferred one in the context of mechanistic interpretation, particularly of AS values. 

The second use of activation parameters is as criteria for mechanistic interpretation. In this application the activation parameters of a single reaction are, by themselves, of little use such quantities acquire meaning primarily by comparison with other values. Thus, the trend of activation parameters in a reaction series may be suggestive. For example, many linear correlations have been reported between AT/ and A5 within a reaction series such behavior is called an isokinetic relationship, and its significance is discussed in Chapter 7. In Section 5.3 we commented on the use of AS to determine the molecularity of a reaction. Carpenter has described examples of mechanistic deductions from activation parameters of organic reactions. 

These are usually very small effects, and, although real, their mechanistic interpretation is seldom clear-cut. 

After an introductory chapter, phenomenological kinetics is treated in Chapters 2, 3, and 4. The theory of chemical kinetics, in the form most applicable to solution studies, is described in Chapter 5 and is used in subsequent chapters. The treatments of mechanistic interpretations of the transition state theory, structure-reactivity relationships, and solvent effects are more extensive than is usual in an introductory textbook. The book could serve as the basis of a one-semester course, and I hope that it also may be found useful for self-instruction. 

Improvements can be achieved by variation of the solvent and the careful location of the acceptor substituents.58 59 Another problem of this synthetic approach is the formation of hexapyrroles as byproducts of the bilin preparation. In the case of the 1,19-dimethylbilenes one of the carbons of the terminal methyl groups has to be expelled, so most of the syntheses make use of the 1-methylbilenes. A mechanistic interpretation of the cyclization step is similar to that for 1,19-dideoxybiladienes-nc (vide infra). In a few cases60 64 bilcne-l-carbaldehydes are used. 

It is worthwhile to briefly discuss the history of investigations into the mechanism of diazotization. Its progression between 1894 and 1958 demonstrates that it may take more than 60 years to correct a false mechanistic interpretation of good experimental results followed by many supporting conclusions. 

Analysis of the decay of the sum of the diazonium ion and (jF)-diazoate concentrations as a function of time reveals that there are two reactions. The first is observed only at the beginning and at relatively low temperatures (20 °C) it is first order in relation to the above sum of concentrations and to the hydroxide ion concentration. The second is a very complex function of the hydroxide ion concentration, so that a mechanistic interpretation was not possible. 

There have been few satisfactory demonstrations that decompositions of hydrides, carbides and nitrides proceed by interface reactions, i.e. either nucleation and growth or contracting volume mechanisms. Kinetic studies have not usually been supplemented by microscopic observations and this approach is not easily applied to carbides, where the product is not volatile. The existence of a sigmoid a—time relation is not, by itself, a proof of the occurrence of a nucleation and growth process since an initial slow, or very slow, process may represent the generation of an active surface, e.g. poison removal, or the production of an equilibrium concentration of adsorbed intermediate. The reactions included below are, therefore, tentative classifications based on kinetic indications of interface-type processes, though in most instances this mechanistic interpretation would benefit from more direct experimental support. 

The kinetic behaviour of metal salts of oxyacids may be influenced by water of crystallization. Where complete-dehydration precedes decomposition, the anhydrous material is the product of a previous rate process and may have undergone recrystallization. If water is not effectively removed, there may at higher temperature be the transient formation of a melt prior to decomposition. The usual problems attend the identification of partial or transient liquefaction of the reactant in the mechanistic interpretation of kinetic data. 

There have been remarkably few reviews of the chemistry of decompositions and interactions of solids. The present account is specifically concerned with the kinetic characteristics described in the literature for the reactions of many and diverse compounds. Coverage necessarily includes references to a variety of relevant and closely related topics, such as the background theory of the subject, proposed mechanistic interpretations of observations, experimental methods with their shortcomings and errors, etc. In a survey of acceptable length, however, it is clearly impossible to explore in depth all features of all reports concerned with the reactivity and reactions of all solids. We believe that there is a need for separate and more detailed reviews of topics referred to here briefly. The value of individual publications in the field, which continue to appear in a not inconsiderable flow, would undoubtedly be enhanced by their discussion in the widest context. Systematic presentation and constructive comparisons of observations and reports, which are at present widely dispersed, would be expected to produce significant correlations and conclusions. Useful advances in the subject are just as likely to emerge in the form of generalizations discerned in the wealth of published material as from further individual studies of specific systems. Perhaps potential reviewers have been deterred by the combination of the formidable volume and the extensive dispersal of the information now available. 

The central role of imperfections in mechanistic interpretations of decompositions of solids needs emphasizing. Apart from melting (which requires redistribution of all crystal-bonding forces, by a mechanism which has not yet been fully established) the decompositions of most solids involve the participation of atypical lattice constituents, structural distortions and/or surfaces. Such participants have, in particular instances, been identified with some certainty (e.g. excitons are important in the decompositions of some azides, dislocations are sites of nucleation in dissociations of a number of hydrates and carbonates). However, the... 

The mechanistic interpretation of the acid-catalyzed ring opening reaction of thiirane oxides125 is based on the push-pull mechanism162 with a transition state in which the bonded hydrogen atom plays a major role (see equation 59). 

This section concerns studies directed toward an elucidation of relative rate constants, k,./kp and kt/kp, and a mechanistic interpretation of these parameters. The data for the HMWF of PIB samples prepared with 7-BuCl and f-BuBr/Et2AlCl/MeCl systems shown in Table 7 were calculated from Mayo plots (see also p. 56 for Mayo Equation) using M data given in Tables 4 and 5. The data of LMWF of PIB prepared with f-BuBr/Et2AlCl/MeCl system at —50 °C shown in Table 7 were calculated from Mayo plots using M data given in Table 5. 

This chapter is concerned initially with kinetic results and mechanistic interpretations of the CO insertion (Section III) and extrusion (Section IV) reactions. A discussion of the stereochemical data follows (Section V), and a comprehensive survey of these reactions by the triads (Section VI) rounds out the review. Carbon monoxide insertion reactions were discussed in 1967 by Basolo and Pearson (21). Since then they have been mentioned in several reviews (49, 118, 203, inter alios) but have not been treated comprehensively. 

The different curves obtained by increasing the number of terms in the Taylor expansion are represented in Figure 3.3 on top of the Gompertz curve itself. The exponential growth model can thus be now justified not only because it fits well the data but also because it can be seen as a first approximation to the Gompertz growth model, which is endowed with a mechanistic interpretation, namely, competition between the catabolic and anabolic processes. 

The descriptors used should not be highly collinear with each other, for two reasons. First, this can lead to statistical instability and overprediction, and second, collinearity makes mechanistic interpretation difficult. For example, Cronin and Schultz [41] have pointed out that although a good correlation could be obtained between the skin sensitization potential and the hydrophobicity of a series of bromoalkanes, a correlation between skin sensitization potential and molecular weight had exactly the same statistics, because hydrophobicity and molecular weight are very highly correlated in homologous series. 

A few data exist on the oxidation of water by Ag(ril) ethylenedibiguanide nitrate . The decomposition is only approximately first-order in oxidant in both water and dilute nitric acid. The activation energies are very different for the two media, (H2O, 27.8 kcal.mole HNO3, 18.65 kcal.mole ). A is about 10 for water and 10 for nitric acid solutions. These data are inadequate for mechanistic interpretation. 

This mechanistic interpretation is supported by the isolation of N-acetylimidazole as a major product and the observation of only first order catalysis by N-methylimidazole irrespective of the pH... 

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