Reaction mechanism, concept

Appealing and important as this concept of a molecule consisting of partially charged atoms has been for many decades for explaining chemical reactivity and discussing reaction mechanisms, chemists have only used it in a qualitative manner, as they can hardly attribute a quantitative value to such partial charges. Quantum mechanical methods (see Section 7.4) as well as empirical procedures (see... 

Our first three chapters established some fundamental principles concerning the structure of organic molecules and introduced the connection between structure and reactivity with a review of acid-base reactions In this chapter we explore structure and reactivity m more detail by developing two concepts functional groups and reaction mechanisms A functional group is the atom or group m a molecule most respon sible for the reaction the compound undergoes under a prescribed set of conditions How the structure of the reactant is transformed to that of the product is what we mean by the reaction mechanism... 

Some stereospecific reactions are listed in Scheme 2.9. Examples of stereoselective reactions are presented in Scheme 2.10. As can be seen in Scheme 2.9, the starting materials in these stereospecific processes are stereoisomeric pairs, and the products are stereoisomeric with respect to each other. Each reaction proceeds to give a single stereoisomer without contamination by the alternative stereoisomer. The stereochemical relationships between reactants and products are determined by the reaction mechanism. Detailed discussion of the mechanisms of these reactions will be deferred until later chapters, but some comments can be made here to illustrate the concept of stereospecificity. 

A special type of substituent effect which has proved veiy valuable in the study of reaction mechanisms is the replacement of an atom by one of its isotopes. Isotopic substitution most often involves replacing protium by deuterium (or tritium) but is applicable to nuclei other than hydrogen. The quantitative differences are largest, however, for hydrogen, because its isotopes have the largest relative mass differences. Isotopic substitution usually has no effect on the qualitative chemical reactivity of the substrate, but often has an easily measured effect on the rate at which reaction occurs. Let us consider how this modification of the rate arises. Initially, the discussion will concern primary kinetic isotope effects, those in which a bond to the isotopically substituted atom is broken in the rate-determining step. We will use C—H bonds as the specific topic of discussion, but the same concepts apply for other elements. 

The period 1930-1980s may be the golden age for the growth of qualitative theories and conceptual models. As is well known, the frontier molecular orbital theory [1-3], Woodward-Hoffmann rules [4, 5], and the resonance theory [6] have equipped chemists well for rationalizing and predicting pericyclic reaction mechanisms or molecular properties with fundamental concepts such as orbital symmetry and hybridization. Remarkable advances in aeative synthesis and fine characterization during recent years appeal for new conceptual models. 

The necessity of the statistical approach has to be stressed once more. Any statement in this topic has a definitely statistical character and is valid only with a certain probability and in certain range of validity, limited as to the structural conditions and as to the temperature region. In fact, all chemical conceptions can break dovra when the temperature is changed too much. The isokinetic relationship, when significantly proved, can help in defining the term reaction series it can be considered a necessary but not sufficient condition of a common reaction mechanism and in any case is a necessary presumption for any linear free energy relationship. Hence, it does not at all detract from kinetic measurements at different temperatures on the contrary, it gives them still more importance. 

The counterflow configuration has been extensively utilized to provide benchmark experimental data for the study of stretched flame phenomena and the modeling of turbulent flames through the concept of laminar flamelets. Global flame properties of a fuel/oxidizer mixture obtained using this configuration, such as laminar flame speed and extinction stretch rate, have also been widely used as target responses for the development, validation, and optimization of a detailed reaction mechanism. In particular, extinction stretch rate represents a kinetics-affected phenomenon and characterizes the interaction between a characteristic flame time and a characteristic flow time. Furthermore, the study of extinction phenomena is of fundamental and practical importance in the field of combustion, and is closely related to the areas of safety, fire suppression, and control of combustion processes. 

Chemical kinetics deals with quantitative studies of the rates at which chemical processes occur, the factors on which these rates depend, and the molecular acts involved in reaction processes. A description of a reaction in terms of its constituent molecular acts is known as the mechanism of the reaction. Physical and organic chemists are primarily interested in chemical kinetics for the light that it sheds on molecular properties. From interpretations of macroscopic. kinetic data in terms of molecular mechanisms, they can gain insight into the nature of reacting systems, the processes by which chemical bonds are made and broken, and the structure of the resultant product. Although chemical engineers find the concept of a reaction mechanism useful in the correlation, interpolation, and extrapolation of rate data, they are more concerned with applications... 

This chapter treats the descriptions of the molecular events that lead to the kinetic phenomena that one observes in the laboratory. These events are referred to as the mechanism of the reaction. The chapter begins with definitions of the various terms that are basic to the concept of reaction mechanisms, indicates how elementary events may be combined to yield a description that is consistent with observed macroscopic phenomena, and discusses some of the techniques that may be used to elucidate the mechanism of a reaction. Finally, two basic molecular theories of chemical kinetics are discussed—the kinetic theory of gases and the transition state theory. The determination of a reaction mechanism is a much more complex problem than that of obtaining an accurate rate expression, and the well-educated chemical engineer should have a knowledge of and an appreciation for some of the techniques used in such studies. 

Table 6.6 lists some reactions of the electron in water, ammonia, and alcohols. These are not exhaustive, but have been chosen for the sake of analyzing reaction mechanisms. Only three alcohols—methanol, ethanol, and 2-propanol—are included where intercomparison can be effected. On the theoretical side, Marcus (1965a, b) applied his electron transfer concept (Marcus, 1964) to reactions of es. The Russian school simultaneously pursued the topic vigorously (Levich, 1966 Dogonadze et al, 1969 Dogonadze, 1971 Vorotyntsev et al, 1970 see also Schmidt, 1973). Kestner and Logan (1972) pointed out the similarity between the Marcus theory and the theories of the Russian school. The experimental features of eh reactions have been detailed by Hart and Anbar (1970), and a review of various es reactions has been presented by Matheson (1975). Bolton and Freeman (1976) have discussed solvent effects on es reaction rates in water and in alcohols. 

Voltammetry, perspectives in modern basic concepts and mechanistic analysis, 32, 1 Volumes of activation, use of, for determining reaction mechanisms, 2, 93... 

Any mathematical function that adequately represents experimental rate data can be used in the rate law. Such a rate law is called an empirical orphenomenologicd rate law. In a broader sense, a rate law may be constructed based, in addition, on concepts of reaction mechanism, that is, on how reaction is inferred to take place at the molecular level (Chapter 7). Such a rate law is called a fundamental rate law. It may be more correct in functional form, and hence more useful for achieving process improvements. 

This chapter provides an introduction to several types of homogeneous (single-phase) reaction mechanisms and the rate laws which result from them. The concept of a reaction mechanism as a sequence of elementary processes involving both analytically detectable species (normal reactants and products) and transient reactive intermediates is introduced in Section 6.1.2. In constructing the rate laws, we use the fact that the elementary steps which make up the mechanism have individual rate laws predicted by the simple theories discussed in Chapter 6. The resulting rate law for an overall reaction often differs significantly from the type discussed in Chapters 3 and 4. 

For the elucidation of chemical reaction mechanisms, in-situ NMR spectroscopy is an established technique. For investigations at high pressure either sample tubes from sapphire [3] or metallic reactors [4] permitting high pressures and elevated temperatures are used. The latter represent autoclaves, typically machined from copper-beryllium or titanium-aluminum alloys. An earlier version thereof employs separate torus-shaped coils that are imbedded into these reactors permitting in-situ probing of the reactions within their interior. However, in this case certain drawbacks of this concept limit the filling factor of such NMR probes consequently, their sensitivity is relatively low, and so is their resolution. As a superior alternative, the metallic reactor itself may function as the resonator of the NMR probe, in which case no additional coils are required. In this way gas/liquid reactions or reactions within supercritical fluids can be studied... 

Time-proven concepts for the reaction mechanisms of homogeneous hydrogenations follow two approaches which, according to Halperrfs step-wise analysis of hydrogenations using Wilkinsorfs catalysr [25] and the cationic catalyst DI-PHOS [26], respectively, can be grouped into the so-called dihydride or unsaturate routes [27] (Fig. 12.9). 

There is no question that, indirectly or directly, Kirrmann and Prevost were influenced by Lowry s theories for explanation of reaction mechanisms. Another important influence was Dupont, with whom they talked at length in the laboratory and who published a paper in 1927 in which he attempted to combine the electron octet theory of valence and Bohr s hydrogen electron model with classical concepts of stereochemistry. Dupont also adopted without reservation Lowry s application of ionic radicals in hydrocarbon chemistry. 66... 

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