Elementary chemical reaction sequences

One conceivable extrapolation of this definition of concertedness is that every elementary chemical reaction is concerted. Every chemical process may be at least conceptually broken up into a sequence of one, two, or more elementary steps or concerted subprocesses. Therefore a reaction may be termed one-step, two-step, or multi-step, and the term "concerted" becomes a redundancy simply equivalent to "one-step . The frequent use of "concerted and "one-step as interchangeable adjectives is one consequence of this extrapolation. 

A mechanism is a description of the actual molecular events that occur during a chemical reaction. Each such event is an elementary reaction. Elementary reactions involve one, two, or occasionally three reactant molecules or atoms. In other words, elementary reactions can be unimolecular, bimolecular, or termolecular. A typical mechanism consists of a sequence of elementary reactions. Although an overall reaction describes the starting materials and final products, it usually is not elementary because it does not represent the individual steps by which the reaction occurs. 

In a termolecular reaction, three chemical species collide simultaneously. Termolecular reactions are rare because they require a collision of three species at the same time and in exactly the right orientation to form products. The odds against such a simultaneous three-body collision are high. Instead, processes involving three species usually occur in two-step sequences. In the first step, two molecules collide and form a collision complex. In a second step, a third molecule collides with the complex before it breaks apart. Most chemical reactions, including all those introduced in this book, can be described at the molecular level as sequences of bimolecular and unimolecular elementary reactions. 

It is apparent from the last example cited in previous section that there is not necessarily a connection between the kinetic order and the overall stoichiometry of the reaction. This may be understood more clearly if it is appreciated that any chemical reaction must go through a series of reaction steps. The addition of these elementary steps must give rise to the overall reaction. The reaction kinetics, however, reflects the slowest step or steps in the sequence. An overall reaction is taken as for an example ... 

Almost all chemical reactions involve a sequence of elementary steps, and do not occur in a single step. 

If we want to understand and describe the influence of environmental factors, especially temperature, on chemical reaction rates, and if we want to see how transformation rates vary as a function of the chemical structure of a compound, we need to take a closer look at these reactions on a molecular level. As mentioned already, a chemical reaction often proceeds in several sequential elementary steps. Frequently, one step in the reaction sequence occurs at a much slower rate than all the others. 

Similar reaction sequences have been identified in other chemically reacting systems, specifically catalytic combustion (52, 53), solid-fuel combustion (54), transport and reaction in high-temperature incandescent lamps (55), and heterogeneous catalysis (56 and references within). The elementary reactions in hydrocarbon combustion are better understood than most CVD gas-phase reactions are. Similarly, the surface reaction mechanisms underlying hydrocarbon catalysis are better known than CVD surface reactions. 

The scales involved in such a reactor should be defined in a relative manner. For a chemist, the molecule is at the start and catalyst (particle) at the end of the scales. To reveal the reaction mechanism over a catalyst particle, a sequence of network of elementary reactions" will be needed. Accordingly, on the basis of, for example, the molecular collision theory (Turns, 2000), the "global reaction" can be derived in terms of global rate coefficient and reaction order. Here, the resultant reaction mechanism is termed "global" by chemists, because the use of it for a specific problem is normally a "black box" approach, without knowing exactly the underlying networks or structures of chemical routes from reactants to products. On the other hand, for a chemical reaction engineer, the catalyst (particle) is often the start and the reactor is the end. The reaction free of inner-particle and outer-particle diffusions, that is,... 

Every catalytic cycle is a sequence of simple chemical reactions. These elementary steps are the building blocks from which you can construct the story behind the reaction (better known as the reaction mechanism). Understanding these steps is often easier in homogeneous catalysis than in heterogeneous catalysis and... 

Chemical reactions occurring because of a single kinetic act, i.e., because of a single collision between two molecules, are defined as elementary reactions. More complex laws of dependence on concentrations can be explained by complex reaction mechanisms, i.e., by the idea that most reactions occur as a sequence of many elementary reactions, linked in series or in parallel. As an example, the following... 

A feasible reaction scheme includes all the reactants and products, and it generally includes a variety of reaction intermediates. The validity of an elementary step in a reaction sequence is often assessed by noting the number of chemical bonds broken and formed. Elementary steps that involve the transformation of more than a few chemical bonds are usually thought to be unrealistic. However, the desire to formulate reaction schemes in terms of elementary processes taking place on the catalyst surface must be balanced with the need to express the reaction scheme in terms of kinetic parameters that are accessible to experimental measurement or theoretical prediction. This compromise between molecular detail and kinetic parameter estimation plays an important role in the formulation of reaction schemes for analyses. The description of a catalytic cycle requires that the reaction scheme contain a closed sequence of elementary steps. Accordingly, the overall stoichiometric reaction from reactants to products is described by the summation of the individual stoichiometric steps multiplied by the stoichiometric number of that step, ai. 

A large group of scientists, including the author, believe that a chemical catalytic process, as well as an enzymatic reaction contains a certain sequence of elementary chemical steps. Each of these steps proceeds by ordinary laws of chemical kinetics. The accelerating action of a catalyst is accounted for by the fact that its active centers become involved in such chemical reactions with substrate molecules, which lead to an increase in the velocity of the process as a whole. Within the framework of this concept, enzymes are characterized by a set of certain specific properties, which have been polished off in the course ofbiological evolution. 

A chemical reaction is the conversion of an EM into an isomeric EM. Generally, a reaction is decomposable into one or more elementary mechanistic steps. Hence such a conversion corresponds to a sequence of intermediate isomeric EM s. Ugi and Dugundji call such a conversion a reaction pathway. Each step of the pathway involves breaking or making a bond. 

One of the major advances in chemistry over the last forty years has been the establishment of the fact that most chemical reactions take place by a complex sequence of elementary steps. The overall course of such a reaction is controlled to a great extent by the properties of the transitory species that participate in these elementary reactions. The properties are difficult to measure because of the short lives and low concentrations of these intermediates and have been inferred usually from the course of the overall reaction. It has been normal practice for a reaction mechanism to be postulated and the rate constants of the individual steps to be estimated by indirect methods based on the analysis of the stable products. 

Almost every chemical reaction in industrial and laboratory practice results not from a single rearrangement or break-up of a molecule or collision of molecules, but from a combination of such molecular events called elementary steps, or steps for short. The steps of a reaction may occur in sequence, reactants reacting to form intermediates which subsequently react to form other intermediates and ultimately a product or products. The sequence of steps then is called a pathway. Almost always, however, one or several of the reactants or intermediates can also undergo alternative reactions that eventually lead to undesired by-products or different but also desired co-products. The combination of steps then is called a network with branches. Pathways from specific reactants to specific products can be defined within networks. Points at which pathways branch are called nodes, and linear portions between nodes or between a node and an end member are called segments. The network may contain parallel pathways from one node to another or to an end member, involving conversion of the same reactants (or intermediates) to the same products (or other intermediates) such pathways form a loop. 

In order to achieve the desirable overall result, however unrealistic it may appear, an organic chemist looks for an opportunity to break down the overall transformation into a sequence of chemically possible steps. Chemically possible means that a well-defined chemical reaction can be utilized to achieve the required elementary transformation. Then it will be within the chemist s control, at least in principle, to select the proper reagents and reaction conditions ( tools and procedures ) for each step that would ensure the efficiency of every required transformation of the whole sequence and hence the viability of the conceived synthetic plan. This ideal scheme is rarely realized completely, but it is generally applicable within reasonable limits. In fact, identification of a set of simple and realistic steps in conjunction with a careful selection of the optimal reagents and reaction conditions for every single step is the underlying approach elaborated for controlled total synthesis. 

A heterogeneous catalytic reaction involves adsorption of reactants from a fluid phase onto a solid surface, surface reaction of adsorbed species, and desorption of products into the fluid phase. Clearly, the presence of a catalyst provides an alternative sequence of elementary steps to accomplish the desired chemical reaction from that in its absence. If the energy barriers of the catalytic path are much lower than the barrier(s) of the noncatalytic path, significant enhancements in the reaction rate can be realized by use of a catalyst. This concept has already been introduced in the previous chapter with regard to the Cl catalyzed decomposition of ozone (Figure 4.1.2) and enzyme-catalyzed conversion of substrate (Figure 4.2.4). A similar reaction profile can be constructed with a heterogeneous catalytic reaction. 

A reaction mechanism is the detailed sequence of elementary reactions that lead to the overall chemical reaction. 

Sequence of elementary reactions which, when added together, gives the net chemical reaction and reproduces its rate law... 

In contrast, any one-pot MCR is a conversion of N > 3 different starting materials into its products, and must contain at least some part of each educt. No MCR can directly convert its educts into its products, as this corresponds to many steps, so that one or two components of their sub-reaction also participate. Therefore, this is always a sequence of elementary reactions. Only one direct chemical reaction of three participating components seems possible, namely the formation of the subsequently rearranging a-adducts of the isocyanides, the a-aminoalkyl cations 5 and the anions 6. Such a three-component reac-... 

Chemical kinetics equations are commonly nonlinear and may represent diverse phenomena of a catastrophe type. Theoretical studies in this area fall into two groups. Purely model considerations belong to the first group. A certain sequence of elementary reactions — the reaction mechanism, permitted from the chemical standpoint (see the Korzukhin theorem, Chapter 4) is postulated, the corresponding system of kinetic equations is found and its solutions are examined. Such a procedure allows us to predict a possible behaviour of chemical systems. The second approach involves the investigation of a mechanism of a specific chemical reaction, having interesting dynamics. 

Within the strictly chemical realm, sequences of pseudo-first-order reactions are quite common. The usually cited examples are hydrations done in water and slow oxidations done in air where one of the reactants (e.g., water or oxygen) is present in great excess and hence does not change appreciably in concentration during the course of the reaction. These reactions are pseudo-first order and behave identically to those in Equation 2.20, although the rate constants over the arrows should be removed as a formality since the reactions are not elementary. 

As described in Section 2, a reaction mechanism refers to a molecular description of how reactants are converted into products during a chemical reaction. In particular, it refers to the sequence of one or more elementary steps that define the route between reactants and products. A prime objective of many kinetic investigations is the determination of reaction mechanism since this is not only chemically interesting in its own right, but it may also suggest ways of changing the conditions to make a reaction more efficient. 

The attack on the problem of mechanism in a chemical reaction begins with the resolution of the reaction into a postulated sequence of elementary reactions. An elementary reaction is one that occurs in a single act. As an example, consider the reaction... 

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