Reaction Enumeration System

The results of the study indicate that the best method of searching explicit schemes in REACCS is by exhaustive enumeration and prior registration of all possible subschemes, analogous to the method adopted by the developers of CASREACT. The same conclusion may not be made for all reaction indexing systems, however both CASREACT and REACCS maintain a separate molecule registry, which hmits the growth of the database when prior registration is used. 

Techniques for the synthesis of reaction paths on the basis of enumeration of all possible reactions are fairly weU developed. The SYNCHEM2 system conducts a backward search of possible reaction paths from a desired product (76). Because the procedure is limited by the rapid growth of... 

The subject has been very thoroughly reviewed [1-3], but certain fundamental aspects can profitably be reconsidered in the light of some recent developments [4-6]. Certainly the most startling of these is the discovery that under conventional conditions the polymerisation of styrene by perchloric and other acids, and by the syncatalytic system stannic chloride-water, is not an ionic process. These polymerisations have been named pseudocationic . This finding is in direct contradiction to the beliefs held previously about this and related reactions. Hence a new survey of the whole field has become necessary which must start with an enumeration of those systems for which the nature of the polymerisation reaction has been established with reasonable certainty. From these boreholes one can then try to assess the nature of the intervening territory and to decide where further detailed exploration would be most profitable. 

Chirality element enumeration is essential for the classification of stereoselective reactions 27>. For instance, in order to distinguish an asymmetrically induced synthesis from other reactions whose stereoselectivity is also due to a chiral reference system, one must compare the number of chirality elements in the starting materials and the products. 

First of all, let us describe all possible auxiliary dynamical systems for the triangle (56). For each vertex, we have to select the fastest outgoing reaction. For Ai, it is always A A2, because of our choice of enumeration (the higher scheme in Figure 4). There exist two choices of the fastest outgoing reaction for two other vertices and, therefore, only four versions of auxiliary dynamical systems for Equation (56) (Figure 4). 

Because of the choice of enumeration, the vectors of logarithms of reaction rate constants form a convex cone in which is described by the system of inequalities lnfc2i> lnfc,y, (/,/)t (2,1). For each of the possible auxiliary systems (Figure 4) additional inequalities between constants should be valid, and we get four correspondent cones in These cones form a partitions of the initial one (we neglect intersections of faces which have zero measure). Let us discuss the typical behavior of systems from these cones separately. (Let us remind that if in a cone for some values of coefficients dp then,... 

We found only three topologically different version of dominant systems for the reversible reaction triangle (i) A] —A2 A3, (ii) A] A2 A3 and (iii) A3 Ai A2. Moreover, there exist only two versions of zero-one asymptotic for eigenvectors the fastest eigenvalue is always —k i (because our choice of enumeration), the correspondent right and left eigenvectors (fast mode) are (1,—1,0) and f = (1,0,0). (The difference between systems (ii) and (iii)... 

This survey has been concerned with the enumeration of all possible mechanisms for a complex chemical reaction system based on the assumption of given elementary reaction steps and species. The procedure presented for such identification has been directly applied to a number of examples in the field of heterogeneous catalysis. Application to other areas is clearly indicated. These would include complex homogeneous reaction systems, many of which are characterized by the presence of intermediates acting as catalysts or free radicals. Enzyme catalysis should also be amenable to this approach. 

The reaction system is one of the most important parts of choosing a basic module for the design of a photochemical reactor. This might be explained by enumerating some of the corresponding parameters and describing their impact on reactor geometry and operational conditions ... 

Next, characteristic properties of components are listed to select appropriate separation method (Table 3.7). Because the trace components belong to different chemical families, we eliminate gas-phase catalytic oxidation or hydrogenation. More specific chemical-based techniques remain. A first one is reversible chemical absorption. As solvents we may enumerate liquid redox systems (chelated iron), caustic washing solutions, amines or special formulations, as Selexol . Since H2S and C02 both have an add character, we may expect that a certain amount of C02 will pass in the off-gas stream. Dry chemical treatment could also be used, as reaction of H2S with iron-sponge or impregnated wood chips. 

The enumeration of all the possible reactions involving radicals and molecules in the ethane system would be a tedious task, but one is not really justified in accepting a mechanism for the ethane pyrolysis until such an exhaustive inquiry has been completed. On the other hand, at our present stage of knowledge, the detailed investigation is impracticable if not impossible. The Rice-Herzfeld principle presents about as practical and complete a guide as is at present warranted for the economical discussion of hydrocarbon reactions. However, even this scheme for the ethane pyrolysis [Eq. (XIII. 10.4)] has been considerably shortened in the discussion already presented [Eq. (XIII.10.5)], and we may now go back and look at some of the reactions which have been neglected in the latter, simplified chain. 

The relative complexity of the extended ir-systems present in these reactants suggest that a variety cf potentially competitive pericyclic processes could be operative in the thermal reaction of a given diene with tropone. Consequently, the question of periselectivity in these reactions has been examined in some detail in recent years and a numl r of useful generalizations have been enumerated for predicting products in a variety of circumstances. 

The effects of emulsifiers in emulsion polymerization systems may be enumerated as follows (l stabilization of the monomer in emulsion, (2) solubilization of monomer in micelles, (3) stabilization of polymer latex particles, (4) solubilization of polymer, (5) catalysis of the initiation reaction, and (6) action as transfer agents or retarders which leads to diemical binding of emulsifier residues in the polymer obtained. 

A related concept is that of laminar mechanisms, introduced by Sinanoglu (1975), Sinanoglu and Lee (1978), and Lee and Sinanoglu (1981). These mechanisms can be constructed a priori (i.e., without regard to a specific set of mechanism steps) given the number of catalysts involved. Sellers (1971, 1972) also presented procedures for construction of a priori mechanisms for certain chemical systems. In both these instances of mechanism construction, the focus was the a priori enumeration of potential pathway shapes, without regard to the specific reactions permitted by the system. These techniques are thus not applicable to the synthesis of pathways or mechanisms from a given set of reaction steps. 

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