Reaction schemes

During polycondensation, it is well known that two main reactions take place  

Herein, the subscript x stands for the number of methylene units used and takes the values x = 2,3,4 for the ethylene, propylene, or butylene ester, respectively 

The forward reactions are facilitated by the by-product removal either by flow of an inert gas or by maintaining reduced pressure, or a combination of the two. 

The rate of change of hydroxyl and carboxyl end groups is described by the following expressions [59]. More details can be found in Ref. [45]  

According to Ma and Agarwal [57, 59], the rapid slowdown in SSP kinetics at high [ ] values can be represented by the transesterification and esterification reactions only when accounting for a part of the carboxyl ([COOH]) and hydroxyl end groups ([OH]) to be rendered temporarily inactive, [OH]j, [COOH]j. Then, the actual concentration of OH and COOH in Equations 4.39 and 4.40 are expressed as [OH]( = [OH] - [OH] and [COOH] = [COOH] - [COOH] . 

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The second source of sulfonic acid uses the following reaction scheme alkylation of benzene by a propylene oligomer then sulfonation of the alkylbenzene. 

Figure Bl.16.4. Part A is the vector representations of the. S state, an intennediate state, and the Jq state of a radical pair. Part B is the radical reaction scheme for CIDNP.

A general reaction scheme for CIDNP is shown in llgnre B1.16.4B. where the radical dynamics in each region... 

Excitable media are some of tire most commonly observed reaction-diffusion systems in nature. An excitable system possesses a stable fixed point which responds to perturbations in a characteristic way small perturbations return quickly to tire fixed point, while larger perturbations tliat exceed a certain tlireshold value make a long excursion in concentration phase space before tire system returns to tire stable state. In many physical systems tliis behaviour is captured by tire dynamics of two concentration fields, a fast activator variable u witli cubic nullcline and a slow inhibitor variable u witli linear nullcline [31]. The FitzHugh-Nagumo equation [34], derived as a simple model for nerve impulse propagation but which can also apply to a chemical reaction scheme [35], is one of tire best known equations witli such activator-inlribitor kinetics ... 

Drawing-, text-, and structure-input tools are provided that enable easy generation of flow charts, textual annotations or labels, structures, or reaction schemes. It is also possible to select different representation styles for bond types, ring sizes, molecular orbitals, and reaction arrows. The structure diagrams can be verified according to free valences or atom labels. Properties such as molecular... 

An R-matrix expresses the bond and electron rearrangement in a reaction. The R-matrix of Figure 3-12 reflects a reaction scheme, the breaking and the making... 

Any more complicated reaction scheme involving several bonds can be classified accordingly. Thus, a comprehensive system for a hierarchical classification of reactions can be built. 

Figure 3-13. The reaction scheme comprising the breaking and the making of two bonds and some examples of reactions following this scheme.

Some systematic studies on the different reaction schemes and how they are realized in organic reactions were performed some time ago [18]. Reactions used in organic synthesis were analyzed thoroughly in order to identify which reaction schemes occur. The analysis was restricted to reactions that shift electrons in pairs, as either a bonding or a free electron pair. Thus, only polar or heteiolytic and concerted reactions were considered. However, it must be emphasized that the reaction schemes list only the overall change in the distribution of bonds and ftee electron pairs, and make no specific statements on a reaction mechanism. Thus, reactions that proceed mechanistically through homolysis might be included in the overall reaction scheme. 

Figure 3-15. The reaction scheme breaking three and making three bonds, and some of the reaction types that fall into this scheme.

Figure 3-16. A reaction scheme that changes the number of bonds at one atom, and some specific examples.

Next, an attempt was made to evaluate the quantitative importance of the various reaction schemes [19]. To this effect, a printed compilation of 1900 reactions dealing with the introduction of one carbon atom bearing a functional group [20] was analyzed and each reaction assigned manually to a corresponding reaction scheme. The results are Hsted in Table 3-3. 

Clearly, this choice of a reference set of organic reactions is arbitrary, not necessarily representative of the whole set of organic reaction types described in the literature, and therefore not free from bias. However, it does give some indication of the relative importance of the various reaction schemes. It is quite clear that the reaction scheme shown in Figure 3-13 (R1 of Table 3-3) comprises the majority of organic reactions in most compilations of reactions it will account for more than 50 % of all reactions. 

Such an analysis of the Hterature for assigning reaction types to different reaction schemes definitely has merits. However, it does not say anything about the importance of a reaction type, such as how frequently it is actually performed in the laboratory. 

Clearly, such statistics are impossible to obtain on a worldwide basis. However, it is quite dear that organic reaction types that follow reaction scheme R1 (Table 3-3, Figure 3-13) are among the most frequently performed. This shifts the balance even further in the direction of this reaction scheme, lending overwhelming importance to it. 

The second most important reaction scheme is the next higher homolog to that shown in Figure 3-13, involving the breaking and making of three bonds. Figure 3-15 shows this reaction scheme and some reaction types that follow it. 

Figure 3-17. Consecutive application of two reaction schemes to model the oxidation of thioethers to sulfoxides.

The two reaction schemes of Figures 3-13 and 3-15 encompass a large proportion of all organic reactions. However, these reactions do not involve a change in the number of bonds at the atoms participating in them. Therefore, when oxidation and reduction reactions that also change the valency of an atom ate to be considered, an additional reaction scheme must be introduced in which free electron pairs are involved. Figure 3-16 shows such a scheme and some specific reaction types. 

Clearly, for symmetry reasons, the reverse process should also be considered. In fact, early versions of our reaction prediction and synthesis design system EROS [21] contained the reaction schemes of Figures 3-13, 3-15, and 3-16 and the reverse of the scheme shown in Figure 3-16. These four reaction schemes and their combined application include the majority of reactions observed in organic chemistry. Figure 3-17 shows a consecutive application of the reaction schemes of Figures 3-16 and 3-13 to model the oxidation of thioethers to sulfoxides. 

It has to be emphasized that these formal reaction schemes of Figures 3-13, 3-15, and 3-16 have the potential to discover novel reactions. Application of these bond- and electron-shifting schemes to specific molecules and bonds may correspond to a known reaction but may also model a completely novel reaction. 

Merges has systematically investigated certain reaction schemes and their realization in chemistry, this led him to instances that were without precedent. He could then verify some of these experimentally and thus discover new reactions [22]. 

Compounds are stored in reaction databases as connection tables (CT) in the same manner as in structure databases (see Section 5.11). Additionally, each compound is assigned information on the reaction center and the role of each compound in the specific reaction scheme (educt, product, etc.) (see Chapter 3). In addition to reaction data, the reaction database also includes bibliographic and factual information (solvent, yield, etc.). All these different data types render the integrated databases quite complex. The retrieval software must be able to recall all these different types of information. 

Again, no further explanation of this reaction scheme is intended here. It should just illustrate the complexity of the problem and present a challenge for further work. 

In principal, synthesis route prediction can be done from scratch based on molecular calculations. However, this is a very difficult task since there are so many possible side reactions and no automated method for predicting all possible products for a given set of reactants. With a large amount of work by an experienced chemist, this can be done but the difficulty involved makes it seldom justified over more traditional noncomputational methods. Ideally, known reactions should be used before attempting to develop unknown reactions. Also, the ability to suggest reasonable protective groups will make the reaction scheme more feasible. 

An alternative approach is to assume, in the light of the experimental evidence just mentioned, that the reactions of cations and neutral molecules have similar values of (or, equivalently, of log ( /l mol and to try to calculate the difference which would arise from the fact that the observed entropy of activation for a minority free base includes a contribution from the acidic dissociation of the conjugate acid in the medium in question (see (5) above). Consider the two following reaction schemes one (primed symbols) represents nitration via the free base, the other the normal nitration of a non-basic majority species (unprimed symbols) ... 

This simple reaction scheme accounts for three experimental observations ... 

Vollmann found that the reaction between l-imino-3-amino isoin dolenine (124) and 2-amino-4-methylthiazole is catalyzed by ammonium chloride and involves the exocyclic nitrogen (285). This reaction (Scheme 82) was later used to prepare dyes (286). 

The guanidine (272) is very reactive in condensation reactions (Scheme 166) (487). 

The 1,3-diester derivative of 2-imino-4-thiazoline (369) is obtained by the Schotten-Bauman reaction (Scheme 213) (263). 

The same 3-amino substituent is reactive in condensation reactions (Scheme 235) (701, 726-729). 2-Imino-3-amino-4-thia2oiine reacts, however, in the nicotinoylation reaction through its imino nitrogen (727). 

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