Classes of Mechanisms

A second way to classify reactions is according to the type of mechanism that is operative. The mechanism-based method of classification is used to organize the material in this text. However, it is important not to lose the forest (the overall transformation) for the trees (the mechanistic steps). Both classification methods have advantages, and it is useful to be able to move freely between them. 

There are four basic types of organic reaction mechanisms—polar, free-radical, pericyclic, and metal-catalyzed or mediated. 

The mechanism classification and the overall transformation classification are orthogonal to each other. For example, substitution reactions can occur by a polar acidic, polar basic, free-radical, pericyclic, or metal-catalyzed mechanism, and a reaction under polar basic conditions can produce an addition, a substitution, an elimination, or a rearrangement. Both classification schemes are important for determining the mechanism of a reaction, because knowing the class of mechanism and the overall transformation rales out certain mechanisms and suggests others. For example, under basic conditions, aromatic substitution reactions take place by one of three mechanisms nucleophilic addition-elimination, elimination-addition, or SrnL If you know the class of the overall transformation and the class of mechanism, your choices are narrowed considerably. 

Iron from a filled orbital. Free-radical reactions usually proceed by a chain reaction, though not always. 

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The metal substrate evidently affords a huge ( 10 and even as high as 10 [84, 85]) increase in the cross-section for Raman scattering of the adsorbate. There are two broad classes of mechanisms which are said to contribute to this enhancenient [, and Ml- The first is based on electromagnetic effects and the second on cheniicaT effects. Of these two classes the fomier is better understood and, for the most part, the specific mechanisms are agreed upon the latter is more complicated and is less well understood. SERS enhancenient can take place in either physisorbed or chemisorbed situations, with the chemisorbed case typically characterized by larger Raman frequency shifts from the bulk phase. 

The three classes of mechanical standards in TEMA are Classes R, C, and B representing varying degrees of mechanical details for the designated process plant applications severity. The code designations [TEMA—1988 Ed] for mechanical design and fabrication are ... 

The third class of mechanisms is that originated by Pichler and Schulz in which chain growth is accomplished by direct insertion of an absorbed CO molecule into a carbon-metal bond produced by hydrogenation of a surface carbonyl (89). A complete representation of this mechanism (see... 

Table 17.1 lists non-oncology compounds from diverse therapeutic, chemical, pharmacological areas and structures that induce clinical hematotoxicity. This demonstrates that bone marrow toxicity is not restricted to a small number of pharmacological or structural classes, thereby making it more difficult to understand specific mechanisms of toxicity. However, there are three classes of mechanisms of hematotoxicity, including antiproliferative, immune-mediated and other. Immune-mediated hematotoxicity and other indirect toxicities (e.g., a decrease of erythropoietin in kidney, leading to an impeded red cell production in the bone marrow) are not discussed in detail in this chapter as it requires involvement of the immune system or remote interactions and in vitro profiling assays have not been developed to detect these mechanisms. 

In this part of the chapter we briefly outline the main classes of mechanisms occurring in chemiluminescent transformations of organic peroxides. The transformations involving isolated 1,2-dioxetanes and related species will not be extensively discussed, since a specific chapter is dedicated to these compounds. We therefore limit ourselves to describing the general decomposition mechanisms of these peroxides, as these are important in the context of the more complex CL systems that we will describe in the last part of this chapter. 

Number of interior roots. System (21) may have both interior (z, 0, i — 1,..., n) and boundary roots (3i z, = 0). As a rule, boundary roots correspond to the zero reaction rate. Some classes of mechanisms can be free of boundary roots at all. For instance, system (21) corresponding to the mechanism of the Basic case type, satisfying the condition (35) does not have boundary roots. All interior roots belong to the algebraic tore. Bemstain theorem (see GeTfand et al., 1994) can be applied to estimate the number of roots in this case. 

I believe that since then Stein s group in Jerusalem has accumulated evidence which suggests that at least under some conditions this is the main reaction path of ferrous ions with hydrogen atoms. In view of this, I wonder specifically what evidence you had bearing on this point. This is important, not only in relation to this class of mechanisms, but also to the more general problem of how the interaction involved in the relatively ionic coordination of a ligand such as water to a metal ion would reflect in weakening a covalent P—H bond. 

For this analysis it is of importance to classify mechanisms (their associated kinetic models here are the sets of quasi-steady-state equations) to answer the question of what class of mechanisms possesses a unique and stable solution for the quasi-steady-state equations, and which one can have several solutions, i.e. several steady states. 

A second result consists in the determination of the class of mechanisms which always have a positive PCB. For its description, let us consider a graph of complex conversions. It nodes are y, and its edges connect those y, and yk for which a list of steps has either y — yk or yk -> y, reactions. In case both reactions take place (y, yk), the two arrows indicate different reac-... 

Hence, in addition to the systems without intermediate interactions, the conditions for the existence of a PCB account for one more class of mechanisms that always have an unique and stable steady state. In conclusion, let us emphasize that, on the basis of the Rozonoer approach [55, 56], Orlov has recently extended the Horn and Jackson results to the non-ideal systems of a rather general type having a PCB [57, 58],... 

There is no doubt that studies for the establishment of new classes of mechanisms possessing an unique and stable steady state are essential and promising. On the other hand, it is of interest to construct a criterion for uniqueness and multiplicity that would permit us to analyze any reaction mechanism. An important contribution here has been made by Ivanova [5]. Using the Clark approach [59], she has formulated sufficiently general conditions for the uniqueness of steady states in a balance polyhedron in terms of the graph theory. In accordance with ref. 5 we will present a brief summary of these results. As before, we proceed from the validity of the law of mass action and its analog, the law of acting surfaces. Let us also assume that a linear law of conservation is unique (the law of conservation of the amount of catalyst). 

The reaction model assumed is one in which free-radical polymerisation is compartmentalised within a fixed number of reaction loci, all of which have similar volumes. As has been pointed out above, new radicals are generated in the external phase only. No nucleation of new reaction loci occurs as polymerisation proceeds, and the number of loci is not reduced by processes such as particle agglomeration. Radicals enter reaction loci from the external phase at a constant rate (which in certain cases may be zero), and thus the rate of acquisition of radicals by a single locus is kinetic-ally of zero order with respect to the concentration of radicals within the locus. Once a radical enters a reaction locus, it initiates a chain polymerisation reaction which continues until the activity of the radical within the locus is lost. Polymerisation is assumed to occur almost exclusively within the reaction loci, because the solubility of the monomer in the external phase is assumed to be low. The volumes of the reaction loci are presumed not to increase greatly as a consequence of polymerisation. Two classes of mechanism are in general available whereby the activity of radicals can be lost from reaction loci ... 

The mechanism based on the chemiosmotic hypothesis of ATP synthesis can also be related to this class of mechanism, because it promotes the separate use of the highly active intermediate particle (H+ ion) with the maximum effectiveness and in each of conjugated reactions. 

The class of mechanical methods used for preparing colloidal dispersions in which particles or droplets are progressively subdivided. See also Condensation Methods. 

Examples of xenobiotics that bind to proteins and fall into this class of mechanism-based inhibitor include tienilic acid, cannabidiol, chloramphenicol, secobarbital, some psoralens, spironolactone, mifepristone, and grapefruit juice. 

Another, class of mechanisms pertains to movements in gross anatomical entities of the head such as scalp, muscle, skull and... 

Supposing you were asked what the mechanisms of the next two reactions might be. This is a rather different sort of problem as you probably don t recognize any of these reagents and you probably cannot fit any of the reactions into one of the classes you have seen so far. You probably don t even see at once which of the three main classes of mechanism you should use ionic pericyclic or radical. 

We may define the stabilisation ratio, s, as that fraction of the total current that oxidises or reduces the solution redox couple. Specifying, for convenience, an n-type semiconductor and considering stabilisation to photogenerated holes, there are two basic classes of mechanism. 

There are three general classes of mechanism most often encountered in alkane reactions (i) radical (ii) electrophilic and (iii) carbenoid. The C—bond-lneaking st in (i) and (ii) are shown in equations (2) and (3). Carbenoid reactions can go either by direct insertion into the C—bond (equation 4), which tends to happen when the carbene in question has singlet character, or by a two-stqi process (equations 5 and 6), in which H-atom abstraction precedes collapse of the radical pair, a pathway which is characteristic of triplet carbenes. ... 

Groutas WC, Kuang R, Venkataraman R, Epp JB, Ruan S, Prakash O. Stmcture-based design of a general class of mechanism-based inhibitors of the serine proteinases employing a novel amino acid-derived heterocyclic scaffold. Biochemistry 1997 36 4739-4750. 

Another system in this class of mechanisms that was also modeled mathematically is the technically important Fischer-Tropsch process on Fe catalysts investigated by Caldwell (217,218). Oscillation had been previously observed in this system by Tsotsis and co-workers (279). Caldwell reduced the mechanism of the Fischer-Tropsch reaction to three reactions ... 

Various subclassifications exist according to the exact nature of the chemical step, which may eventually be a succession of elementary steps with formation of intermediate products. As explained earlier for the displacement of endergonic electron transfer steps, the C step occurs because it is continuously pulled to the right by the further chemical reaction of the Y species. Note that Chapter 28 is devoted to this class of mechanisms. 

A second class of mechanism-based inhibitors are those that in the course of their cleavage by the enzyme release an aglycone that rearranges to give a reactive species. Two examples may be cited. Salicortin [164], a natural product, is hydrolyzed by )6-glucosidases to produce, by rearrangement, a reactive quinone methide as shown inO Fig. 75 [165]. 

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