Orientation of Reactants

Reactions between species that require no special orientation during collision to result in a reaction tend to be more rapid than reactions between species that must be properly aligned for a collision to result in a reaction. This so-called orientation effect partially explains why many collisions between reacting species are not successful in producing a reaction. For example, the spherical Ag ion and the spherical Cl ion combine very rapidly to form the precipitate AgCl(s) when present in a 

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Many reactions catalyzed by the addition of simple metal ions involve chelation of the metal. The familiar autocatalysis of the oxidation of oxalate by permanganate results from the chelation of the oxalate and Mn (III) from the permanganate. Oxidation of ascorbic acid [50-81-7] C HgO, is catalyzed by copper (12). The stabilization of preparations containing ascorbic acid by the addition of a chelant appears to be negative catalysis of the oxidation but results from the sequestration of the copper. Many such inhibitions are the result of sequestration. Catalysis by chelation of metal ions with a reactant is usually accomphshed by polarization of the molecule, faciUtation of electron transfer by the metal, or orientation of reactants. 

The above kinetic scheme of the bimolecular reaction simplifies physical processes that proceed via the elementary bimolecular act. To react, two reactants should (a) meet, (b) be oriented by the way convenient for the elementary act, and (c) be activated to form the TS and then react. Hence, not only translational but also rotational diffusion of particles in the solution and polymer are important for the reaction to be performed. So, the more detailed kinetic scheme of a bimolecular reaction includes the following stages diffusion and encounter the reactants in the cage, orientation of reactants in the cage due to rotational diffusion, and activation of reactants followed by reaction [5,13]. 

In liquids all orientations of reactants A and B are energetically equivalent and P = Air) 2 A0aA0b. Therefore, the ratio of rate constants in polymer and liquid is... 

X and Y. This behavior was not observed, suggesting that the substrate had considerable freedom of motion at the micellar surface. However, product selectivity is observed in some reactions in aqueous surfactants, and may be related to orientation of reactants in micelles (Sections 9 and 10). 

Figure 6.9 shows five of the many possible ways in which NO and NO3 can collide. Only one of these five possibilities has the correct collision geometry for a reaction to occur. As shown in the figure, only a certain orientation of reactants prior to collision leads to the formation of two molecules of nitrogen dioxide. 

In Heading 1.4 we have already seen that three different kinds of control elements [3] may be considered 1) chemoselective control elements (controlling chemical reactivity), 2) regioselective control elements (controlling the orientation of reactants) and 3) stereoselective control elements (controlling the spatial arrangement of atoms within the molecule), which may control either the relative (diastereoselective) or the absolute spatial arrangement (enantioselective control elements). 

A summary of aniline N-methylation mechanistic features on Cui xZnxFe204 ferrospinel catalysts is given in Figure 27. It was possible, due to in-situ IR studies, to observe a dissociative adsorption and possible orientation of reactants on the catalyst surface, their conversion to product at low temperatures, and desorption-limited kinetics, all under conditions that are close to the reaction conditions. Although Cu is the active center for the aniline A-methylation reaction, and IR studies reveal that Zn acts as the main methyl species source. 

The orientation of reactant molecules in a collision can determine whether or not a reaction takes place. A perpendicular collision between N2 and 02 does not tend to result in formation of a product molecule. 

Tetrahydrothiopyran and its simply substituted derivatives are to be regarded as cyclic sulfides, and in consequence their chemistry is analogous to that of their acyclic counterparts, which is extensively covered in the standard texts (B-79MI22500). Oxidation, alkylation, halogenation, etc. reactions differ from the corresponding phenomena in the noncyclic species only in so far as the stereochemical constraints imposed by a more rigid framework control the orientation of reactants, transition states and products. 

Several reviews deal with the solid-state reactions of simple inorganic salts and of organic compounds.1-8 The essential differences between solid-state reactions and reactions in solution can be ascribed to the fact that solid-state reactions occur within the constraining environment of the crystal lattice. The reactant crystal lattice can control both the kinetic features of a reaction, and the nature of the products. In many solid-phase reactions the separation distances and mutual orientations of reactants in the solid determine the product. Such reactions are said to be topo-chemically controlled.9 Topochemical control of a reaction product is analogous to kinetic control in solution. The product is not necessarily the thermodynamically most stable product available to the system, but is rather the one dictated by the reaction pathway available in the constraining environment of the solid. 

Figure 8 Geometrical parameters describing the relative orientation of reactant double bonds. T and T are the tt-orbitals of the double bonds, and A, B, and C are the point atoms. The distance between T and T represents the extent of overlap of T and T. ...

Increasing the rate of the reaction by increasing the frequency of reaction-promoting contacts, or favorable orientation of reactants (88) ... 

The key element of the strategy described in this section is the provision of a binding site for the substrate close to a catalytic center. Of the mechanistic effects described by Jencks (I), the following contribute proximity and orientation of reactants and covalent catalysis. 

If the coefficients are calculated for 1-methoxy-1,3-butadiene the termini are -1-0.3 and —0.58 (or —0.3 and -1-0.58). For acrylonitrile the coefficients are -1-0.2 and —0.66 (or —0.2 and -1-0.66). The cycloaddition reaction proceeds so that the coefficients match, in terms of both phase (essential) and coefficient magnitude -1-0.3 with -1-0.2 and —0.58 with —0.66. Thus, the preferred orientation of reactants (diene and dienophile) for the initial bonding interaction is displayed and this orientation agrees with the regioselectivity reported in Fig. 8.33. 

Figure 9 Proposed orientation of reactants in phthalide enolate-imine additions leading to (a) cis or (b) trans...

Two miscellaneous methods for preparing the diaza-crowns are presented here. Okahara and coworkers, in an attempt to prepare aza-9-crown-3, reacted an azatriethylene glycol with benzenesulfonyl chloride. The small crown was not obtained, but rather two diaza-18-crown-6 compounds resulting from different orientations of reactants in the dimerization pathway (method K-1) (Kuo et al., 1978). 

The original definition of regioselectivity and the subsequent lUPAC recommendation of that term were reexamined and shown to encompass two conceptually distinct ideas. Where the focus of attention is onsite selectivity, the correct term should be situselectivity/toposelectivity in this respect, the term regioselectivity is inapplicable and should be abandoned. On the other hand, where the effect of relative orientations of reactants is concerned and vectorial reversal is being considered, the term regioselectivity may still be used however, one should realize that regioselectivity is indeed a specific manifestation of the wider concept of vectoselectivity, as defined in this chapter. 

The complexation mode stoichiometries, conformations, and dynamics The possibility of simultaneous formation of complexes with different stoichiometries must also be considered. Cooperative binding, a bi-molecular process, has been observed, as well as the uni-molecular process, producing a 1 1 complex in either a head-to-tail (HT), tail-to-tail (TT) or head-to-head (HH) state. Moreover, a study of the fluorescence decay of a number of aniUnonaphthalene sulfonates in the presence of CD supported the view that a 1 1 complex can be present in several slightly different conformations. There are, in fact, various conformations and orientations of reactants as well as varying mobilities in reactant-CD complexes. 

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