Kinetic consequences of reaction pathways

Although the kinetic rate law is helpful in determining the mechanism of a reaction, it does not always provide sufficient information. In cases of ambiguity, other evidence must be used to find the mechanism. This chapter will describe a number of examples in which the rate law and other experimental evidence have been used to find the mechanism of a reaction. Our goal is to provide two related types of information (1) the type of information that is used to determine mechanisms, and (2) a selection of specific reactions for which the mechanisms seem to be fairly completely determined. The first is the more important, because it enables a chemist to examine data for other reactions critically and to evaluate the proposed mechanisms. The second is also helpful, because it provides part of the collection of knowledge that is required for designing new syntheses. Each of the substitution mechanisms is described with its 

Intimate Mechanism Octahedral Reactant Octahedral Reactant 

1 Slow Reactions (Inert) Moderate Rate Fast Reactions (Labile) 1 

Strong-field d (square planar) Weak-field d d (f, d ° 

This chapter describes examples in which the rate law is used to propose reaction mechanisms. We provide two types of information (1) the information used to propose mechanisms and (2) specific reactions for which mechanisms are known with fairly high levels of confidence. The first is necessary to critically examine data for other reactions. The second is helpful since it forms a knowledge base to shed light on new reactions. Each substitution mechanism, D, I, and A, will be described with its rate law.  

FIGURE 12.3 Energy Profiles for Dissociative and Associative Reactions, (a) Dissociative mechanism. The intermediate has a lower coordination number than the reactant, (b) Associative mechanism. The intermediate has a higher coordination number than the reactant. 

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Kinetic Consequences of Reaction Pathways 441 TABLE 12.2 Classification of Substitution Mechanisms for Octahedral Complexes... 

As demonstrated in the work of Grimbs et al. [296] using a model of the human erythrocyte, the ranking obtained by any of the measures above is i) consistent independent of the specific measure and ii) corresponds closely to a ranking obtained from an explicit kinetic model of the pathway, and it is in excellent agreement with prior knowledge of the metabolic system. Consequently, we expect that the methods described here and in [296] provide a suitable starting point to locate crucial parameters and reactions in cellular metabolism, as well as in cases where the construction of explicit kinetic models is not (yet) feasible. 

The role of a catalyst is, first, to form a complex molecule with reactant molecule coordination through its appropriate frontal orbital and thus fully weaken the H H bond and triple N=N bond. For example, H-H changes to two coordinated hydrogens and then may easily form new bond with the activated reaction intermediates. In other words, catalyst can participate in the reaction and form unstable intermediate complexes with reactants and form products at last. The activation energies required for every step are much lower than that for the reaction without catalyst, and therefore the reaction rate is accelerated. Consequently, the reaction pathway is changed by catalysts. For the case of ammonia synthesis reaction on Fe (111), Ertl, a winner of the Nobel chemistry prize in 2007, proposed a thermochemical kinetic profile, as shown in Fig. 2.1. 

A disadvantage of this technique is that isotopic labeling can cause unwanted perturbations to the competition between pathways through kinetic isotope effects. Whereas the Born-Oppenheimer potential energy surfaces are not affected by isotopic substitution, rotational and vibrational levels become more closely spaced with substitution of heavier isotopes. Consequently, the rate of reaction in competing pathways will be modified somewhat compared to the unlabeled reaction. This effect scales approximately as the square root of the ratio of the isotopic masses, and will be most pronounced for deuterium or... 

It is known that when the kinetic behaviour cannot be studied in the absence of the molecular complex (and the reactivity of the free reagent is unknown), the kinetic law by itself cannot indicate whether the complex observed is on the reaction pathway or is a non-productive equilibrium267. In S/yAr reactions discussed here, the usual experimental conditions [ArF]o < [RNH2]o are used. In some cases, the k0bs values obtained under experimental conditions [ArF]o > [RNH2]o may be assumed to involve only the uncatalysed process. As a consequence, it is possible to say that the presence of complexes (in apolar solvent) clearly produces an enhancement of the reactivity and a positive catalysis, while in polar solvents256 the presence of complexes depresses the rate of substitution. 

The rule of HPLC in the study of etuyme systems is expected to burgeon from these small beginnings. The technique can be used to analyze complex mixtures of reaction products with high speed and preci-Moii. As u consequence, the existence ul cuinpeting reactions niuy be lup-idly discovered and the kinetics of the reactions associated with the various pathways conveniently analyzed. 

In terms of the development of an understanding of the reactivity patterns of inorganic complexes, the two metals which have been pivotal are platinum and cobalt. This importance is to a large part a consequence of each metal having available one or more oxidation states which are kinetically inert. Platinum is a particularly useful element of this pair because it has two kinetically inert sets of complexes (divalent and tetravalent) in addition to the complexes of platinum(O), which is a kinetically labile center. The complexes of divalent and tetravalent platinum show significant differences. Divalent platinum forms four-coordinate planar complexes which have a coordinately unsaturated 16-electron d8 platinum center, whereas tetravalent platinum is an 18-electron d6 center which is coordinately saturated in its usual hexacoordination. In terms of mechanistic interpretation one must therefore consider both associative and dissociative substitution pathways, in addition to mechanisms involving electron transfer or inner-sphere atom transfer redox processes. A number of books and articles have been written about replacement reactions in platinum complexes, and a number of these are summarized in Table 13. 

Platinum(IV) is kinetically inert, but substitution reactions are observed. Deceptively simple substitution reactions such as that in equation (554) do not proceed by a simple SN1 or 5 2 process. In almost all cases the reaction mechanism involves redox steps. The platinum(II)-catalyzed substitution of platinum(IV) is the common kind of redox reaction which leads to formal nucleophilic substitution of platinum(IV) complexes. In such cases substitution results from an atom-transfer redox reaction between the platinum(IV) complex and a five-coordinate adduct of the platinum(II) compound (Scheme 22). The platinum(II) complex can be added to the solution, or it may be present as an impurity, possibly being formed by a reductive elimination step. These reactions show characteristic third-order kinetics, first order each in the platinum(IV) complex, the entering ligand Y, and the platinum(II) complex. The pathway is catalytic in PtnL4, but a consequence of such a mechanism is the transfer of platinum between the catalyst and the substrate. 10 This premise has been verified using a 195Pt tracer.2011... 

Two examples of the dynamic trajectories (the time dependence of the kinetic energy) for the molecule RDX are presented in the Fig. 3 and 4, where the Fig. 3 illustrates the reaction pathway Rl and the Fig. 4 shows the trajectory for the reaction R2. Arrows mark the points on the dynamic trajectory corresponding to the rupture of C-N and N-N02 bonds. Fig. 5 shows the example of trajectory for p-HMX, where the decomposition process is the combination of Rl and R2 reactions. Anyway the character of dynamic trajectories and consequently the character of molecular decomposition is very similar for RDX and p-HMX. 

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