Definition of Elementary Steps

Initiation consists of ion-generation and cationation [1, 43]. Ion-generation may be due to adventitious impurities HX or to purposely added initiator RX. For simplicity, we consider ion-generation only in the forward direction  

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For more detailed description see also parts 2.1 (Table of Symbols for Scheme 1) and 2.2.1 (Definition of Elementary Steps)... 

Macroscopic definition of elementary steps Consider a thermal reaction of the form -> products... 

A knowledge of the kinetic parameters and, in particular, the orders of reaction of a catalysed reaction is important to the accurate definition of the reaction mechanism. However, catalytic hydrogenation reactions proceed through a series of elementary steps, only one of which may be ratedetermining. In consequence, the observed rate expressions give little or no direct information about most of the steps involved and kinetics alone are not sufficient for a precise description of the mechanism. 

In the isomerisation of the tetra-substituted olefin 3,4-dimethylhex-3-ene over palladium—alumina [146], it has been shown that double bond migration is a necessary precursor to cis—trans isomerisation. This has been interpreted as showing that the mechanism involves a series of elementary steps, each of which is stereospecific, although no definite conclusions were drawn as to whether an addition—abstraction or an abstraction-addition mechanism was involved. 

In contrast to outer-sphere reactions, the simple observation that a reaction occurs by an inner-sphere mechanism necessarily introduces an element of structural definition. The relative dispositions of the oxidizing and reducing agents are immediately established and, except for structurally flexible bridging ligands such as NC5H4(CH2) C5H4N, the internuclear separation between redox sites can be inferred from known bond distances. Even so, bimolecular inner-sphere reactions necessarily occur by a sequence of elementary steps (Scheme 2) and the observed rate constant may include contributions from any of the series of steps. 

One-step reactions between stable molecules are very rare. This is only natural since a stable molecule is, by definition, quite unreactive, and complicated rearrangements of bonds are usually required to go from reactants to products. This means that most directly observed reactions do not proceed in a single elementary step. Rather, a sequence of elementary steps is necessary, and reactants or products of these are not the stable reactants or products but are highly reactive intermediates that shall be called active centers. 

Proton-coupled electron transfer (PCET) describes any elementary step (or series of elementary steps) in which both a proton and an electron are exchanged [1-9]. Within this broad definition, further distinctions may be drawn if the proton and... 

As explained before, a chemical reaction can seldom be described by a single elementary step, and hence we need to adapt our definition of activation for an overall reaction. Since we are not particularly interested in the effects of thermodynamics we define the apparent activation energy as... 

Besides, the structure, nature and reactivity of the chemisorbed molecule could not be unambiguously identified because the physical tools used could not lead easily to a complete understanding of the quasi molecular character of surface chemisorbed species and move precisely to the definition of the elementary steps occurring during the molecular transformations taking place on the surfaces. 

Since all of the above-mentioned interconversion reactions are reversible, any kinetic analysis is difficult. In particular, this holds for the reaction Sg - Sy since the backward reaction Sy -+ Sg is much faster and, therefore, cannot be neglected even in the early stages of the forward reaction. The observation that the equilibrium is reached by first order kinetics (the half-life is independent of the initial Sg concentration) does not necessarily indicate that the single steps Sg Sy and Sg Sg are first order reactions. In fact, no definite conclusions about the reaction order of these elementary steps are possible at the present time. The reaction order of 1.5 of the Sy decomposition supports this view. Furthermore, the measured overall activation energy of 95 kJ/mol, obtained with the assumption of first order kinetics, must be a function of the true activation energies of the forward and backward reactions. The value found should therefore be interpreted with caution. 

A definition of the cycle rate is clear for steady states because stationary rates of all elementary reactions in cycle coincide. There is no common definition of the cycle rate for nonstationary regimes. In practice, one of steps is the step of product release (the "final" step of the catalytic transformation), and we can... 

For our purpose elementary steps can be chosen to include any reaction that cannot be further broken down so as to involve reactions in which the specified intermediates are produced or consumed. Ideally, elementary steps should consist of irreducible molecular events, usually with a molecularity no greater than two. Such steps are amenable to treatment by fundamental chemical principles such as collision and transition state theories. Often such a choice is not feasible because of lack of knowledge of the detailed chemistry involved. Each of these elementary reactions, even when carefully chosen, may itself have a definite mechanism, but theory may be unable to elucidate this finer detail [Moore (2)]. 

The assumption of a definite location for the negative charge on the j8-carbon requires that 59a and 60a both be on the lowest energy path for substitution and isomerization. That this location should be formed anti derives from orbital symmetry, i.e. microscopic reversibility suggests that anti entry of one halide and syw-departure of the other are improbable. We deduce, therefore, that the substitutions in (142) require at least three elementary steps, e.g. cis-1 - 59a -> 60b -v cis-2 isomerizations with exchange require at least three steps, e.g. cis-l 59a —> 59b - trans-2 isomeriza-... 

A chemical transformation that takes place via exactly one transition state is called an elementary reaction. This holds regardless of whether it leads to a short-lived intermediate or to a product that can be isolated. According to the definition, an n-step reaction consists of a sequence of n elementary reactions. It takes place via n transition states and (n 1) intermediates. 

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