Termolecular steps

Elementary reactions are generally unimolecular or bimolecular, depending on whether they entail the reaction of one species or two. Occasionally a termolecular step occurs, particularly between atoms or small molecules in the gas phase. Solution reactions that might appear to be termolecular usually prove really to be a succession of two simpler ones. 

This rate expression is consistent with the observed kinetics, so this combination of a slow termolecular step with a rapid bimolecular step is a plausible mechanism based on the information we have been given. 

This is the most common category of elementary reactions and can be illustrated by unimolecular, bimolecular, and termolecular steps. 

Termolecular steps are rare, but may appear to arise from two rapid bimolecular steps in sequence. 

One B molecule and two A molecules are needed to give the species for the slow step. The three molecules do not collide simultaneously since we are disregarding the very rare termolecular steps. There must then be some number of prior fast steps to furnish at least one intermediate needed for the slow step. The second B molecule which appears in the reaction equation must be consumed in a fast step following the slow step. 

The verification of the mechanism will tell nothing about the details of reaction (19), as discussed the apparent termolecular step can still be composed of a rapid equilibrium involving either (NO)2 or ONOO, followed by a rapid reaction of the intermediate with 02 or NO, respectively. However, it is interesting to compare the predicted rate of the unlikely mechanism... 

The investigations of early workers107 -252-269-423-426-426 -428,437 on the nitric oxide-chlorine system, while not immediately understandable, were concordant in their results. These results were critically reviewed and extended by Welinsky and Taylor,446 who recalculated the apparently inconsistent results of Trautz and co-workers and showed that the anomalous results they obtained in the presence of excess nitric oxide were due to incorrect analysis. Trautz had concluded that, in accordance with his general kinetic theory, the reaction proceeded by two consecutive steps, reactions (2) and (3). However, Welinsky and Taylor felt there was no basis for assuming any mechanism other than a single termolecular step. In Figure 5-1 values are shown for the third-order rate constant 1 for the reaction... 

These equations clearly reduce to the original model with the termolecular step in the limit e — k — 0. 

Mechanism B could involve either a single termolecular step rapid reaction of solvent with an intermediate, slowly formed pentacovalent... 

The disadvantage of general base catalysis is that the first, rate-determining, step is termolecular. It is inherently unlikely that three molecules will collide with each other simultaneously and in the next section wc shall reject such an explanation for amide hydrolysis. In this case, however, if ROH is the solvent, it will always be present in any collision so a termolecular step is just about acceptable. 

In principle such a termolecular step is not distinguishable from the two fast consecutive steps, rea( tions 2 and 4. 

Define what is meant by unimolecular and bimolecular steps. Why are termolecular steps infrequently seen in chemical reactions ... 

Termolecular step a reaction involving the simultaneous collision of three molecules. (15.6)... 

The mechanisms of reactions such as (e) are difficult to elucidate. Reaction via a direct termolecular step, H2 addition to undetectable amounts of dimer, or the process outlined in equation (i) have all been considered for cobalt(II) systems. 

Agreement of the rate equation with observation, however, cannot be taken as evidence supporting the assumed single-step mechanism of the surface reaction. It is highly improbable that two adsorbed hydrogen atoms react simultaneously with adsorbed aldehyde in a single, termolecular step. Much more likely is a two-step mechanism... 

Why is a bimolecular step more reasonable physically than a termolecular step ... 

The frequency of occurrence of three-body collisions is very much smaller than that of two-body collisions. Consequently, if a termolecular step is essential to the progress of the reaction, the reaction is very slow. 

Termination reactions which are first order in chain centres thus emerge as the favoured candidate to be influential in shaping the kinetics of fast chain reactions. These may occm by association of the centres with a major component of the reacting system in a bimolecular or termolecular step, or by some other bimolecular or even xmi-molecular step which consumes a chain centre without regenerating... 

Each of these reactions is called an elementary step, a reaction whose rate law can be written from its molecularity. Molecularity is defined as the number of species that must collide to produce the reaction indicated by that step. A reaction involving one molecule is called a unimolecular step. Reactions involving the collision of two and three species are termed bimolecular and termolecular, respectively. Termolecular steps are quite rare because the probability of three molecules colliding simultaneously is very small. Examples of these three types of elementary steps and the corresponding rate laws are shown in Table 15.7. Note from Table 15.7 that the rate law for an elementary step follows directly from the molecularity of that step. For example, for a bimolecular step, the rate law is always second order, either of the form [A] for a step with a single reactant or of the form [A][B] for a step involving two reactants. 

The first time you met third-order kinetics (where it arose from a combination of more than one step, see p. 261) we pointed out how unlikely real termolecular steps are. 

Thus, mechanism B, which consists solely of bimolecular and unimolecular steps, is also consistent with the information that we have been given. This mechanism is somewhat simpler than the first in that it does not requite a termolecular step. 

We seek a reaction mechanism that is consistent with this rate law. One possibility is that the reaction occurs in a single termolecular step ... 

If the termolecular step in the Brusselator proceeds via an activated U2 complex, we have the following kinetic scheme for the Brusselator DDRW ... 

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