Single-step, third-order reaction

Although the order of a single mechanistic step can be predicted from the molecularity, the molecularity of a step, or steps, cannot be predicted from the order of the overall reaction. There are a number of complications which make it impossible to conclude automatically that a first-order reaction is unimolecular, that a second-order reaction is bimolecular, or that a third-order reaction is termolecular. In many cases, the reaction is a sequence of steps, and the overall rate may be governed by the slowest step. Experimental conditions might interchange the relative speeds of different steps, and the... 

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... 

The reaction doesn t occur in a single elementary step because, if it did, the rate law would be third order Rate = k[H2][ICl]2. The observed rate law will be obtained if the rate-determining step involves the bimolecular reaction of H2 and IC1. 

We and others have demonstrated that association of short strands containing a single guanine-repeat seems to obey a fourth-order kinetics model. Third or fourth-order reactions are not common in biochemistry, and the practical consequences of this reaction order are important. A fourth-order reaction does not imply that an elementary kinetic step involves a four-body collision. Such mechanism is extremely unlikely and other processes could lead to this fourth order. The structure of these elusive intermediates remains unknown Stefl et have recently demonstrated that a Hoogsteen G-G duplex is an improbable intermediate. Its identification will be experimentally difficult, as numerical simulations indicate that it may not be present at detectable levels. 

Termolecular reactions, such as A + B + C — P do not usually consist of a single tiimolecular step, and consequently are not usually third order. Instead, the reaction is likely to proceed via two or more elementary steps, such as A -f B - X, foUowedby X -i- C — P. If one step in such a reaction is much slower than the others, the rale of the complete reaction is equal to the rate of the slow... 

A third, though far less studied possibility, is the one-step synthesis of macrocycles by a multicomponent reaction (MCR). MCR is a process in which three or more reactants are combined in a single reaction vessel to produce a product that incorporates substantial portions of all the components [29]. They have, by definition, sustainable chemistry and are inherently (a) chemo- and regioselective, a prerequisite for a successful MCR since at least three reactive functional groups are involved and they have to react in an ordered and selective fashion (b) atom-economic [30] since most of them involve addition rather than substitution reactions. 

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