Clock reaction, mechanism

The simplest manifestation of nonlinear kinetics is the clock reaction—a reaction exliibiting an identifiable mduction period , during which the overall reaction rate (the rate of removal of reactants or production of final products) may be practically indistinguishable from zero, followed by a comparatively sharp reaction event during which reactants are converted more or less directly to the final products. A schematic evolution of the reactant, product and intenuediate species concentrations and of the reaction rate is represented in figure A3.14.2. Two typical mechanisms may operate to produce clock behaviour. 

When chemists investigate the mechanism of a reaction, they are not so lucky. Determining the mechanism of a chemical reaction is a bit like figuring out how a clock works just by looking at its face and hands. For this reason, reaction mechanisms are proposed rather than definitively stated. Much of the experimental evidence that is obtained to support a mechanism is indirect. Researchers need a lot of creativity as they propose and test mechanisms. 

J. Chem. Ed. 1955,32, 78. See also J. J. Fortman and J. A. Schreier, "Some Modified Two-Color Formaldehyde Clock Salutes for Schools with Colors of Gold and Green or Gold and Red, J. Chem. Ed. 1991, 68, 324 M. G. Burnett, The Mechanism of the Formaldehyde Clock Reaction, J. Chem. Ed. 1982,59, 160 and P. Wameck, The Formaldehyde-Sulfite Clock Reaction Revisited, ... 

In addition to their use for the calibration of rates for radical reactions, radical clocks can be employed to distinguish between ionic and radical pathways. In the simplest embodiment of this idea, a suitable clock reaction that undergoes a known fast rearrangement with easily identifiable products is incorporated into the reaction system to be studied. This approach has been exploited in the pioneering work of Newcomb and co-workers in studies of the mechanism of cytochrome P450 oxidation reactions [13]. Newcomb has developed a range of ultrafast radical clocks able to detect radical species with lifetimes of 80-200 fs. 

Others authors propose alternative ways to conduct or discuss an experiment that is already commonly used in the teaching of chemical kinetics. For example the presentation of a videotaped clock (iodine-azide) reaction which is suitable for videotaping and which has an easily determined mechanism (Haight Jones, 1987) the determination of a reaction mechanism of the blue bottle reaction (Engerer Cook, 1999). Others may be used to discuss thermodynamics and kinetics simultaneously, e.g., from the study of the surface of nanomaterials (such as a gold colloid monolayer) (Keating, Musick, Keefe Natan, 1999), and from the study of a chemical equihbrium in solution (Leenson, 1986). 

The system used in the Landolt clock reaction, IOj/SOl , when treated with [Fe(CN)6] , shows oscillations in a CSTR. A modification of the mechanism proposed previously resolves a number of problems. A slightly different system, I03/HS03/S20i produces a small number of high-amplitude pH oscillations (up to 2 units of pH) over a narrow range of conditions in a closed system. ... 

Chlorite ion oxidizes iodine to iodate, a well-known clock reaction, equation (25). The proposed mechanism postulates the intermediate ICIO2,... 

A second requirement is that the system contain some sort of feedback, i.e., that the product of some reaction exert an influence upon its own rate of production. Probably the simplest type of feedback and the one most commonly found in chemical oscillators is autocatalysis. The simple model (Lotka, [16]) containing two coupled autocatalytic reactions shown in Table 1 was probably the first chemical" mechanism to give sustained oscillations. While autocatalysis is far more prelavent in biological than in chemical systems, autocatalytic chemical reactions do exist many of them are listed as "clock reactions in collections of lecture demonstrations. 

In a closed system, the chlorite iodide reaction is a clock reaction if 1 < [I ]o/[C102 ]o < 4. Its kinetics were determined and the basic elements of a mechanism were proposed first for the batch reaction by Kern and Kim [22]. Figure 1 shows kinetic curves (iodine absorbance vs time) of the reaction with different initial ratios of the reactants. In the first part of each curve, iodine is produced at an accelerating rate. During this period, the stoichiometry is... 

Zigmond, 1988). The ATP-hydrolysis that accompanies actin polymerization, ATP —> ADP + Pj, and the subsequent release of the cleaved phosphate (Pj) are believed to act as a clock (Pollard et ah, 1992 Allen et ah, 1996), altering in a time-dependent manner the mechanical properties of the filament and its propensity to depolymerize. Molecular dynamics simulations suggested a so-called back door mechanism for the hydrolysis reaction ATP ADP - - Pj in which ATP enters the actin from one side, ADP leaves from the same side, but Pj leaves from the opposite side, the back door (Wriggers and Schulten, 1997b). This hypothesis can explain the effect of the toxin phalloidin which blocks the exit of the putative back door pathway and, thereby, delays Pi release as observed experimentally (Dancker and Hess, 1990). 

More than two dozen radical clocks are now known. They span a range of lifetimes from 10 to 10"7 s.12 The investigator must be aware of the possibility that the clock rearrangement is due to a side reaction or that the radical induced an efficient chain mechanism (Chapter 8). Also, radicals are not the only entities that can rearrange in this fashion. Carbanions, for example, have been shown to rearrange under certain conditions. 

The elucidation of mechanisms of reactions of Sml2 have involved polarography, kinetics, radical clocks and trapping techniques (radical cyclisation) [19, 20]. The reagent is able to reduce alkyl halides and ketones/aldehydes, as shown in Scheme 10.25, in non-chain radical reactions. 

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