Clock reactions

In the bromate-iron clock reaction, there is an autocatalytic cycle involvmg the species intennediate species HBrO. This cycle is comprised of the following non-elementary steps ... 

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. 

Figure A3.14.2. Characteristic features of a clock reaction, illustrated for the Landolt reaction, showing (a) variation of product concentration witii induction period followed by sharp reaction event (b) variation of overall reaction rate witli course of reaction.

The Landolt reaction (iodate + reductant) is prototypical of an autocatalytic clock reaction. During the induction period, the absence of the feedback species (Irere iodide ion, assumed to have virtually zero initial concentration and fomred from the reactant iodate only via very slow initiation steps) causes the reaction mixture to become kinetically frozen . There is reaction, but the intemiediate species evolve on concentration scales many orders of magnitude less than those of the reactant. The induction period depends on the initial concentrations of the major reactants in a maimer predicted by integrating the overall rate cubic autocatalytic rate law, given in section A3.14.1.1. 

The radicals formed by imimolecular rearrangement or fragmentation of the primary radicals arc often termed secondary radicals. Often the absolute rate constants for secondary radical formation are known or can be accurately determined. These reactions may then be used as radical clocks",R2° lo calibrate the absolute rate constants for the bimolecular reactions of the primary radicals (e.g. addition to monomers - see 3.4). However, care must be taken since the rate constants of some clock reactions (e.g. f-butoxy [3-scission21) are medium dependent (see 3.4.2.1.1). 

The rate constant for p-scisskm is dependent on ring substituents. Rate constants for radicals X-Q.H. CCh are reported to increase in the series where X is / -Fi There is qualitative evidence that the relative rales for p-scission and addition are insensitive to solvent changes. For benzoyloxy radicals, similar relative reactivities are obtained from direct competition experiments10 as from studies on individual monomers when p-scission is used as a clock reaction.399,401... 

The reaction between nitroxides and carbon-centered radicals occurs at near (but not at) diffusion controlled rates. Rate constants and Arrhenius parameters for coupling of nitroxides and various carbon-centered radicals have been determined.508 311 The rate constants (20 °C) for the reaction of TEMPO with primary, secondary and tertiary alkyl and benzyl radicals are 1.2, 1.0, 0.8 and 0.5x109 M 1 s 1 respectively. The corresponding rate constants for reaction of 115 are slightly higher. If due allowance is made for the afore-mentioned sensitivity to radical structure510 and some dependence on reaction conditions,511 the reaction can be applied as a clock reaction to estimate rate constants for reactions between carbon-centered radicals and monomers504 506"07312 or other substrates.20... 

Self-Test 13.4A The reaction S2082 (aq) + 31 (aq) — 2 S042 (aq) + I3 (aq) is commonly used to produce triiodide ion in a clock reaction," in which a sudden color change signifies that a reagent has been used up. Write the rate law for the consumption of persulfate ions and determine the value of k, given the following data ... 

The "iodine clock" reaction Rate of reduction of iodate by sulfite was increased... 

I. Title Chemical Kinetics An Iodine Clock Reaction... 

ChemicalEducation Resources, Inc., "ChemicalKinetics iodine Clock Reaction," Palmyra, PA, 1988. 

Many types of reaction may be used. The simple recording of the time required for a noticeable change is particularly applicable to clock reactions. 

LFP-Clock Method. In this method, rate constants for the radical clock reactions are measured directly by LFP, and the clocks are used in conventional competition kinetic studies for the determination of second-order rate constants. The advantages are that the clock can be calibrated with good accuracy and precision in the solvent of interest, and light-absorbing reagents can be studied in the competition reactions. The method is especially useful when limited kinetic information is available for a class of radicals. 

The rate constants for reaction of Bu3SnH with the primary a-alkoxy radical 24 and the secondary ce-alkoxy radical 29 are in reasonably good agreement. However, one would not expect the primary radical to react less rapidly than the secondary radical. The kinetic ESR method used to calibrate 24 involved a competition method wherein the cyclization reactions competed with diffusion-controlled radical termination reactions, and diffusional rate constants were determined to obtain the absolute rate constants for the clock reactions.88 The LFP calibrations of radical clocks... 

The concept of this method is illustrated in Scheme 3.1, where the clock reaction (U R ) is the unimolecular radical rearrangement with a known rate constant ( r)- The rate constant for the H atom abstraction from RsSiH by a primary alkyl radical U can be obtained, provided that conditions are found in which the unrearranged radical U is partitioned between the two reaction channels, i.e., the reaction with RsSiH and the rearrangement to R. At the end of the reaction, the yields of unrearranged (UH) and rearranged (RH) products can be determined by GC or NMR analysis. Under pseudo-first-order conditions of silane concentration, the following relation holds UH/RH = (A H/A r)[R3SiH]. A number of reviews describe the radical clock approach in detail [3,4]. 

Cubic autocatalysis and clock reactions in closed vessels... 

The induction period, followed by a sharp increase in rate is, however, the most characteristic feature of autocatalysis in closed vessels. One manifestation of this behaviour is the clock reaction . An experimental system which is a typical chemical clock and which also exhibits cubic autocataiysis is the iodate-arsenite reaction. In the presence of excess iodate, the system which is initially colourless eventually undergoes a sudden colour change to brown (or blue in the presence of starch). The potential of an iodide-sensitive electrode shows a barely perceptible change during most of the induction period, but then rises rapidly, reaching a peak at the point of colour change. 

Thus, apart from the constant factor c, the rate of change of concentration with position along the tube has the same form as the rate of change of concentration in time. With autocatalytic processes, for instance, this allows for a clock reaction in space rather than in time—if the reaction has an associated colour change, there can be a sharp band at a point xa related to the clock time ta by xa = ctCi. 

In the formaldehyde clock reaction, formaldehyde is added to a solution containing HSO, SO -, and phenolphthalein indicator. Phenolphthalein is colorless below a pH of 8 and red above this pH. The solution remains colorless for more than a minute. Suddenly the pH shoots up and the liquid turns pink. Monitoring the pH with a glass electrode gave the results in the graph. 

Graph of pH versus time in the formaldehyde clock reaction. 

Procedure All solutions should be fresh. Prepare a solution of formaldehyde by diluting 9 mL of 37 wt% formaldehyde to 100 mL. Dissolve 1.5 g of NaHSO,10 and 0.18 g of Na,SO, in 400 mL of water, and add 1 mL of phenolphthalein solution (Table 11-4). Add 23 mL of formaldehyde solution to the well-stirred buffer solution to initiate the clock reaction. The time of reaction can be adjusted by changing the temperature, concentrations, or volume. 

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, ... 

E. H. White, An Efficient Chemiluminescent Clock Reaction, J. Chem. Educ., 34, 275 (1957). 

Bassam Z. Shakhashiri, "Color Variations of the Landolt Reaction," Chemical Demonstrations, A Handbook for Teachers of Chemistry, Vol. 4 (The University of Wisconsin Press, Madison, 1992), pp. 26-28. Three pairs of colorless solutions are mixed. After 10 seconds the three mixtures turn red, yellow, and blue, respectively. Each clock reaction is the reduction of iodate by bisulfite. The observed colors are related to the amount of triiodide produced, which depends on the relative amounts of iodate and bisulfite reacted, and whether or not starch is present. 

Ealy, "The Starch-Iodine Clock Reaction," Chemical Demonstrations, A Sourcebook for Teachers, Vol. 1 (American Chemical Society, Washington, DC, 1988), pp. 107-108. 

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