Clock Reaction Research Articles

A Clock Reaction Based on Molybdenum Blue

Clock reactions are rare kinetic phenomena, so far limited mostly to systems with ionic oxoacids and oxoanions in water. We report a new clock reaction in cyclohexanol that forms molybdenum blue from a noncharged, yellow molybdenum complex as precursor, in the presence of hydrogen peroxide. Interestingly, the concomitant color change is reversible, enabling multiple clock cycles to be executed consecutively. The kinetics of the clock reaction were experimentally characterized, and by adding insights from quantum chemical calculations, a plausible reaction mechanism was postulated. Key elementary reaction steps comprise sigmatropic rearrangements with five-membered or bicyclo[3.1.0] transition states. Importantly, numerical kinetic modeling demonstrated the mechanism's ability to reproduce the experimental findings. It also revealed that clock behavior is intimately connected to the sudden exhaustion of hydrogen peroxide. Due to the stoichiometric coproduction of ketone, the reaction bears potential for application in alcohol oxidation catalysis.

A facile synthesis of 1D nano structured selenium and Au decorated nano selenium: catalysts for the clock reaction

Elemental selenium nanowires (Se NWs) are obtained from the easily available, less-toxic, low-cost reagents sodium selenite and ethylenediamine (en) through a modified hydrothermal reaction (MHT). The reaction supports the habitual growth of selenium nanowires and the amine molecules act as a reducing agent. Then, an interesting galvanic exchange reaction deposits Au nanoparticles on the as-synthesized Se evolving an Au decorated composite material. But a similar galvanic exchange produces Ag2Se because of the lower diffusion activation energy of Ag. Selenium nanowires, even as metalloids, have been found to catalyze the reduction of methylene blue (MB) using NaBH4 as a reducing agent. The blue colour of MB vanishes due to the formation of leucomethylene blue (LMB). On shaking the reaction mixture, the blue colour reappears by aerial oxidation. This oscillation of reversible colour change is taken as a clock reaction. The reversible reaction is monitored by a UV-visible spectrophotometer with respect to the absorbance of MB at λmax = 663 nm. Common reducing agents such as hydrazine hydrate, ammonium thiocyanate and glucose do not show this stunning phenomenon. Sodium borohydride fails to assist the reduction process in the presence of bulk selenium. The reduction process proceeds through two steps. Initially, the rate is slower due to an ‘induction period’ and then it becomes facile. A higher catalyst dose reduces the time lag, i.e., the induction period. The Se NWs performed well for about 30–50 cycles for a clock reaction set up. Finally, the composite materials have been found to exhibit a faster clock reaction cycle in comparison to neat Se NWs.

Methylene Blue–Cu2O Reaction Made Easy in Acidic Medium

Truncated Cu2O cubes with well-defined morphology have been successfully synthesized using a stable Cu(II)–EDTA precursor and glucose as the reducing agent in alkaline conditions under 5 min of microwave irradiation. The truncated cubic Cu2O particles as a solid powder were characterized by different physical methods. Curiously enough, upon the addition of Cu2O particles, the blue-colored methylene blue (MB) in aqueous acidic (pH ≈ 1.0) solution successively bleaches to colorless leuco-MB (LMB). The redox reaction generates a colorless solution which easily reverts back to blue in air. The reaction has been quantitatively monitored by UV–vis spectrophotometry. The reversible color change, i.e., oscillation between a blue MB solution and colorless LMB solution happens to be a periodic phenomenon for more than 50 cycles and is reproducibly demonstrated as a simple “clock reaction”. Acidic (pH ≈ 3.0) conditions favor zeroth-order kinetics for the underlying redox phenomena. Dilute H2SO4 has been proven to be the best choice to provide a passive reaction medium, and the undisturbed reaction mixture showed oscillatory behavior even after one month.

The Effect of Temperature and Ionic Strength on the Oxidation of Iodide by Iron(III): A Clock Reaction Kinetic Study

A laboratory exercise has recently been reported in which the students use the initial rates method based on the clock reaction approach to deduce the rate law and propose a reaction mechanism for the oxidation of iodide by iron(III) ions. The same approach is used in the exercise proposed herein; the students determine the dependence of the reaction rate on ionic strength and temperature in a straightforward manner. The previously proposed mechanism is further supported by the results, the rate-determining step is identified, and the activation parameters Ea, Δ⧧H, and Δ⧧S are determined for the overall reaction of the first two steps of the mechanism. The significance of such kinetic studies in elaborating reaction mechanisms is clearly demonstrated to the students.

On the Complexity of Kinetics and the Mechanism of the Thiosulfate–Periodate Reaction

The thiosulfate-periodate reaction has been studied spectrophotometrically in a slightly acidic medium at 25.0 ± 0.1 °C in an acetate/acetic acid buffer by monitoring the absorbance in the 250-600 nm wavelength range at a constant ionic strength adjusted by the buffer component sodium acetate. In agreement with a previous study, we found that the reaction cannot be described by a single stoichiometric equation, tetrathionate and sulfate are simultaneously formed, and its ratio strongly depends on the pH. As expected at certain initial concentration ratios of the reactants, the reaction behaves as a clock reaction, but after its appearance, iodine is slowly consumed mainly because of the moderate tetrathionate-iodine reaction. It is also enlightened that the initial rate of the reaction is completely independent of the pH, which apparently contradicts a previous study, which postulates a "supercatalytic" behavior of the hydrogen ion on the title reaction. Significant buffer assistance that may change the absorbance-time profiles was also observed. On the basis of the kinetic data, a robust 28-step kinetic model with 22 fitted parameters is proposed and discussed to explain adequately all of the important characteristics of the kinetic curves.

Ring-opening radical clock reactions: many density functionals have difficulty keeping time

The ability of several density-functional theory methods to describe the kinetics and energetics of a series of ring-opening reactions of cyclopropyl and cyclobutyl-type radicals was explored. PBE, B971 and B3LYP perform quite well in their ability to replicate experiment, based upon the ring opening of cyclopropylcarbinyl, two α-trialkylsilyloxycyclopropylmethyl radicals, pentamethylcyclopropylcarbinyl, cyclobutylcarbinyl and 1-cyclobutylethylcarbinyl. The other functionals tested, which includes BLYP, CAM-B3LYP, BHandHLYP, B2PLYP and B2PLYP-D, as well as functionals designed for kinetics applications, namely MPW1K, BMK and M06-2X, all perform poorly. The latter of these functionals display some integration grid dependencies.

A Microscale Approach to Chemical Kinetics in the General Chemistry Laboratory: The Potassium Iodide Hydrogen Peroxide Iodine-Clock Reaction

A microscale laboratory for teaching chemical kinetics utilizing the iodine clock reaction is described. Plastic pipets, 3 mL volume, are used to store and deliver precise drops of reagents and the reaction is run in a 24 well plastic tray using a total 60 drops of reagents. With this procedure, students determine the rate of reaction and the order of reaction as a function of iodide ion concentration or hydrogen peroxide concentration. Six reaction concentrations are run concurrently in about an hour. The students also calculate the value of the rate constant and determine the concentration of an unknown iodide ion sample. The procedure has been adopted to run at a lower temperature to determine the energy of activation. The experiment teaches the principles of chemical kinetics to first-year college students and can also be used in second-year or advanced-placement high school chemistry classes.

Alternate solutions for two particular third order kinetic rate laws

Alternate solutions to third order rate law expressions which are first order in one reactant and second order in another for the generalized reaction \({{aA}+{bB}\mathop \rightarrow \limits^{k_A }{P}}\) with a limiting reactant and for simple cubic autocatalysis \({{A}+{ 2B}\mathop \rightarrow \limits^{k_A }{3B}}\) are presented and discussed. These solutions are expressed in terms of the Lambert function W[x] with argument x(t) = α exp(α − βt), where α and β are empirical parameters. These expressions permit the concentrations of the reactant species to be explicitly represented as dependent functions of time. For the generalized reaction, the solution involves considering exactly which species is limiting, so that the correct branch of the Lambert function can be determined. For simple cubic autocatalysis, the solution is shown to be consistent with the characteristics associated with clock reactions. An exact expression for the “induction time” associated with a clock reaction described by this mechanism is also derived.

Clock Reaction: Outreach Attraction

Chemistry students are often introduced to the concept of reaction rates through demonstrations or laboratory activities involving the well-known iodine clock reaction. For example, a laboratory experiment involving thiosulfate as an iodine scavenger is part of the first-year general chemistry laboratory curriculum at Dalhousie University. With the necessary materials, expertise, and laboratory space readily available, this undergraduate clock reaction experiment was modified to make it accessible to grade 9 students. The revised, qualitative experiment has been performed as an on-campus outreach initiative with eight grade 9 classes, reaching a total of 250 students, teachers, parents, and aides. The activity was facilitated by graduate student volunteers, who assisted the grade 9 students as they were exposed to a colorful adventure involving kinetics, chemical reactions, and catalysis. In general, adapting undergraduate experiments offers researchers and educators a positive and efficient way to introduce younger students to a university chemistry laboratory.

Classical Versus Nonstatistical Behavior in the C²-C6/Ene Cyclization of Enyne-Allenes: Intramolecular Kinetic Isotope Effects and Radical Clock Reactions

The effects of aryl groups on the mechanism of the C²-C6/ene cyclization of enyne-allenes were studied by means of radical clock openings and intramolecular kinetic isotope effects. Upon attachment of a single aryl group at either the alkyne or the allene terminus, the thermal reaction proceeds by a stepwise mechanism that shows significant nonstatistical dynamic effects. Despite this, we were able to intercept the intermediate diradical intramolecularly by using the ultrafast diphenylcyclopropyl radical clock reaction. When aryl groups were present at both the alkyne and allene termini, the intramolecular kinetic isotope effects were consistent with a classical stepwise mechanism. The present study thus demonstrates the shift in reaction mechanism from a nonstatistical stepwise mechanism to a classical stepwise behavior, depending on the substituents.

Time‐lapse thiol‐acrylate polymerization using a pH clock reaction

A time-lapse polymerization exhibits a programable timebetween the mixing of the reagents and the onset of poly-merization. In fact, a free-radical polymerization systemcontaining an inhibitor can be thought of this way. Anotherapproach was invented by Norling that consisted of aperoxide or persulfate initiating system using iron or copperwith a special chelating agent and an organic reducingagent.

N-Heterocyclic Carbene Boryl Radicals: A New Class of Boron-Centered Radical

Reduction of xanthates by N-heterocyclic carbene boranes (NHC-boranes) has been suggested to occur by a radical chain mechanism involving heretofore unknown NHC-boryl radicals. In support of this suggestion, both the expected borane dithiocarbonate product and an unexpected borane xanthate product have now been isolated. These are the first NHC-boranes with boron-sulfur bonds, and their structures have been secured by spectroscopic and crystallographic means. The first rate constants for H-atom transfer from an NHC borane complex were determined by using the ring opening of a substituted cyclobutylcarbinyl radical as a clock reaction. The rate constant for reaction of the NHC-borane with a secondary alkyl radical at ambient temperature is 4 x 10(4) M(-1) s(-1), and the Arrhenius function displayed an entropic term (log A term) that was typical for a bimolecular reaction. The B-H bond dissociation energy of an NHC-borane complex has been estimated at 88 kcal/mol. The putative NHC-boryl radical in these transformations has been detected by EPR spectroscopy. Spectral analysis suggests that it is a pi-radical, analogous to the benzyl radical.

Oxyhalogen–sulfur chemistry — Kinetics and mechanism of oxidation of methionine by aqueous iodine and acidified iodate

The oxidation of methionine (Met) by acidic iodate and aqueous iodine was studied. Though the reaction is a simple two-electron oxidation to give methionine sulfoxide (Met–S=O), the dynamics of the reaction are, however, very complex, characterized by clock reaction characteristics and transient formation of iodine. In excess methionine conditions, the stoichiometry of the reaction was deduced to be IO3– + 3Met → I– + 3Met–S=O. In excess iodate, the iodide product reacts with iodate to give a final product of molecular iodine and a 2:5 stoichiometry: 2IO3– + 5Met + 2H+→ I2 + 5Met–S=O + H2O. The direct reaction of iodine and methionine is slow and mildly autoinhibitory, which explains the transient formation of iodine, even in conditions of excess methionine in which iodine is not a final product. The whole reaction scheme could be simulated by a simple network of 11 reactions.

Modeling the Complex Bromate−Iodine Reaction

In this article, it is shown that the FLEK model (ref 5 ) is able to model the experimental results of the bromate-iodine clock reaction. Five different complex chemical systems, the bromate-iodide clock and oscillating reactions, the bromite-iodide clock and oscillating reactions, and now the bromate-iodine clock reaction are adequately accounted for by the FLEK model.

SQUID magnetometry as a tool for following a clock reaction in solution

SQUID magnetometry, normally used to characterise the properties of solids, was used to follow a clock reaction in solution, namely the auto-catalytic oxidation of [Co(II)EDTA]2- by H2O2, in real time and it was shown that, in combination with other methods (e.g., magnetic resonance proton relaxation studies and UV-vis absorption analysis), SQUID magnetometry can be a powerful method in elucidating and interpreting the time-profile of chemical reactions so as long as reactants, intermediates and products have suitably large differences in their respective magnetic susceptibilities.

Nonlinear Oxidation of Thiourea in an Unbuffered Medium

AbstractThe reaction between periodate and thiourea has been studied in an unbuffered medium. A variety of interesting dynamical behavior including variable stoichiometries, characteristic "clock reaction" and oligooscillations or single peak oscillations in pH, the concentrations of both iodide and iodine were found in a closed reactor. When studied in an open reactor, the above reaction system displays fascinating damped oscillations and bistability in an unbuffered aqueous solution. A 12‐step simplified mechanism has also been proposed here. Our investigation illustrates that it is critical to include a rapid H+‐mediated preequilibrium step in the suggested mechanism. The modified mechanism gives a better agreement between the calculated results and experimental observations.

Kinetics and Mechanism of Oxidation of N, N′-Dimethylaminoiminomethanesulfinic Acid by Acidic Bromate

The major metabolites of the physiologically active compound dimethylthiourea (DMTU), dimethylaminoiminomethansesulfinic acid (DMAIMSA), and dimethylaminoiminomethanesulfonic acid (DMAIMSOA) were synthesized, and their kinetics and mechanisms of oxidation by acidic bromate and aqueous bromine was determined. The oxidation of DMAIMSA is much more facile and rapid as compared to a comparable oxidation by the same reagents of the parent compound, DMTU. The stoichiometry of the bromate-DMAIMSA reaction was determined to be 2BrO 3 (-) + 3NHCH 3(NCH 3)CSO 2H + 3H 2O --> 3SO 4 (2) (-) + 2Br (-) + 3CO(NHCH 3) 2 + 6H (+), with quantitative formation of sulfate. In excess bromate conditions, the stoichiometry was 4BrO 3 (-) + 5NHCH 3(NCH 3)CSO 2H + 3H 2O --> 5SO 4 (2) (-) + 2Br 2 + 5CO(NHCH 3) 2 + 6H (+). The direct bromine-DMAIMSA reaction gave an expected stoichiometric ratio of 2:1 with no further oxidation of product dimethylurea (DMU) by aqueous bromine. The bromine-DMAIMSA reaction was so fast that it was close to diffusion-controlled. Excess bromate conditions delivered a clock reaction behavior with the formation of bromine after an initial quiescent period. DMAIMSOA, on the other hand, was extremely inert to further oxidation in the acidic conditions used for this study. Rate of consumption of DMAIMSA showed a sigmoidal autocatalytic decay. The postulated mechanism involves an initial autocatalytic build-up of bromide that fuels the formation of the reactive oxidizing species HBrO 2 and HOBr through standard oxybromine reactions. The long and weak C-S bond in DMAIMSA ensures that its oxidation goes directly to DMU and sulfate, bypassing inert DMAIMSOA.

Kinetics and Mechanism of Iodide Oxidation by Iron(III): A Clock Reaction Approach

A simple method for studying the kinetics of a chemical reaction is described and the significance of reaction orders in deducing reaction mechanisms is demonstrated. In this student laboratory experiment, oxidation of iodide by iron(III) ions in an acidic medium is transformed into a clock reaction. By means of the initial rates method, it is found that the reaction is second order with respect to iodide and first order with respect to iron(III) ions. Having established the rate law, the students are encouraged to propose a possible mechanism for the reaction investigated.

Rate Constants and Arrhenius Functions for Ring Opening of a Cyclobutylcarbinyl Radical Clock and for Hydrogen Atom Transfer From the Et3B−MeOH Complex

Kinetics of ring opening of the 1-cyclobutyldodecyl radical (1) were studied, and an Arrhenius function over the temperature range -20 to 47 degrees C was determined. The radical clock reaction has kinetics described by log k = 13.2 - 13.5/2.313 RT (in kcal/mol), and k = 1.5 x 10(3) s(-1) at 20 degrees C. Previous kinetic studies of hydrogen atom transfer trapping of radical 1 by the triethylborane-methanol complex at variable temperatures (J. Org. Chem. 2007, 72, 5098) were analyzed with the newly obtained kinetic data.

Nanoparticle-Catalyzed Clock Reaction

Bulk Cu2O or cuprite is the only stable copper(I) compound present in plentiful amount in earth's crust. It is a challenging job to take bulk Cu2O to a nanoregime and to stabilize it in solution. No wonder that Cu2O in its nanoregime would act as a photocatalyst. We report a new synthetic protocol for the first time to obtain monodispersed, stable, exclusively cubic Cu2O nanoparticles in surfactant-free condition and its catalytic action for methylene blue (MB)−hydrazine reaction in aqueous medium. The blue color of the dye, MB, faded away upon the addition of hydrazine, producing colorless leuco methylene blue (LMB) indicating the progress of the redox reaction. The rate of this redox reaction has been found to be enhanced in the presence of the nanocatalyst, Cu2O. The success of the reaction demonstrates a simple ‘clock reaction'. An oscillation between a blue MB color and colorless solution due to formation of LMB is observed on periodic shaking. This oscillation continues for over 15 cycles. Studies on the effect of bulk Cu2O and nanoparticles of CuO and Cu(0) have not been successful for demonstration of the ‘clock reaction'. Thus, the importance of Cu2O nanoparticles in the clock reaction is established beyond doubt. The Cu2O nanoparticles were characterized by different physical methods. TEM studies authenticate the cube shaped monodispersed particles. The electrochemical studies indicate that nano-Cu2O shows a couple of redox peaks which correspond to the redox Cu(II)/Cu(I) system. Kinetic studies authenticate a first-order reaction mechanism. Further, quantum chemical calculations reveal that the nanoparticles reduce the activation energy by ∼17 kcal/mol, thereby making the reaction 2.4 × 107 times faster compared to the gas phase.