Benzylmalonic acid reaction with iodine and behavior in a Briggs–Rauscher oscillator

Benzylmalonic acid (BzMA) reacts via an enol mechanism with aqueous iodine to form rather stable iodobenzylmalonic acid. In the presence of iodate, which oxidizes iodide to di‐iodine, the reaction goes to completion. The kinetics of the reaction BzMA + I₂ + IO₃^? + H⁺ have been studied and the results were interpreted with a suitable mechanism. In a mixture with acidic iodate, hydrogen peroxide, and manganous ions, BzMA can serve as a substrate for a Briggs–Rauscher‐type oscillating reaction. The behavior of this oscillator has been studied in detail. A mechanistic interpretation of the oscillations based on a new kinetic model is also given. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 357–365, 2002     

Model study of synchronization and other phenomena in light perturbation of the Briggs–Rauscher reaction

Calculations are performed to model several phenomena observed when a photosensitive oscillating chemical reaction, the Briggs–Rauscher (BR) reaction, is subject to irradiation in a continuous-flow stirred-tank reactor. The BR mechanism proposed earlier by De Kepper and Epstein is supplemented by additional steps involving the photogeneration and subsequent reaction of iodine atoms. The calculations show qualitative agreement with the following experimentally observed phenomena: (a) change in the period and amplitude of oscillation as a function of the intensity of constant illumination, (b) synchronization between periodically varying illumination and the period of chemical oscillation, (c) phase shifts induced by single light pulses, and (d) dark steady states which are excitable by single light pulses and become oscillatory at appropriate levels of constant illumination.     

Inhibitory Effects by Soy Antioxidants on the Oscillations of the Briggs‐Rauscher Reaction

Inhibitory effects by addition of aqueous extracts of soy flour to an active Briggs‐Rauscher mixture are reported. The effect consists of an immediate cessation of oscillations, but, after some time, the oscillatory behavior is regenerated with amplitude and frequency different from those observed in a reference mixture. The inhibition time depends linearly on the concentration of substances contained in the extract in a wide range of concentration. The inhibitory effects are due to the high free‐radical scavenging activity of substances contained in the soy flour. Two preponderant products contained in the soy flour extract were identified and characterized as malonyldaidzin and malonylgenistin. The antioxidant activity of these isoflavones contained in the extracts was determined on the basis of the inhibition time. A qualitative mechanistic explanation of the inhibitory effects is given. Our findings are decisive indirect evidence of involvement and important role played by HOO^. radicals in establishing oscillations in the Briggs‐Rauscher (BR) system. The linear relationship between the inhibition time and the whole‐mass concentration of antioxidant contained in soy extracts added to a BR mixture is an indication of the possibility to develop and implement an analytical procedure for monitoring the activity of antioxidant scavengers of free radicals based on the oscillating Briggs‐Rauscher reaction.     

A Potentiometric Study of the Briggs–Rauscher Oscillatory Reaction in a Solution of Nonionic Surfactants and tert‐Butanol

The concentration effect of nonionic surfactants (Triton X‐100, Brij 30, Brij 58, Tween 20, and Tween 80) and tert‐butanol was investigated on the Briggs–Rauscher oscillatory reaction in a stirred batch reactor at 25 ± 0.1°C in both the absence and presence of nonionic surfactant and tert‐butanol as well. The addition of Triton X‐100, Brij 58, and Tween 20 influenced the oscillatory parameters in a similar fashion: a decrease of the induction period until its disappearance, an increase of the oscillation period, an increase of the oscillation amplitude, an increase of the duration of the oscillation, and a gradual increase of the oscillation numbers. The addition of Brij 30 has no significant effect on the oscillation parameters of the Briggs‐Rauscher oscillatory reaction. The effect of tert‐butanol on the Briggs–Rauscher oscillatory reaction is very similar to the effect of Brij 58; however, we were unable to observe the disappearance of the induction period on the studied concentration range. The addition of Tween 80 to the reaction mixture has a similar effect as the addition of Triton X‐100, Brij 58, or Tween 20, except for the induction period, which in the case of Tween 80, it becomes larger. The observed effects are explained in terms of micellar catalysis or inhibition, that is, in a different extent of individual reactants of solubilization.     

Perturbations of an Acetone‐Based Briggs–Rauscher System by α‐Tocopherol (Vitamin E)

The malonic acid (MA)‐based oscillating Briggs–Rauscher reaction (BR) in batch mode has been shown to be sensitive to various hydrophilic polyphenol antioxidants. Several of these have been shown to cause cessation of oscillations for a period of time before a restart occurs. The length of time before oscillations restart is related to the type of antioxidant and its concentration. Procedures have been devised to use this method as a tool for measuring antioxidant activity from pure compounds and from extracts of natural sources. The antioxidant activity has been related to the reaction of the antioxidants with HOO^. radicals present in the oscillating system. Vitamin E (α‐tocopherol), a typical highly lipophilic antioxidant containing an phenolic OH group, is soluble in acetone that also is a suitable substrate for the BR reaction. Perturbations of a highly concentrated acetone‐based BR oscillator by acetonic solutions of vitamin E were studied. The inhibitory effects were found similar to those provoked by hydrophilic polyphenols in the MA‐based oscillator, but to obtain reasonable inhibition times, the concentration of vitamin E must be at the mM level instead μM . However, there is a region of concentrations where there is a nearly linear relation between concentration and inhibition time. A comparison with a hydrophilic diphenol (2,6‐dihydroxybenzoic acid) in the acetone‐based oscillator showed that the inhibitory reaction is much slower in this system than in the MA one. We were able to model the perturbations by vitamin E assuming its reaction with HOO^. radicals by using the FCA mechanism previously reported with some little modifications.     

Kinetics of the bromate–bromide reaction at high bromide concentrations

At bromide concentrations higher than 0.1 M, a second term must be added to the classical rate law of the bromate–bromide reaction that becomes ?d[BrO₃^?]/dt = [BrO₃^?][H⁺]²(k₁[Br^?] + k₂[Br^?]²). In perchloric solutions at 25°C, k₁ = 2.18 dm³ mol^?3 s^?1 and k₂ = 0.65 dm⁴ mol^?4 s^?1 at 1 M ionic strength and k₁ = 2.60 dm³ mol³ s^?1and k₂ = 1.05 dm⁴ mol^?4 s^?1 at 2 M ionic strength. A mechanism explaining this rate law, with Br₂O₂ as key intermediate species, is proposed. Errors that may occur when using the Guggenheim method are discussed. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 39: 17–21, 2007     

The Briggs‐Rauscher Reaction as a Test to Measure the Activity of Antioxidants

A new method for monitoring the relative activity of antioxidants is presented, and its advantages and limits are discussed. The method is based on the previously reported inhibitory effects of free‐radical scavengers on the oscillations of the Briggs‐Rauscher reaction. The effect consists of an immediate cessation of oscillations, an inhibition time that linearly depends on the concentration of the antioxidant added, and subsequent regeneration of oscillations. Here the effects of ten antioxidants (pyrocatechol (=benzene‐1,2‐diol), ferulic acid (=3‐(4‐hydroxy‐3‐methoxyphenyl)prop‐2‐enoic acid), caffeic acid (=3‐(3,4‐dihydroxyphenyl)prop‐2‐enoic acid), 2,6‐, 3,4‐, 2,4‐, 3,5‐, and 2,5‐dihydroxybenzoic acids, homovanillic acid (=4‐hydroxy‐3‐methoxybenzeneacetic acid), and resorcinol (=benzene‐1,3‐diol)) were studied in detail. Relative antioxidant activities of these substances with respect to resorcinol were determined in different ways on the basis of inhibition times. The limits of the calculated values of relative activity based on the Briggs‐Rauscher reaction are the same as those obtained with other analytical procedures and are discussed here. The new method is inexpensive: reagents and apparatus are commonly used in all chemical laboratories. The thermochemical behavior of the Briggs‐Rauscher reaction and the dependence of inhibition time on the temperature were also carefully investigated and taken into account. A semiquantitative mechanistic interpretation of the inhibitory effects based on a suitable kinetic model is given.     

Determination of Butylated Hydroxyanisole by Briggs‐Rauscher Oscillating Reaction Catalyzed by a Macrocyclic Nickel (II) Complex

A novel analytical approach for quantitative measurement of butylated hydroxyanisole (BHA) is dis‐ cussed in this paper. Such a method depends on the inhibitory effect of BHA on a Briggs‐Rauscher (B‐R) oscillating reaction. Unlike the classical B‐R system which involves Mn²⁺ as the catalyst, such a B‐R sys‐ tem is catalyzed by a macrocyclic nickel (II) complex [NiL](ClO₄)₂, where L in the complex is an unsatu‐ rated ligand 5,7,7,12,14,14‐hexemethyl‐1,4,8,11‐tetraazacyclotetradeca‐4,11‐diene. By perturbation of BHA on the system, the oscillation was inhibited in the presence trace amounts of BHA and the inhibition time was found to be proportional to the concentration of BHA over the range 1.00×10^?7–1.20×10^?4 mol/L. Two calibration curves were obtained: the first linear regression is over the range of 1.00×10^?7–2.00×10^?6 mol/L, and the second linear regression is over the range between 2.00×10^?6 and 1.20×10^?4 mol/L, with a lowest limit of detection of 4.00×10^?8 mol/L. UV spectra measurements were employed to clarify the possible perturbation mechanism caused by BHA on the B‐R oscillating reaction.     

Inhibitory Effects by Antioxidants on the Oscillations of the Briggs‐Rauscher Reaction in Mixed EtOH/H2O Medium

The Briggs‐Rauscher (BR) oscillating reaction in mixed 20% EtOH/H₂O (v/v) medium is studied together with the inhibitory effects by the addition of hydroalcoholic solutions of antioxidants on its oscillatory regime. As in aqueous BR mixtures, the inhibitory effect consists of an immediate cessation of oscillations, an inhibitory time that linearly depends on the concentration of the antioxidant added, and subsequent regeneration of oscillations. The effects of several water‐insoluble and water‐soluble antioxidants were investigated: at a parity of concentration of antioxidant added, inhibition times in the mixed EtOH/H₂O medium are 2–3 times lower than those reported previously in aqueous solution. However, the mechanism of the BR reaction, as far as that of the inhibitory effect, seems to be the same in either aqueous or mixed medium. The findings reported and discussed here are an indication that the analytical procedure to assess the activity of free radical scavengers based on the BR reaction can be extended to lipophilic antioxidants.     

Determination of iodide and bromide by ion chromatography with post-column reaction detection

During an investigation into the mechanism of the biosynthesis of thyroid hormones, it became necessary to determine traces of iodide and bromide in biological matrices as well as in food. A vydac 302-IC anion-exchange column with methanesulphonic acid as the mobile phase was used for the ion chromatographic separation of iodide and bromide. A post-column reaction detector was developed based on the reaction between iodide or bromide, chloramine-T and 4,4'-bis(dimethylamino)diphenylmethane. Methods with minimal sample preparation are described for determination of iodide or bromide in serum, milk, salt and water. The detection limit is ca. 20 pg iodide and 15 ng bromide injected.     

Negative ion chemical ionization mass spectrometry—binding of molecules to bromide and iodide anions

The analytical potential of negative ion chemical ionization (NICI) mass spectrometry utilizing dibromodifluoro-methane (CF₂Br₂) and iodomethane (CH₃I)/methane (CH₄) as reagent gases is examined. The NICI mass spectrum of CF₂Br₂ contains Br^?, [HBr₂]^? and [CF₂Br₃]^? anions. Weak acids (i.e. those acids with approximately ΔH°_(acid) values between 1674 and 1464 kJ mol^?1) react with Br^? to produce minor yields of the hydrogen?bonded bromide attachment [MH + Br]^? anion or are unreactive. Strong acids (i.e. those acids with approximately ΔH°_(acid) > 1464 kJ mol^?1) produce primarily [MH + Br]^? anions with a minor yield of proton transfer [M ? H]^? anion. The NICI spectrum of CH₃I/CH₄ is dominated by I^?. Weak acids react with I^? to yield minor amounts of [MH + 1]^? or are unreactive. Strong acids produce only [MH + l]^? anions. From a consideration of the gas-phase basicity of the halide anion and the binding energy of the hydrogen-bonded halide attachment adduct, thermochemical data are used as a potential guide to rationalize or predict the ions observed in NICI mass spectra.     

Mechanistic details of the Belousov–Zhabotinsky oscillations. II. The organic reaction subset

Our previous mechanistic model for the Belousov–Zhabotinsky reaction has been revised to include a more realistic set of reactions for the oxidation pathways of the organic intermediates. A few other rate constants have also been modified to include new information. The revised mechanism reproduces the essential experimental observations, although the periods of oscillation are somewhat too long and oscillations cease at malonic acid concentrations about ten times greater than the observed lower limit. However, the essential features of the mechanism are clearly understood.     

Reactions in microemulsion media. Nucleophilic displacement reaction of benzyl chloride with bromide ion

A kinetic and synthetic study of the reaction of benzyl chloride with bromide ion has been performed in microemulsions composed of cetyltrimethylammonium bromide, 1-butanol, water, potassium bromide, and hexane; rates and yields of benzyl bromide formation decreased with increasing hexane content.     

Mechanistic details of the Belousov–Zhabotinskii oscillations

A detailed chemical mechanism for the cerium catalyzed bromination and oxidation of malonic acid by bromate has been developed, and complete numerical solutions have been obtained for the temporal behavior of the concentrations of the intermediate species. The many points of similarity with the experimentally observed initial decay, long quiescence, and abrupt transition to sustained oscillation establish confidence in the essential validity of the proposed mechanism. Close examination of the time dependence of these species concentrations as well as the magnitude of the several reaction channels permits the clarification of the switching mechanism causing theoscillatory behavior of this system. The results appear to exhibit limit cycle behavior, in accordance with theoretical predictions.     

Studies on the oxidation of α-amino acids by N-Bromo oxidants: Kinetics of the reaction of bromide ion with N-Bromoacetamide

The rate of oxidation of amino acids (AA) by N-Bromoacetamide (NBA) was studied in aqueous buffered medium at 35°C. The rate of disappearance of [NBA] is catalyzed by the Br⁻ produced from the reduction of NBA. Analysis of the autocatalyzed reaction gives the kinetic data for the oxidation of bromide ion by NBA. The results suggest that the protonated NBA reacts with Br⁻ to form Br₂ which rapidly oxidizes amino acids. The rate constant for the reaction between protonated NBA and Br⁻ at 35°C is estimated. © 1996 John Wiley & Sons, Inc.     

Determination of bromide ion residues in broccoli after fumigation with methyl bromide by nonsuppressed ion-chromatography

A nonsuppressed ion chromatography method using conductivity detection was performed to determine the concentration of bromide ions in broccoli following fumigation with methyl bromide (MB). After fumigation by MB with concentration up to 40 g/m3, bromide residue (BR) was considerably increased from the trace amount on unfumigated broccoli to about 12 ppm on fumigated broccoli. For probing the location of BR, broccoli was divided into stems and florets. The BRs were measured separately, and the results indicated that BRs only appear in the florets of broccoli.     

Fragment–molecule adduct ion formation in positive methane chemical ionization mass spectrometry1

Fragment–molecule (F⁺M) adduct ions of a variety of heterocyclic systems have been observed in positive methane chemical ionization spectra at relatively low sample concentrations, i.e. in the microgram range. The relative abundance of these adduct ions increases with sample concentration. Possible mechanisms of formation of F⁺M are discussed. Formation of such ions should be recognized as an artifact, otherwise such F⁺M adduct ions in chemical and electron ionization may be mistaken for impurities present in a sample.     

Modulation of selectivity in a fluorocarbene cyclopropanation reaction: a catalytic role for bromide ion

Phenylfluorocarbene is diverted to phenylfluorobromomethide carbanion (PhCFBr⁻) by adding bromide ion. The carbanion adds to acrylonitrile leading, after expulsion of bromide, to 1-fluoro-1-phenyl-2-cyanocyclopropane.     

Determination of thioureas by reaction with phenacyl bromide

Thioureas react quantitatively with phenacyl bromide in ethanolic solution to give hydrobromides of aminothiazoles/iminodihydrothiazoles which can be titrated with sodium carbonate, Thymol Blue being used as indicator.