I(+1) Transfer from Diiodomalonic Acid to Malonic Acid and a Complete Inhibition of the CO and CO2 Evolution in the Briggs−Rauscher Reaction by Resorcinol

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• 作者：Lavinia Onel ; Gelu Bourceanu ; Maria Wittmann ; Zoltá ; n Noszticzius ; Gabriella Szabó
• 刊名：Journal of Physical Chemistry A
• 出版年：2008
• 出版时间：November 20, 2008
• 年：2008
• 卷：112
• 期：46
• 页码：11649-11655
• 全文大小：163K
• 年卷期：v.112,no.46(November 20, 2008)
• ISSN：1520-5215

文摘

A recent report on an intense CO₂ and CO evolution in the Briggs−Rauscher (BR) reaction revealed that iodination of malonic acid (MA) is not the only important organic reaction in the classical BR oscillator. To disclose the source of the gas evolution, iodomalonic (IMA) and diiodomalonic (I₂MA) acids were prepared by iodinating MA with nascent iodine in a semibatch reactor. The nascent iodine was generated by an iodide inflow into the reactor, which contained a mixture of MA and acidic iodate. Some CO₂ and a minor CO production was observed during these iodinations. It was found that in an aqueous acidic medium the produced I₂MA is not stable but decomposes slowly to diiodoacetic acid and CO₂. The first-order rate constant of the I₂MA decarboxylation at 20 °C was found to be k₁ = 9 × 10⁻⁵ s⁻¹, which is rather close to the rate constant of the analogous decarboxylation of dibromomalonic acid under similar conditions (7 × 10⁻⁵ s⁻¹). From the rate of the CO₂ evolution, the I₂MA concentration can be calculated in a MA−IMA−I₂MA mixture as only I₂MA decarboxylates spontaneously but MA and IMA are stable. Following CO₂ evolution rates, it was proven that I₂MA can react with MA in the reversible reaction I₂MA + MA ↔ 2 IMA. The equilibrium constant of this reaction was calculated as K = 380 together with the rate constants of the forward k₂ = 6.2 × 10⁻² M⁻¹s⁻¹ and backward k₋₂ = 1.6 × 10⁻⁴ M⁻¹s⁻¹ reactions. The probable mechanism of the reaction is I(+1) transfer from I₂MA to MA. The presence of I(+1) in a I₂MA solution is demonstrated by its reduction with ascorbic acid. To estimate the fraction of CO₂ coming from the decarboxylation of I₂MA in an oscillatory BR reaction, the oscillations were inhibited by resorcinol. Unexpectedly, all CO₂ and CO evolution was interrupted for more than one hour after injecting a small amount of resorcinol (10⁻⁵ M initial concentration in the reactor). Finally, some implications of the newly found I(+1) transfer reactions and the surprisingly effective inhibition by resorcinol regarding the mechanism of the oscillatory BR reaction are discussed. The latter is explained by the ability of resorcinol to scavenge free radicals including iodine atoms without producing iodide ions.