Onel Lavinia, Bourceanu Gelu, Wittmann Maria, Noszticzius Zoltán, Szabó Gabriella
Department of Physical and Theoretical Chemistry, Faculty of Chemistry, AlICuza University, Iasi, Romania.
J Phys Chem A. 2008 Nov 20;112(46):11649-55. doi: 10.1021/jp8064163. Epub 2008 Oct 23.
A recent report on an intense CO 2 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 I 2MA decarboxylation at 20 degrees C was found to be k1 = 9 x 10(-5) s(-1), which is rather close to the rate constant of the analogous decarboxylation of dibromomalonic acid under similar conditions (7 x 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 k 2 = 6.2 x 10 (-2) M (-1)s(-1) and backward k-2 = 1.6 x 10(-4) M(-1)s(-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.
最近一份关于布里格斯 - 劳舍尔（BR）反应中强烈的二氧化碳和一氧化碳逸出的报告显示，丙二酸（MA）的碘化并非经典BR振荡器中唯一重要的有机反应。为了揭示气体逸出的来源，在半间歇反应器中用新生态碘对MA进行碘化制备了碘丙二酸（IMA）和二碘丙二酸（I₂MA）。新生态碘通过向含有MA和酸性碘酸盐混合物的反应器中流入碘化物产生。在这些碘化过程中观察到了一些二氧化碳和少量一氧化碳的产生。研究发现，在酸性水介质中，生成的I₂MA不稳定，会缓慢分解为二碘乙酸和二氧化碳。I₂MA在20℃下脱羧的一级速率常数为k₁ = 9×10⁻⁵ s⁻¹，这与在类似条件下二溴丙二酸类似脱羧的速率常数（7×10⁻⁵ s⁻¹）相当接近。根据二氧化碳逸出速率，可以计算MA - IMA - I₂MA混合物中I₂MA的浓度，因为只有I₂MA会自发脱羧，而MA和IMA是稳定的。通过跟踪二氧化碳逸出速率，证明了I₂MA可以与MA在可逆反应I₂MA + MA ⇌ 2 IMA中发生反应。该反应的平衡常数计算为K = 380，同时还得到了正向反应速率常数k₂ = 6.2×10⁻² M⁻¹s⁻¹和逆向反应速率常数k⁻₂ = 1.6×10⁻⁴ M⁻¹s⁻¹。该反应可能的机理是I（+1）从I₂MA转移到MA。通过用抗坏血酸还原I₂MA溶液证明了其中I（+1）的存在。为了估计在振荡BR反应中来自I₂MA脱羧的二氧化碳比例，用间苯二酚抑制振荡。出乎意料的是，注入少量间苯二酚（反应器中初始浓度为10⁻⁵ M）后，所有二氧化碳和一氧化碳的逸出都中断了一个多小时。最后，讨论了新发现的I（+1）转移反应以及间苯二酚令人惊讶的有效抑制作用对振荡BR反应机理的一些影响。后者可以通过间苯二酚清除包括碘原子在内的自由基而不产生碘离子的能力来解释。
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