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To the core of autocatalysis in cyclohexane autoxidation

机译：环己烷自氧化中自催化的核心

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摘要

Despite their industrial importance, the detailed reaction mechanism of autoxidation reactions is still insufficiently known. In this work, complementary experimental and theoretical techniques are employed to address the radical-chain initiation in the autoxidation of cyclohexane with a particular focus on the "lighting-off" of the oxidation by (added) cyclohexanone. We used a newly developed method to quantify the intrinsic rate of chain initiation as well as the rate enhancement by cyclohexanone and several other (oxygenated) molecules. On the basis of first principles, the hitherto assumed perhemiketale mechanism was found to be many orders of magnitude too slow to account for the observed initiation enhancement by the ketone. Instead, it is shown that the pronounced chain-initiation enhancement by the ketone is attributable to a newly proposed concerted reaction between cyclohexyl hydroperoxide and cyclohexanone, in which the (OH)-O-center dot radical breaking away from the hydroperoxide abstracts an alpha H atom from the ketone, thereby energetically assisting in the cleavage of the RO-OH bond. Ibis reaction is highly efficient in generating radicals as it quasi-excludes geminate in-cage recombination. As a result, the ketone oxidation product at a level of 1 mol% increases the initiation rate by one order of magnitude, and so acts as a highly efficient "autocatalyst" in autoxidation reactions. An analogous reaction with cyclohexanol, although estimated to be even faster, has only a marginal effect on the overall kinetics, owing to the fast subsequent formation of HO2 center dot radicals that very rapidly terminate with other ROO center dot radicals. Finally, solid evidence is presented that, also in absence of oxygenates, ROOH initiation is actually a bimolecular reaction, involving concerted H abstraction from the alkane substrate by the nascent (OH)-O-center dot.
机译：尽管它们具有工业重要性，但是自氧化反应的详细反应机理仍不为人所知。在这项工作中，采用互补的实验和理论技术来解决环己烷自氧化反应中的自由基链引发问题，尤其侧重于（添加的）环己酮氧化的“起燃”作用。我们使用一种新开发的方法来量化内在的链引发速率以及环己酮和其他几种（氧化的）分子的速率提高。在第一原理的基础上，发现迄今假定的perhemiketale机制太慢了多个数量级，以至于不能解释所观察到的酮的引发增强作用。相反，表明酮显着增强的链引发作用归因于新提出的环己基氢过氧化物与环己酮之间的协同反应，其中从氢过氧化物上脱开的（OH）-O-中心点自由基提取了一个αH酮中的碳原子，从而在能量上有助于RO-OH键的裂解。宜必思反应在产生自由基方面非常高效，因为它几乎排除了笼中新芽的重组。结果，以1mol％的水平的酮氧化产物将引发速率提高了一个数量级，因此在自氧化反应中用作高效的“自催化剂”。与环己醇的类似反应，尽管据估计甚至更快，但由于随后很快形成的HO2中心点自由基很快与其他ROO中心点自由基终止，因此对整体动力学的影响很小。最后，有力的证据表明，即使在不存在含氧化合物的情况下，ROOH引发实际上也是一个双分子反应，涉及通过新生的（OH）-O-中心点从烷烃底物上协同地夺取H。

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