次氯酸与不饱和脂肪酸氧化机制和转化产物的理论研究

摘要 目的：揭示次氯酸与不饱和脂肪酸的氧化反应机制及转化产物。方法：运用Gaussian 16软件包，采用密度泛函方法M06-2X(D3)，结合6-31+G(d)基组，在SMD液相水模型水平下进行计算。结果：次氯酸与单不饱和脂肪酸油酸的氧化反应是先形成氯鎓离子中间体，氯鎓离子再与水分子反应生成氯醇，第一步氯鎓离子的形成是控速步骤，其反应活化自由能~8 kcal/mol。环氧化合物和短链的醛是两种转化产物，前者由氯醇脱氯化氢而来，而后者由环氧化合物和氯醇通过系列与次氯酸根的反应而得到，生成它们的控速步骤的反应活化自由能分别为23 和24 kcal/mol。选取两个乙基为取代基的乙烯为油酸模型，其与次氯酸反应的活化自由能仅比油酸高1 kcal/mol。计算得到次氯酸与亚油酸、顺-9，反-11 亚油酸、梓树酸和花生四烯酸模型氧化反应生成氯醇的活化自由能分别是~10、13、16和14 kcal/mol。结论：氯鎓离子中间体机制是次氯酸与不饱和脂肪酸氧化反应的主要机制，反应的活化自由能通常低于15 kcal/mol，意味着此氧化反应动力学上容易发生。氧化产物氯醇能转化为环氧化合物和短链的醛，但活化自由能较高，约23和24 kcal/mol。选取距离双键3个碳以内的结构为不饱和脂肪酸模型，它能够很好地反映不饱和脂肪酸的反应活性。

Theoretical Investigation of Oxidation Mechanisms and Transformation Products of Hypochlorous Acid Reacting with Unsaturated Fatty Acids

ABSTRACT Objective: To reveal the oxidation mechanisms and transformation products of HOCl reacting with unsaturated fatty acids. Methods: Gaussian 16 software package was used, in which density functional method M06-2X(D3) in conjunction with 6-31+G(d) basis set along with implicit SMD solvent model were selected. Results: In the oxidation reaction of HOCl with oleic acid, a monounsaturated fatty acid, chloronium ion intermediate was initially generated, and then it reacted with water molecule to yield chlorohydrin. The initial chloronium ion intermediate formation was the rate-limiting step with the activation free energy of ~8 kcal/mol. Epoxide and truncated aldehyde were two transformation products. The former was produced from dehydrochlorination of chlorohydrins, while the latter was yielded from epoxide and chlorohydrin through a series of reactions with hypochlorite. Their activation free energies of the rate-limiting steps were ~23 and 24 kcal/mol, respectively. Ethylene substituted with two ethyl groups was selected as the oleic acid model, and its activation free energy in reacting with HOCl was higher than that of oleic acid by only 1 kcal/mol. The activation free energies of HOCl reacting with linoleic acid, cis-9, trans-11 LCA, catalpic acid, and arachidonic acid models to generate chlorohydrin were ~10, 13, 16, and 14 kcal/mol, respectively. Conclusion: The chloronium ion intermediate formation is the main mechanism in the oxidation reaction of HOCl and unsaturated fat acid and the activation free energy is generally lower than 15 kcal/mol, which indicates that this oxidation reaction is kinetically feasible to occur. The oxidation product chlorohydrin can transform into epoxide and truncated aldehyde, but the activation free energies are relatively high with the values of ~23 and 24 kcal/mol, respectively. The structure within 3 carbon atoms from the double bond in the unsaturated fat acid can be selected as the model, which can well reflect the reaction activity of unsaturated fat acid.