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首页> 外文期刊>Chemistry: A European journal >A Theoretical Analysis of the Free-Energy Profile of the Different Pathways in the Alkaline Hydrolysis of Methyl Formate In Aqueous Solution
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A Theoretical Analysis of the Free-Energy Profile of the Different Pathways in the Alkaline Hydrolysis of Methyl Formate In Aqueous Solution

机译：水溶液中甲基甲酸酯碱水解中不同途径的自由能谱的理论分析

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The free-energy profile for the different reaction pathways available to the hydroxide ion and methyl formate in aqueous solution is reported for the first time. The theoretical analysis was carried out by using the clustercontinuum method recently proposed by us for calculating the free energy of solvation of ions. Unlike the gas-phase reaction, our results are consistent with the fact that the reaction occurs mainly by nucleophilic attack of the hydroxide on the carbonyl carbon to yield a tetrahedral intermediate (B_(AC)2 mechanism). However, an additional pathway, in which the hydroxide ion acts as a general base and a water molecule coordinated to this ion acts as the nucleophile, is also predicted to be important. The relative importance of these pathways is calculated to be 87% and 13%, respectively. The tetrahedral intrmediate of the hydrolysis reaction has an estimated lifetime of 10 nanoseconds, and its conjugate acid has a pK_a of 8.8. This tetrahedral intermediate is predicted to proceed to products by two pathways: elimination of methoxide ion (84%) and by water catalyzed elimination of methanol (16%). The less common reaction pathway, which involves attack of the hydroxide ion on the formylhydrogen (decarbonylation mechanism) and leads to water, carbon monoxide, and methanol, is calculated to be only 3 kcal mol~(-1) less favorable than the B_(AC)2 mechanism. By comparison, direct attack of the hydroxide ion on the methyl group (B_(AL)2 or S_N2 mechanism) leading to an acyl-oxygen bod cleavage has a very high free energy of activation and is not expected to be important. The theoretically observed activation free energy at 298.15 K is calculated to be 15.5 kcal mol~(-1), in excellent agreement with the experimentally measured value of 15.3 kcal mol~(-1). This present model allows for a clear distinction between contributions due to solvation and those due to intrinsic (gas-phase) effects and proves to yield results in very good agreement with available experimental data.
机译：首次报道了水溶液中氢氧根离子和甲酸甲酯可利用的不同反应途径的自由能曲线。使用我们最近提出的簇连续谱方法进行理论分析，以计算离子的溶剂化自由能。与气相反应不同，我们的结果与以下事实一致：反应主要通过氢氧化物对羰基碳的亲核攻击而发生，从而产生四面体中间体（B_（AC）2机理）。然而，另外的途径也很重要，在该途径中氢氧根离子充当一般的碱，而与该离子配位的水分子充当亲核试剂。这些途径的相对重要性分别计算为87％和13％。水解反应的四面体中间体的估计寿命为10纳秒，其共轭酸的pK_a为8.8。预计该四面体中间体会通过两种途径生成产物：消除甲醇离子（84％）和通过水催化消除甲醇（16％）。较不常见的反应途径涉及氢氧根离子袭击甲酰氢（脱羰机理）并导致水，一氧化碳和甲醇，计算得出的反应途径仅比B _（-）低3 kcal mol〜（-1）。 AC）2机制。相比之下，氢氧根离子对甲基的直接攻击（B_（AL）2或S_N2机理）导致酰基-氧键裂解，具有很高的活化自由能，因此并不重要。理论上在298.15 K处观察到的活化自由能经计算为15.5 kcal mol〜（-1），与实验测得的15.3 kcal mol〜（-1）非常吻合。本模型可以清楚地区分溶剂化作用与内在作用（气相）的影响，并证明所产生的结果与可用的实验数据非常吻合。

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