通过DFT计算和MO理论推导了苯硝化反应的修正三步反应机理

Hongchang Shi
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引用次数: 1

摘要

LC-wHPBE/6-311++G(d,p)水平的密度泛函理论(DFT)计算发现,教科书中苯在混合酸中的三步硝化机理存在严重错误。生成氮离子(NO2+)的第1步不是自发的,NO2+是由Lewis碰撞产生的,需要克服混合酸或硝酸中Ea = 18或22 kcal/mol的势垒。利用量子化学计算获得刘易斯碰撞的Ea是本研究的一个重点。反应体系(NO2+ + H2O) + HSO4⎺或+ NO3⎺或+ nH2O (n≥1)可通过聚(≥3)分子酸化使NO2+自发转化为HNO3。硫酸能使[H2O]大幅度还原,[NO2+]大幅度增加。因此,在混合酸中的硝化速度比在硝酸中的硝化速度快得多。第二步,C6H6 + NO2+是亲电加成,遵循过渡态理论,需要克服一个低势垒,ΔE* = 7 kcal/mol。第二步的产物是σ-络合物C6H6-NO2+。亲电加成的实质是C6H6的HOMO-1电子向NO2+的LUMO转移。步骤3为无屏障自发Lewis酸碱中和反应,生成目标产物硝基苯C6H5NO2。NO2+和σ-络合物是硝化反应的活性中间体。苯硝化速率控制步骤不是生成σ-络合物的步骤2,而是生成NO2+的步骤1。通过DFT计算得到各步硝化反应的势垒Ea和ΔE*,反应热量ΔHσ和ΔHp,得到总硝化反应热量ΔH = -35 kcal/mol。与实验结果ΔH = -34 kcal/mol一致。在此基础上,提出了修正的苯硝化三步反应机理。
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A corrected benzene nitration three-step mechanism derived by DFT calculation and MO theory
Density-functional theory (DFT) calculations at the LC-wHPBE/6-311++G(d,p) level found that the textbook three-step nitration mechanism of benzene in mixed acids was seriously wrong. Step 1 of generating nitronium ion (NO2+) is not spontaneous, the NO2+ is generated by Lewis collision, and needs to overcome a barrier Ea = 18 or 22 kcal/mol in mixed acid or in nitric acid. Obtaining the Ea of the Lewis collision by quantum chemical calculations is a highlight of the study. The reaction system (NO2+ + H2O) + HSO4⎺ or + NO3⎺ or + nH2O (n ≥ 1) can make NO2+ spontaneously change to HNO3 through a poly(≥3)-molecular acidification. Sulfuric acid can greatly reduce [H2O] and increase [NO2+]. Therefore, the nitration rate in mixed acid is much faster than that in nitric acid. Step 2, C6H6 + NO2+, is an electrophilic addition, follows the transition state theory, and needs to overcome a low barrier, ΔE* = 7 kcal/mol. The product of Step 2 is the σ-complex C6H6-NO2+. The essence of the electrophilic addition is the transfer of HOMO-1 electrons of C6H6 to LUMO of NO2+. Step 3 is a spontaneous Lewis acid-base neutralization without any barrier, and generates the target product nitrobenzene C6H5NO2. NO2+ and σ-complex are the two active intermediates in nitration. The benzene nitration rate control step is not Step 2 of generating σ-complex, but is Step 1 to generate NO2+. The DFT calculation obtains the barriers Ea and ΔE*, the reaction heats ΔHσ and ΔHp of each step of the nitration, resulting in the total nitration reaction heat ΔH = -35 kcal/mol. It is consistent with the experimental ΔH = -34 kcal/mol. Based on the results, a corrected benzene nitration three-step mechanism proposed.
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