芳族硝基化合物的电还原反应性能及其机理研究

Study on Electroreduction Performance and Mechanism of Substituted Aryl Nitrobenzene

采用循环伏安、线性扫描以及恒电位电解等测试方法,对o,m,p-氯硝基苯,o,m,p-硝基甲苯、o,m-硝基苯甲酸等八种芳族硝基化合物在硫酸-乙醇水溶液中的电还原性能及其机理进行了研究。结果表明,芳族硝基化合物在苯环上的取代基位置不同以及在相同位置上取代基的种类不同均可使各芳族硝基化合物的电还原性能发生较大差异;吸电子取代基在电还原过程中电位较正,推电子取代基的电还原电位较负;邻位取代基在反应过程中由于空间位阻效应,苯环上的硝基在电还原时电位往负方向移动。在电还原过程中,还原峰电流和表观活化能的大小均与取代基的电子效应和空间位阻相关,同时取代硝基氯苯得3个电子将还原生成氧化偶氮苯类化合物:邻、间硝基甲苯、硝基苯甲酸得4个电子将还原生成酚类化合物;对硝基甲苯得6个电子将还原生成胺类化合物,在此基础上,探讨了各芳族硝基化合物的电还原机理。

The electroreduction performance and mechanism of substituted aryl nitrobenzene, such as o,m,p-chloronitrobenzene, o,m,p-nitrotoulene and o,m-nitrobenzoic acid in sulfuric acid-ethanol solution were studied by means of quasi-steady-state polarization, cyclic voltammery and constant potential electrolysis. Experimental results show that the different substituted location and different substituted group on benzene cycle would result in the diversity of reduction properties of substituted aryl nitrobenzene. Substituted nitrobenzenes with withdrawing group will be reduced at more positive potential, while those with repelling group will be reduced at more negative potential. In addition, the reduction potential of nitro group of o-nitrobenzene shifts more negative due to the effect of space resistance. The value of current density and the apparent active energy of electroreduction of substituted nitrobenzene depend on the electronic effect of substituted group and space resistance, o,m,p-chloronitrobenzene will be electroreduced to azoxy compounds by taking three electrons, o,m-nitrotoulene and o,m-nitrobenzoic acid will be reduced to phenol compounds by taking four electrons, and p-nitrotoulene will be reduced to the aniline by taking six electrons. According to these results, the electroreduction mechanism of the substituted aryl nitro-compounds was studied in detail.