Study On The Redox Mechanism Of Phenol Compounds By FT-IR Spectroelectrochemistry

In the paper, the progress on redox of phenol compounds and IR spectroelectrochemistry was reviewed, then the redox mechanism of hydroquinone (QH2) and p-aminophenol (p-AP) was studied by the techniques of cyclic voltammetry (CV), in situ FT-IR spectroelectrochemistry (FT-IR), cyclic voltabsorptometry (CVA), derivative cyclic voltabsorptometry (DCVA) and reconstructed i-E curves. The main achievement was summarized as follows:1. The electrochemical behavior of QH2was studied by cyclic voltammetry (CV). Based on in situ FT-IR spectroelectrochemistry (FT-IR), we can track simultaneously the concentration changes of relevant redox species (reactant, intermediate, final product) during electrochemical process. The reasonable electrochemical oxidation mechanisms of QH2in acetonitrile, neutral unbuffer and basic solutions were proposed.(1) In acetonitrile solution, the electrochemical behavior of hydroquinone shows a couple of irreversible redox peaks in cyclic voltammogram. One negative-going band at1510cm-1and three positive-going bands at1656,1595,1318cm-1, corresponding to QH2,Q respectively, are observed in FT-IR3D map. It suggests that the oxidation of QH2is2e,2H+process and the final oxidation product is benzoquinone (Q).(2) In neutral unbuffer solution, it displays an anodic peak (A1) and two cathode peaks (C1, C2) for one cycle. For continuous three cycles, a new oxidation peak (A2), corresponding to the negative potential reduction peak (C2), appears. In IR3D map, six IR absorption peaks,1510;1656,1318;1341;1495,1272cm-1which are associated with QH2, Q, Q-, Q2-respectively, are observed. The results indicate that the final oxidation product is also Q. In reduction process, part of Q is reduced to QH2at more positive potential (C1) and the other is to Q2-at more negative potential (C2).(3) In basic solution, when1equiv of OH-was added to the solution, two anodic peaks which are connected with QH2, Q2-are observed in CV. If2equiv of OH-was added to the solution, only a couple of redox peaks (Q2-/Q) appear. In addition, we can observe the change of hydrogen bond by the negative-going bands at2140cm-1which is due to the absorption of Q2-… D2O or Q-… D2O. So the results indicate that this process is hydrogen bond coupled-electron transfer. Based on reconstructed current-potential (i-E) curve, we can confirm that the electron transfer of Q2-shows two-step one-electron transfer process.2. The electrochemical behavior of p-AP was studied by CV, FT-IR technique and reasonable redox mechanism of p-AP in acetonitrile and mixed solution was proposed.(1) The electrochemical character of p-aminophenol in acetonitrile media was investigated by cyclic voltammetry (CV). Two couples of redox peaks and two shoulder peaks are observed in cyclic voltammogram. By in situ FT-IR spectroelectrochemistry, several IR absorption peaks,1518,1256,1210;1610,1503,1272;1664,1649,1595,1287cm-1, corresponding to OHC6H4NH2;(OHC6H4NH2)+. and dimer;(OHC6H4NH2)2+and C6H4ONH, are observed. Based on cyclic voltabsorptometry (CVA) and derivative cyclic voltabsorptometry (DCVA) techniques, we can track the changes of each species during electrochemical process. The results indicate that p-AP is oxidized in two-step one-electron transfer, and moreover, amino group is first oxidized than the hydroxyl group. The final oxidation product is quinonimine, and meanwhile the formation of dimer is detected. The dimer can be oxidized at more positive potential to form the final oxidation product and reduced at more negative potential to form a new reduction product. By calculating relative energy, three possible structures of the dimer are proposed. The specific oxidation course is listed as follows: (2) The addition of H2O to acetonitrile has obvious change in the electrochemical behavior of p-AR. The potential of redox peaks has a negative shift. It is found that the bands at1272;1664,1287cm-1which are related to intermediate and final product, shift to higher frequency in the concentration (4%) of proton donors. Furthermore, obvious wide-band absorber is observed between the band at1700and2000cm-1. The results indicate that the hydrogen bond is formed between intermediate, final product and H2O. Moreover, final product is easier to form hydrogen bond. By calculating the electrochemical parameters, the final product combines with about1.8H2O to form hydrogen.