Elementary Reactions of Phenol Oxidation

Change in visible spectra of the reaction system with time (a) and d-d absorption change of polymer-Cu catalyst with time (b)1S4  

Step 1. Coordination of the substrate immediately after mixing the Cu(II) catalyst with the substrate, a rapid change in the absorption is observed within several decaseconds [Fig. 27(b)] this is believed to be caused by the coordination of the substrate to the Cu(II) complex. We measured this rapid change spectroscopically by the stopped-flow method, and calculated the apparent rate constant k. When an insoluble polymer complex is used as a catalyst, a decrease in the XOH concentration in the liquid phase corresponds to the coordination of XOH to the Cu catalyst in the solid phase155.  

Step 2. Electron-transfer the decrease in the d-d absorption subsequent to the steep rise of the absorbance in Fig. 27(b) corresponds to the formation of Cu(I) ions, i.e. electron transfer from the substrate to Cu(II) ions. The d orbitals of Cu(I) are perfectly occupied, so that d-d absorption spectrum does not appear in the Cu(I) complex. The rate constant ke is obtained from the falling curve of the d-d absorption. The fraction of Cu(II) ions is also determined from the magnetic susceptibility of the reaction system. The Cu(II) ion contains an unpaired electron and is paramagnetic, while the Cu(I) ion is diamagnetic. Magnetic measurement is applicable particularly to an insoluble catalyst system155.  

Step 3. Dissociation of the activated substrate both before and after the dissociation step of the activated substrate the complex is composed of Cu(I), so that neither spectral nor magnetic measurements on the Cu catalyst are effective. However, this step can be monitored by ESR spectrometry on the substrate. The ESR signals due to the activated phenol (phenoxy radical) appear at about g = 2.003. 

Step 4. Reoxidation of the catalyst the catalyst is regenerated by oxidizing the Cu(I) complex with oxygen. The rate constant k0 of the catalyst-recycle step 

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