In this study, we have clarified the chlorination mechanism of precursor phenol in the homogeneous and heterogeneous phase using ab initio molecular orbital calculation. Simultaneously, we have analyzed the catalytic roles of copper on this chlorination reaction. The results obtained in this study are as follows: (1) the chlorination of precursor phenol in the homogeneous phase progresses via the direct condensation of phenol and Cl₂, whereby desorbing HCl, (2) the chlorination of precursor phenol in the heterogeneous phase occurs via the adsorption and surface reaction of precursor phenol. The catalytic role of copper on the chlorination of precursor phenol is that Cl–Cl bond strength is weakened due to the back donation from copper surface.