Studies On Electrochemical Debromination Mechanism Of Bromobenzoic Acids By In Situ FTIR Spectroscopy

Halogenated organic compounds(HOCs)are frequently found in many surface and ground waters.Most of HOCs are toxic due to the halogen contained in their structure,while halogen-free organic compounds are usually much less toxic,and can be disposed by more convenient and economic manners.Bromobenzoic acids play an important role in numerous organic processes and the electrochemical debromination reaction of these compounds is an important area of electrochemistry.So in this paper,bromobenzoic acids were chosen as model molecules to investigate the electrochemical debromination mechanism by in situ FTIR.First,the electrochemical hydrodebromination(EHB)mechanism of 2,5-dibromobenzoic acid(2,5-DBBA)were studied on Ag electrode in aqueous solution.Compared with titanium and graphite electrodes,silver electrode exhibited a high electrocatalytic activity for the debromination reaction of 2,5-DBBA.On the basis of in situ FTIR data,the EHB reaction of 2,5-DBBA on Ag cathode might be represented as a sequence of electron additions and bromine expulsions.From potential at approximately-1100 mV,2,5-DBBA received an electron to form 2,5-DBBA radical anion,which lost a bromine ion in the 2-position to form 3-bromobenzoic acid(3-BBA)free radical.Then the free radical received a proton to give 3-BBA.Finally,3-BBA further took off another bromine ion to produce benzoic acid free radical and the end product benzoic acid was obtained by receiving another electron and a proton with the potential shifting to more negative values.These results were also in good agreement with the results of computational studies.Second,the EHB reaction of 2-BBA were studied on Ag and Cu electrode.The results of in situ FTIR showed that the EHB reaction also can be occurred on Cu electrode,but the adsorption ways on these two electrodes were different.2-BBA,bring negatively charged,repelled Cu electrode by virtue of the electrostatic repulsion.But the electrostatic repulsion between 2-BBA and Ag electrode was decreased due to the extraordinary affinity of Ag for halide anions,which assumed to hinge on an attenuated radical intermediate R…X…Ag.Last,the electrooxidation mechanism of 3-BBA and 2-BBA on Pt electrode were investigated by in situ FTIR spectroscopy.The results suggested the mechanism of both compounds were very similar.From potential at 1100 mV,one electron was removed from carboxylate ion and bromobenzene was obtained after decarboxylation.Then the intermediate product,bromobenzene,was debrominated through the generation of adsorbed hydroxyl radicals on anode.Hydroxyl radicals were added to produce dihydroxybenzene with the potential shifting to more negative values.Finally,dihydroxybenzene was electrooxidized to benzoquinone.In addition,maleic acids、fumaric acid were also detected as the reaction products.