Electrochemical reduction of benzoic acid and substituted benzoic acids in some room temperature ionic liquids

Abstract

Using cyclic voltammetry, the electrochemical reduction of benzoic acid (BZA) has been studied at Pt and Au microelectrodes (10 and 2 μm diameter) in six room temperature ionic liquids (RTILs), namely [C2mim][NTf2], [C4mim][NTf2], [C4mpyrr][NTf2], [C4mim][BF4], [C4mim][NO3], and [C4mim][PF6] (where [Cnmim]+ = 1-alkyl-3-methylimidazolium, [NTf2]− = bis(trifluoromethylsulfonyl)imide, [C4mpyrr]+ = N-butyl-N-methylpyrrolidinium, [BF4]− = tetrafluoroborate, [NO3]− = nitrate, and [PF6]− = hexafluorophosphate). In all cases, a main reduction peak was observed, assigned to the reduction of BZA in a CE mechanism, where dissociation of the acid takes place before electron transfer to the dissociated proton. One anodic peak was observed on the reverse sweep, assigned to the oxidation of adsorbed hydrogen, and a reductive “prepeak” (assigned to the reduction of protons forming adsorbed hydrogen, H•(ads)) was observed before the main reduction peak.On extending the cathodic window, an electrochemically reversible peak was observed in [C4mpyrr][NTf2] and [C4mim][BF4], assigned to the reduction of the radical anion of BZA forming the dianion (C6H5•C(O−)2), thought to be stable in RTIL media. The voltammetry of 4-methoxy, 4-chloro, 4-bromo and 4-dialkylamino substituted BZAs yielded a main reduction peak, with two anodic peaks on the reverse sweep, (assigned to the oxidation of adsorbed hydrogen H•(ads) and the oxidation of C6H5CO2− to form (C6H5)• and CO2). At more negative potentials, the electrochemically reversible radical anion reduction peak was also observed. Potential-step chronoamperometry revealed diffusion coefficients of ca. 2 × 10−11 m2 s−1 for all BZA derivatives studied in [C4mpyrr][NTf2], with a slight tendency to decrease with increasing size of substituent. The solubilities of the substituted BZAs were much lower than that of the nonsubstituted form. In addition, the oxidation of 4-dimethylamino- and 4-diethylaminobenzoic acid yielded a sharp anodic peak, suggesting the possible formation of substituted polyaniline on the Pt electrode surface.