Indirect reduction of o- and m-nitrobenzoic acids at a Ti/ceramic TiO2 cathode

Abstract

The Ti/ceramic TiO2 cathode was used successfully for the catalytic reduction (indirect) of several compounds such as 5-nitrosalicylic acid [1], isomeric nitroanilines [2,3], isomeric chloronitrobenzenes [4], 1-nitroso 2-naphthol [5] and maleic acid [6] to the corresponding amines, diamines or to succinic acid, in earlier studies. The Tig+/Ti 3+ redox species at the electrode was responsible for the catalytic reduction. In continuation of the above work, the results of galvanostatic electrolysis data on the reduction of oand m-nitrobenzoic acids (o-NBA and m-NBA) to the corresponding aminobenzoic acids (o-ABA and rn-ABA) are reported. Also reported, are cyclic voltammetric studies. o-Aminobenzoic acid (anthranilic acid) is usually made from phthalic anhydride via sodium phthalamate. Oxidative decarboxylation, carried out using sodium hypochlorite solution at 60-100 °C followed by acidification, yields 88% of o-ABA. Other synthetic routes include the amination of o-chlorobenzoic acid and the reduction of o-NBA, m-ABA is prepared by the reduction of m-NBA [7]. Electrolytic reduction of oand m-NBA in a divided cell (porous diaphragm) with Pb anode and Sn, Pb, Cu, graphite or amalgamated cathode (catholyte:HC1 0.58-4.46 N; Anolyte 15% H2804) in the current density range 2-14Adm -2 at 30-50 °C (for o-NBA) and 40-60 °C (for m-NBA) yields oand m-ABA in 92-98% depending on the conditions [8]. Reduction of oand m-NBA in 30% H2SO4 at Cu cathode and Pb anode yields a mixture of o-ABA (m-ABA) and 5-hydroxyanthranilic acid (5-aminosalicylic acid) [9]. Electrolytic reduction of o-NBA at a controlled potential of -0.9V against SCE at a silver cathode in HC1-KC1 at pH 1 in 30% EtOH at 20 °C yields oABA and that of m-NBA at a potential of -0.75 V against SCE in 1N H2SO 4 in 15% EtOH below 15 °C yields m-ABA. However, at higher temperature and higher acid strength (50% H2804; 100 °C for o-NBA and 2N H2SO4; 95°C for m-NBA) at potentials of -0.1V and -0.3V, 2,5-NH2OHC6H3COOH and 3,6-NH2OHC6H3COOH are formed respectively [10]. o-ABA (vitamin L1) enhances milk production in cows. It serves as an intermediate for making dyes, tranquilizers and antiphlogistics, and as odorant for toiletries [11] and as an ascaricide in swine [7]. mABA is used as a dye intermediate [11].