Facile and Economical Reduction of Nitro Aromatic Compounds using Aluminium and Ultrasound Waves

Abstract

The reduction of nitro group to amine is one of the most important transformations in organic synthesis since the amino group is often used for further derivatization towards valuable products such as dye stuffs, textile auxiliaries, pharmaceuticals, agrochemicals, photographic materials, polymers, explosives, fibers, surfactants, cosmetics etc. Mono, di and tri nitro aromatic compounds were reduced using aluminium turnings and aluminium foil which are benign and cheap along with ultrasound waves in moderate yield under mild conditions. 2,4,6-triaminophenol was synthesized which was used as a standard in case of TLC analysis of picric acid. 1,3- dinitrobenzene was first synthesized using traditional nitrating mixture of acids. Picric acid was reduced for the first time using this milder approach. Picric acid was reduced with yield of 61%. Optimization of reaction conditions was done with nitro benzene, 1,3-dinitrobenzene and picric acid using methanol, acetone, their combination with water and pure water as reaction medium was also used. First of all optimization was done with nitrobenzene then with 1,3-dinitrobenzene and in the end with picric acid. For the solubility of organic substrates in pure water CTAB was used. Aluminium turnings were more efficient as compared to aluminium foil because of purity. Methanol showed shortest reaction time and greatest yield as compared to acetone and pure water. Aromatic amines were characterized by IR, NMR, HPLC and TLC. Presence of amino group in the products and absence of nitro groups was depicted by IR analysis. HPLC was done in order to check the purity of aromatic amine formed as a result of reduction. Single sharp peak confirmed the purity of product. NMR was used to confirm the structure of aromatic amines. TLC was used in order to monitor progress of reaction, completion and comparison with standards for product confirmation. Facile, mild, greener and economical procedure has been optimized for reduction of nitro aromatic compounds to obtain moderate yields. Shorter reaction times, milder reagents, easy availability of reagents and cost effectiveness associated with this method make this protocol industrially important.