Microwave-Aided Nitration of Phenol with Inorganic Nitrates: Inquiry-Based Learning Experiments

Abstract

Current trends in organic synthesis as well as in chemistry education highlight the principles of Green chemistry and call for new synthetic procedures that conform to the actual ecological and economic requirements according to the Green deal and other global challenges. This includes, for instance, replacement of aggressive and toxic reagents, optimisation of synthetic protocols to achieve the highest possible yields within the shortest reaction times, lowering the reaction temperature, solvent recycling and waste minimisation. Considering the present technological advances, replacement of classical heating by microwave irradiation turns out to be an important tool of Green chemistry that permits significant reduction of the reaction time and increase of the yield under certain conditions. In chemistry education alike, several teaching experiments have also been reported in the latest literature to show advantages of the microwave-aided synthesis of various organic compounds or to elucidate basic chemical reactivity principles (e.g. direct carboxamide synthesis, aromatic sulphonation with rearrangement). In the present study, we design a Green chemistry education project focused on microwave-aided nitration of phenol by a set of inorganic metal nitrates (i.e. sodium nitrate, calcium nitrate, copper(II) nitrate, iron(III) nitrate) in concentrated acetic acid. These inquiry-based learning experiments proceed very easily, and the reaction conditions can be controlled to achieve the first, the second or even the third degree of nitration. Along with a necessary minimum of chemistry knowledge and skills, the proposed educational experiments on microwave-aided synthesis encourage students to explore the influence of the inorganic nitrate structure on the phenol nitration products. The basic relationship between the properties of reactants and the course of this interesting organic reaction can be easily monitored by universal pH papers and thin-layer chromatography, and subsequently explained through inductive reasoning. As such, these student-centred experiments are suitable for implementation in inquiry-based chemistry education at universities or high schools oriented in natural sciences.