Regioselective nitration of phenol under continuous microwave irradiation

This study deals with the nitration of phenol in a multimode MW reaction system operating under continuous microwave (MW) irradiation. Reactions were carried out with two nitric acid (HNO3)/phenol molar ratios: 1.25 and 1.1, under the following conditions: applied (nominal) MW power, Pnom = 600 W, adsorbed power P = 16.0 kW dm−3, reaction temperature, TR = 68–90 °C, reaction time, t = 5–9 min, heating rate 0.1–0.25 °C s−1. At the end of the synthesis, para/ortho (p/o) isomer distribution of nitrophenol product was determined by high-pressure liquid chromatography (HPLC). Furthermore, conventional (CH) experiments were conducted also under the same conditions. Then, MW and CH methods were compared in terms of product yield and isomer distribution. According to results, yield and para-selectivity of MW-assisted synthesis were higher than the CH counterpart although the reaction temperature of the MW method was lower than that of the CH method.