Ultrasound-Assisted Regioselective Nitration of Phenols Using Dilute Nitric Acid in a Biphasic Medium

A simple and convenient methodology for the regioselective nitration of phenols using dilute nitric acid as a nitrating agent under sonication is described. The protocol eliminates the use of any additive and requires lower concentration of nitric acid (9 wt %). The effect of various reaction parameters such as agitation speed, solvent, phase hold-up ratio, substrate concentration, nitric acid concentration, and temperature on the reaction system has been studied. The present methodology shows a considerable enhancement in the reaction rate along with improved para-selectivity compared with the reactions performed under silent conditions. The kinetic analysis of nitration of phenol both with and without sonication has been investigated by studying reaction parameters such as substrate and nitric acid concentration. The increased rate of reaction and selectivity are explained on the basis of ultrasonically generated cavitational effects. The work presented hereby clearly indicates the methodology to be followed for investigating the optimum extent of intensification achieved using ultrasound as compared to conventional methods. It will be useful for the design engineers to investigate a particular application in question.