Regioselective nitration of phenols and anisols in microemulsion

A nitration reaction was performed in a cationic surfactant-based microemulsion system with four different aromatic compounds, and the ortho-to-para ratio was examined. The results have been compared with a reference reaction performed in a two-phase system without surfactant and with previous work done with reactions in microemulsions. We report almost selective para-nitration of the chosen compounds when the reactions were performed in the microemulsion. Reaction in a two-phase system gave approximately equal ortho- and para nitration. The difference in reaction rate between the microemulsion system and the two-phase system was examined as well. Using diluted HNO3 the reaction in microemulsion went to completion in 30 min. The two-phase system demanded concentrated HNO3; with dilute HNO3, there was almost no reaction after 24 h. 1H-NMR techniques were used to characterise the microemulsion and to determine the ortho-to-para ratio of the reaction products.