Mini Review A Review on Selective Synthesis and Yield Enhancement of Ortho-Nitrophenol in the Presence of Phase-Transfer Catalysts
Mini Review: Ortho-Nitrophenol by Phase-Transfer Catalysts
Current studies were performed to review the synthesis and yield enhancement of ortho- nitrophenol in the presence of phase-transfer catalysts. Selective synthesis and yield enhancement of ortho- nitrophenol can be varied in the presence of phase-transfer catalysts under different synthetic mechanisms. The phenols and substituted phenols can be selectively nitrated at their ortho/para positions by direct or indirect nitration and by varying the reaction parameters such as dilution of acid, substrate and the strength of the catalyst. High selectivity can be achieved by indirect nitration with diluted nitric acid in the presence of tetra-butyl ammonium bromide (TBAB) and acetyl trimethylammonium bromide (CTAB). TBAB has been reported to be the most effective catalyst for the conversion of phenolic compounds into o-nitrophenol. However, this regio-selectivity will be reversed to para-position by using sodium bromide as a surfactant. The use of nitronium tetra fluoroborate suspended in acetonitrile process gives highest regio-selective nitration with a 95% yield of ortho product. Performing the nitration under sonication conditions lowers the reaction time and increases the ortho/para regio-selectivity. Under sonication conditions, maximum conversion into ortho derivative (95%) is achieved by using TBAB as catalyst in 20 min but maximum para-selectivity is achieved by using CTAB and NaBr catalysts. The use of PdCl2, Pd(PPh3)2Cl2 and Pd(OAc)2, H-beta zeolite, catalysts for selective nitration has also been reported. Greater than 85% yield of o-nitrophenol was observed by using microwaves system technology.