Ionic Liquid Induced p-Nitrophenol prototropism

Abstract

The manuscript “Ionic Liquid Induced p-Nitrophenol Prototropism” deals with the acid-base properties of p-Nitrophenol molecule in the presence of two different Ionic Liquids (ILs). Prototropic equilibria of p-Nitrophenol (PNP) have been studied in water, ethanol, carbon tetrachloride, chloroform, cyclohexane and IL added aqueous solution with the help of Uv-vis absorbance spectroscopy. It was found that PNP mainly exists in neutral form when dissolved in ethanol, carbon tetrachloride, chloroform and cyclohexane. Interestingly, ground state ionization of PNP takes place in water and results into the formation of p-Nitrophenolate anion as a result of proton transfer to water and hence two absorbing species have been detected in the absorbance spectra. Inter-conversion of neutral to anionic and vice versa can be achieved with the addition of small amount of NaOH/HCl. Furthermore, in water/ethanol binary mixture both neutral and anionic forms exist. Interactions of surfactants sodium dodecyl sulfate (SDS) and cetrimonium bromide (CTAB) with neutral and ionic forms of PNP is also explored. To assess the potential role of ILs as a solubilizing media that facilitates proton-transfer reactions, p-Nitrophenol prototropism is investigated in water in the presence of small amount of added ILs. Interestingly, addition of small amount of hydrophilic IL, 1-butyl-3-methylimidazolium tetrafluoroborate [bmim][BF₄] or hydrophobic IL, 1-butyl-3-methylimidazolium hexafluorophosphate [bmim][PF₆] into PNP dissolved in high pH water causes protonation of the already present p-Nitrophenolate anion. Protonation of p-Nitrophenolate is attributed due to presence of acidic C2-H protons in [bmim]⁺ cations associated with both ILs. Furthermore, the extent of protonation induced by addition of [BF₄]⁻ based IL is found to be much pronounced as compared to [PF₆]⁻ based IL having same cation due to difference in hydrolytic properties of [BF₄]⁻ based ILs when compared with [PF₆]⁻ based ILs. These findings could contribute to the technological applications for detection, removal and sensing of environmentally hazardous p-Nitrophenol and its ionic forms from water. The results presented in this work lead to a better understanding of prototropic behavior of PNP in IL based media as well as different organic solvents based media.