Ion pair formation between tertiary aliphatic amines and perchlorate in the biphasic H2O / CH2Cl2 system

The ability of selected alyphatic amines, namely tripropylamine (TPrA), trisobutylamine (TisoBuA), and tributylamine (TBuA), to form ion pairs with perchlorate anion in biphasic aqueous/dichloromethane mixtures has been demonstrated by GC with flame ionization and mass detectors, and by NMR measurements. Aliphatic amines are not present in the organic phase when the aqueous solution is at acidic pH and no perchlorate is added, whereas they are transferred with high efficiency to the dichloromethane phase when the aqueous solution is added with perchlorate 0.1 M. The extraction efficiency was modeled by theoretical equations, obtained from the mass balances and mass action laws, which were used to fit the experimental GC data. The fitting procedures allowed to determine values for KP (partition constant of the free alyphatic amines), KIP (formation constant of the ion pairs), and KPIP (partition constant of the ion pair), for TPrA, TisoBuA, TBuA at 25 °C. NMR spectra in dichloromethane before and after partition showed the presence of R3NH+ in the organic phase after partition, and no evidences of such species before partition. Ion pairs were shown to form in dichloromethane also when perchlorate anion is replaced by other common monocharged inorganic anions such as nitrate, chloride, chlorate, dihydrogenophosphate, and iodate. The extracted fraction of the ion pair by these ions was however lower than that observed for perchlorate. No ion pairs formed when dichloromethane is replaced by n-hexane, suggesting that aliphatic amine ion pairs can form in polar organic solvents but not in non-polar ones. The ion pair formation affects the distribution of aliphatic amines between organic and aqueous phase, and it must therefore be considered when partition coefficients and solubility data are computed for these compounds in biphasic aqueous/organic mixtures.