Tetraalkylammonium Picrates in the Dichloromethane–Water System: Ion-Pair Formation and Liquid–Liquid Distribution of Free Ions and Ion Pairs

摘要

Equilibria concerning picrates of tetraalkylammonium ions (Me4N+, Et4N+, Pr4N+, Bu4N+, Bu3MeN+) in a dichloromethane?water system have been investigated at 25 °C. The 1:1 ion-pair formation constants (K IP,o o) in dichloromethane at infinite dilution were conductometrically determined. The distribution constants (K D o) of the ion pairs and the free cations between the solvents were determined by a batch-extraction method. The K IP,o o value varies in the cation sequence, Bu4N+ ≈ Pr4N+ ≈ Et4N+ < Bu3MeN+ < < Me4N+; this trend is explained by the electrostatic cation?anion interaction taking into account the structures of the ion pairs determined by density functional theory calculations. For the ion pairs of the symmetric R4N+ cations, there is a linear positive relationship between log10 K D o and the number of methylene groups in the cation (N CH 2). The ion pair of asymmetric Bu3MeN+ has a higher distribution constant than that expected from the above log10 K D o versus N CH 2 relationship. These cation dependencies of log10 K D o for the ion pairs are explained theoretically by using the Hildebrand-Scatchard equation. For all the cations, the log10 K D o value of the free cation increases linearly with N CH 2; the variation of log10 K D o is discussed by decomposing the distribution constant into the Born-type electrostatic contribution and the non-Born one, and attributed to the latter that is governed by the differences in the molar volumes of the cations. The cation dependencies of the ion-pair extractability and ion pairing in water are also discussed.