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

Abstract

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.