Bathochromic Shift in Absorption Spectra of Ion-Pair Complexes of Alkali Picrates with 18-Crown-6 Derivatives Extracted into Low-Polar Organic Solvents

Abstract

Ion-pair extractions of alkali picrates (MPi) by three crown compounds (benzo-15-crown-5 (B15C5), 18-crown-6 (18C6) and benzo-18-crown-6 (B18C6)) and acyclic polyether (1, 2-bis[2-(2-methoxyethoxy)ethoxy]benzene (ACB18C6)) were performed in H2O/CHCl3, C6H6/H2O and H2O/CCl4 systems at 25°C. The absorption maxima (λmax) of the 1:1ion-pair complexes of MPi with the crown compounds were measured in water-saturated organic phases at 25°C. The magnitude λmax value increased with an increase in the crystal ionic radii of the metals trapped in the crown ether cavities (Li_??_Na<K≤Rb_??_Cs), the ring sizes of the crown compounds, (B15C5<B18C6 for Na complexes), and solvent polarity (CCl4<C6H6<CHCl3). The λmax-differences among B18C6, 18C6 and ACB18C6 were not observed in CHCl3. The apparent overall extraction constant, [M(B18C6)+Pi-]o/[M+][B18C6]o[Pi-], for K+ was larger than that of Rb+. The sequence of selectivity of B18C6 for K+ was CHCl3<C6H6<CCl4. This result corresponds to the strength of the interaction between K(B18C6)+ and Pi- in a given solvent system.