Discussion on complexation reactions between dicyclohexyl-18-crown-6 (DCH18C6) with Na+, K+, Cs+, Rb+, and Tl+ metal cations in acetonitrile-water binary mixtures |
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Authors: | M H Soorgi G H Rounaghi and M S Kazemi |
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Institution: | (1) Department of Chemistry, Faculty of Sciences, Ferdowsi University of Mashhad, Mashhad, Iran |
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Abstract: | The complex formation between Na+, K+, Cs+, Rb+ and Tl+ metal cations with macrocyclic ligand, dicyclohexyl-18-crown-6 (DCH18C6) was studied in acetonitrile-water (AN-H2O) binary systems at different temperatures using conductometric method. DCH18C6 forms 1:1 complexes with these metal cations.
The stability constants of the complexes were obtained from fitting of molar conductivity curves using a computer program
Genplot. The results show that the selectivity order of DCH18C6 for the metal cations in acetonitrile-water mixtures (AN =
25.3 and 50.4 mol %) is: Tl+ > K+ > Rb+ > Cs+ > Na+. A non-linear behaviour was observed between the log K
f of the complexes versus the composition of the mixed solvent which it related to changes of acidity, basicity, polarity and
also polarizability of AN-H2O mixtures with the composition of this binary solution. The values of standard enthalpy changes (ΔH
s0) for complexation reactions were obtained from the slope of the van’t Hoff plots and the changes in the standard entropy
(ΔS
s0) were calculated from the relationship: ΔG
s,298.150 = ΔH
s0 − 298.15ΔS
s0. The obtained results show that in most cases, the complexes are enthalpy stabilized but entropy destabilized.
Original Russian Text ? M.H. Soorgi, G.H. Rounaghi, M.S. Kazemi, 2008, published in Zhurnal Obshchei Khimii, 2008, vol. 78,
No. 10, pp. 1627–1632. |
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