Study of Tetraethylammonium and Lithium Chlorides Dissociation in Acetonitrile Solutions
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y of Tetraethylammonium and Lithium Chlorides Dissociation in Acetonitrile Solutions A. A. Vidiakinaa, N. A. Bogacheva, M. Yu. Skripkina, and A. S. Mereshchenkoa,* a St.
Petersburg State University, St. Petersburg, 199034 Russia *e-mail: [email protected]
Received May 27, 2020; revised June 11, 2020; accepted June 15, 2020
Abstract—Solutions of tetraethylammonium and lithium chlorides in acetonitrile were studied by electronic absorption spectroscopy. It was shown that at concentrations up to 100 mmol/L tetraethylammonium chloride completely dissociates. The association constant of lithium chloride in acetonitrile (Kass = 4800 mol–1 L) was determined by the spectrophotometric method. The electronic absorption spectra of the Li+Cl– ionic pair and Cl– anion in acetonitrile were studied. Keywords: tetraethylammonium chloride, lithium chloride, association constant
DOI: 10.1134/S1070363220090170 one chemical form. Therefore, the salt either completely dissociates, or it is entirely in an associated state. To confirm the hypothesis of complete Et4NCl dissociation, we examined the absorption spectra of Et 4NClO 4 and LiCl salts in acetonitrile. The optical density of a 200 mM solution of tetraethylammonium perchlorate in acetonitrile is less than 0.05 in the wavelength range of 190–300 nm in a 1 cm thick cell. Hence the NEt4+ cation does not absorb in this wavelength range (190–300 nm) and the band with a maximum at 197 nm corresponds to the absorption of either the chloride ion, if Et4NCl completely dissociates, or the Et4N+Cl‒ ionic pair, if
1703
D/I, cm–1
Lithium and tetraalkylammonium cations are objects that fundamentally differ in their behavior in solutions. Lithium cations are characterized by rather high ionic solvation energies in polar media. Tetraalkylammonium cations, having a relatively small size of the hydrocarbon substituent, interact extremely weakly with most of the commonly used solvents. Studies of the forms of existence of salts with these ions in solutions are mainly related to aqueous systems [1, 2]. Data for non-aqueous systems are sketchy. However, non-aqueous solutions of lithium halides are used as an electrolyte in lithium-ion batteries [3], and tetraalkylammonium salts have proven to be effective interfacial catalysts for the activation of anionic nucleophiles [4] (reactions are carried out in aprotic organic solvents). We have studied the ionic association of lithium and tetraethylammonium chlorides in acetonitrile by electron absorption spectroscopy. To study the association of Et4N+ and Cl‒ ions, the electronic absorption spectra of Et4NCl solutions of various concentrations (0.1–5 mmol/L) were measured in the wavelength range from 190 to 230 nm (Fig. 1). In all the spectra, there is only one absorption band with a maximum at λ = 197 nm. To estimate the dissociation degree of tetraethylammonium chloride in acetonitrile, the dependence of the optical density at the edge of the band (210 nm) on the salt concentration was plotted. In the concentration range 0.1–100 mmol/L, the Boug
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