Solvent Effects on Ion-Pair Distribution and Dimerization of Tetraalkylammonium Salts

Summary.  The distribution of tetraalkylammonium ions (C _n H _2n+1 )₄N⁺ (R ⁺, TAAn ⁺, n = 4–7) with picrate ion (pic ⁻) and inorganic anions X ⁻, (Cl⁻, Br⁻, ClO⁻ ₄), into various inert organic solvents was studied at 25.0°C. The distribution data were analyzed by taking into consideration the distribution of ion pairs, R ⁺ · X ⁻, and the dimerization of the ion pairs, (R ⁺ · X ⁻)₂, in the organic phase. The ion-pair, distribution constant, K _dist, increases with increasing chain length of the tetraalkylammonium ion and with increasing ionic radius of the anion. The values of K _dist show a good correlation with the E _T value of solvent, i.e. the solvation ability with respect to the anion, and smoothly increase with increasing E _T. The effect of the solvent on the dimerization constants, K _dim, is markedly different between the ion pairs of picrate ion and inorganic anions. In the case of picrate, K _dim significantly decreases with decreasing length of the alkyl chain of the tetraalkylammonium ion, but hardly changes by changing the solvent. On the other hand, in the case of ion pairs of inorganic anions the value of K _dim decreases with decreasing E _T and is almost constant for all anions. These results were reasonably explained by the difference of the solvation of the anion moieties of the monomeric and dimeric ion pairs. Received May 15, 2001. Accepted (revised) July 18, 2001     

Ion-Pair Solvent Extraction of EDTA Anions with Tetraalkylammonium Ions in Various Organic Solvents

Summary.  The ion-pair solvent extraction behavior of ethylenediaminetetraacetate (EDTA) anion by various tetraalkylammonium ions was investigated at 25.0 ± 0.1°C. The extraction of EDTA exceeded 90% from the basic aqueous solution into the organic solvents such as n-hexane and benzene derivatives containing tri-n-octylmethylammonium chloride, but EDTA was hardly extracted from acidic solution. Among the chemical species of EDTA in aqueous solution, edta ⁴⁻ is the most extractable one. On the other hand, the extraction of EDTA was less than 1% into chloroform and 1,2-dichloroethane even from the basic aqueous solution. The effect of the structure of alkylammonium ion was also examined. Tetra-n-hexylammonium and tetra-n-octylammonium ions could not extract EDTA even from the basic aqueous solution, while the use of tri-n-octylmethylammonium and di-n-lauryldimethylammonium ions enhances the extraction of EDTA. These results suggest that the steric hindrance in the ion-pair of alkylammonium and EDTA anion in the organic phase affects the extractability of EDTA containing ion-pair. The solution structure of ion-pair in the organic phase was calculated by MMFF force field and the steric effect in the ion-pair was also suggested. From the extraction constants obtained, the possibility of the extraction separation of EDTA has been shown. Present address: Chemistry Department, Heilongjiang University, Harbin, China     

Ion-Pair Solvent Extraction of EDTA Anions with Tetraalkylammonium Ions in Various Organic Solvents

The ion-pair solvent extraction behavior of ethylenediaminetetraacetate (EDTA) anion by various tetraalkylammonium ions was investigated at 25.0 ± 0.1°C. The extraction of EDTA exceeded 90% from the basic aqueous solution into the organic solvents such as n-hexane and benzene derivatives containing tri-n-octylmethylammonium chloride, but EDTA was hardly extracted from acidic solution. Among the chemical species of EDTA in aqueous solution, edta ⁴⁻ is the most extractable one. On the other hand, the extraction of EDTA was less than 1% into chloroform and 1,2-dichloroethane even from the basic aqueous solution. The effect of the structure of alkylammonium ion was also examined. Tetra-n-hexylammonium and tetra-n-octylammonium ions could not extract EDTA even from the basic aqueous solution, while the use of tri-n-octylmethylammonium and di-n-lauryldimethylammonium ions enhances the extraction of EDTA. These results suggest that the steric hindrance in the ion-pair of alkylammonium and EDTA anion in the organic phase affects the extractability of EDTA containing ion-pair. The solution structure of ion-pair in the organic phase was calculated by MMFF force field and the steric effect in the ion-pair was also suggested. From the extraction constants obtained, the possibility of the extraction separation of EDTA has been shown.     

Equilibrium Study on Ion-Pair Formation in Water and Distribution Between Water and m-Xylene of Tetraalkylammonium Picrates

The 1:1 ion-pair formation constants (K _IP) of tetraalkylammonium (Me₄N⁺, Et₄N⁺, Pr₄N⁺, Bu₄N⁺, and Bu₃MeN⁺) picrates in water were determined by capillary electrophoresis at 25°C. The ion-pair extraction constants (K _ex,ip) of the picrates from water to m-xylene were determined by a batch-extraction method at 25°C, and the distribution constants (K _D) of the neutral ion-pairs were calculated from the relationship K _D = K_ex,ip/K _IP. The tetraalkylammonium ion having more methylene groups generally forms a slightly more stable ion-pair with the picrate ion in water, which is attributed to the lower hydration of the cation. For Me₄N⁺, Et₄N⁺, Pr₄N⁺, and Bu₄N⁺, the distribution of the ion pair into m-xylene increases in that order, and a linear relationship was found between log K _D and the number of methylene groups in the cation. This is consistently explained by the regular solution theory. It was also revealed that the ion pairs have a strong specific interaction with water. The ion pair of Bu₃MeN⁺ has a higher distribution constant than that expected from the relationship between log K _D and the number of methylene groups for the symmetrical tetraalkylammonium ions. The cation dependence of the ion pair extractability is mostly governed by that of the distribution of the ion pair.     

Solvent Effect on Rotational Correlation Times of Symmetric Tetraalkylammonium Ions

The overall rotational correlation times of symmetric tetraalkylammonium ions, R ₄N⁺ (R = ethyl, n-propyl, n-butyl, and n-pentyl), in various solvents were determined by the measurements of the ¹³C NMR spin-lattice relaxation times and the nuclear Overhauser enhancement factors of each α-carbon, considering the contribution of the internal rotation around the N—C bond. Except in water, the observed solvent dependencies of the rotational correlation times, τ_r, showed good correlations with those predicted from an electrohydrodynamic (Hubbard–Onsager–Felderhof) model. The correlation times of R ₄N⁺ increased as the size of the alkyl groups became larger. In the case of the n-Bu₄N⁺ and the n-Pen₄N⁺ ion, the τ _r values were similar to or even higher than those predicted by the HOF model under the stick hydrodynamic boundary condition, in spite of the fact that the ions were too small to allow the solvent to be regarded as a hydrodynamic or a dielectric continuum. A comparison of the results with the rotations of other pseudotetrahedral ions, e.g., tetraphenylborate and tetraphenylarsenium ions and with the translation of the R ₄N⁺ ions suggests that a considerable part of the rotational friction for R ₄N⁺ is brought about by pushing aside the solvent in the spaces between the alkyl groups of R ₄N⁺. A significant slowing in the rotation in water was observed for the n-Pr₄N⁺, n-Bu₄N⁺, and n-Pen₄N⁺ions; the extent of this effect increased with increasing size of the alkyl group. The increase in friction was related to the hydrophobic hydration of the R ₄N⁺ ions.     

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

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. An erratum to this article can be found at     

Solvent Effect on the Rate of Dimerization of Cyclopentadiene

The rate of dimerization of cyclopentadiene and some other cycloaddition reactions is determined by electrophilic solvation of the substrate with some contribution of other solvation factors, primarily nonspecific solvation. The corresponding dependence is described by multiparameter equations.     

Chalcogen Bonding Ion-Pair Cryptand Host Discrimination of Potassium Halide Salts

A series of chalcogen, halogen and hydrogen bonding heteroditopic macrobicyclic cryptands are reported and their potassium halide ion-pair recognition properties investigated. Saliently, the co-bound potassium cation was determined to be crucial in switching on the bromide and iodide recognition properties of the respective cryptand receptor. Importantly, the nature of the sigma-hole mediated interaction employed in the anion recognition component is demonstrated to significantly augment the ion-pair binding behaviour, markedly so for the halogen bonding analogue. Most notably the incorporation of a chelating chalcogen bonding donor motif significantly improves the selectivity towards KBr over KI, relative to halogen and hydrogen bonding analogues.     

Activation Strain Analyses of Counterion and Solvent Effects on the Ion-Pair SN2 Reaction of and CH3Cl

We have computationally studied the bimolecular nucleophilic substitution (S_N2) reactions of M_nNH₂⁽ⁿ⁻¹⁾ + CH₃Cl (M⁺ = Li⁺, Na⁺, K⁺, and MgCl⁺; n = 0, 1) in the gas phase and in tetrahydrofuran solution at OLYP/6-31++G(d,p) using polarizable continuum model implicit solvation. We wish to explore and understand the effect of the metal counterion M⁺ and of solvation on the reaction profile and the stereochemical preference, that is, backside (S_N2-b) versus frontside attack (S_N2-f). The results were compared to the corresponding ion-pair S_N2 reactions involving F⁻ and OH⁻ nucleophiles. Our analyses with an extended activation strain model of chemical reactivity uncover and explain various trends in S_N2 reactivity along the nucleophiles F⁻, OH⁻, and , including solvent and counterion effects. © 2019 Wiley Periodicals, Inc.     

Apparent Molar Volumes of Symetric and Asymetric Tetraalkylammonium Salts in Dilute Aqueous Solutions

The apparent molar volumes, V_φ of tetramethylammonium, tetraethylammonium, tetrabutylammonium, butyltriethylammonium, dibutyldiethylammonium, and tributylethylammonium bromides have been measured at 298.15K in the concentration range from 0.01 to 0.04mol⋅kg⁻¹. The concentration dependence of V_φ is given using the Redlich and Meyer relation. The apparent molar volume at infinite dilution, V∘_φ, and the empirical constant, B_V, have been calculated. The CH₂-group contribution has been obtained by the additivity rule. The results were interpreted in terms of solute–solvent interactions.     

The Determination of Methenamine and its Salts in Tablet Dosage Forms Using Ion-Pair Extraction

Abstract

This paper reports a simple, accurate, reproducible, stability-indicating procedure which may be used to quantitatively determine methenamine base, methenamine mandelate or methenamine hippurate in tablet dosage forms. The procedure initially involves the separation of methenamine from the dosage form and from formaldehyde, the decomposition product, by a cartridge ion-pair extraction process. The methenamine may then be assayed by a number of methods. In this paper the methenamine was hydrolyzed to formaldehyde and the formaldehyde determined by the Nash procedure. The method compares favorably with the USP XXI procedure for methenamine tablets. Advantages of the procedure include the ability to use a single method for the base and salt forms of the drug and the potential application to stability and quality control studies of methenamine dosage forms.     

Determination of Hydrophobic Organic Salts by Pseudo-Single-Phase Ion-Pair Titration in an Emulsion Medium

It was shown that hydrophobic organic salts can be determined by pseudo-single-phase ion-par titration in an oil-in-water emulsion stabilized with a nonionic surfactant. Conditions were proposed for determining anionic and cationic surfactants and some hydrophobic salt pharmaceuticals in an emulsion stabilized with Triton X-305 using molybdenum(VI)–Pyrogallol (Bromopyrogallol) Red complexes for the detection of the titration end-point.     

Ion-Pair Chromatography of Metal Complexes of Unithiol in the Presence of Quaternary Phosphonium Salts

The chromatographic behavior of heavy-metal (Pb, Cd, Cu, Hg, and Ni) complexes of unithiol was studied by ion-pair reversed-phase chromatography using new ion-pair reagents: tetrabutylphosphonium and tributylhexadecylphosphonium bromides. The dependence of the retention of metal unithiolates on the nature and concentration of ion-pair reagents, concentrations of an organic solvent and inorganic salts in the mobile phase, and the pH of the mobile phase was studied. It was found that the retention of complexes increases upon increasing the hydrophobicity and concentration of the ion-pair reagent and upon decreasing the concentrations of acetonitrile and inorganic salts (NaClO₄ and NaNO₃). Optimal conditions of the separation of Pb, Cd, Cu, Hg, and Ni unithiolates were selected, and the possibility of their separate determination in a mixture was demonstrated. Concentrations of metals in process water were determined.     

Activation Mechanisms of Trialkylaluminum in Alkali Metal Alkoxides or Tetraalkylammonium Salts / Propylene Oxide Controlled Anionic Polymerization

The anionic polymerization of propylene oxide (PO) initiated by alkali metal alkoxides is in non polar solvents a very slow and non controlled reaction process. Transfer reaction to monomer is predominant, allowing only the preparation of low molar masses PPO. The influence of the addition of trialkylaluminium to either an alkali metal alkoxide or a tetraalkylammonium salt used as initiator for PO polymerization in hydrocarbon media was investigated. A strong enhancement of the polymerization rate accompanied by a drastic decrease of the transfer reactions is observed, allowing the synthesis of PPO with well controlled molar masses. At constant monomer and alkali metal alkoxide concentrations, the polymerization rate increases with increasing trialkylaluminium concentration. Results indicate that the trialkylaluminium derivative is involved in the formation of two distinct complexes, one with the alkali metal alkoxide or the tetraalkylammonium salt and another one with the PO monomer which is strongly activated towards nucleophilic active species. Significant differences between the alkali metal and tetraalkylammonium based initiators are observed. In particular much less trialkylaluminum activator is needed with the ammonium salt to get the same rate of propagation and controlled polymerization.     

Hydrogen‐Bonding Catalysis of Tetraalkylammonium Salts in an Aza‐Diels–Alder Reaction

A piperidine‐derived tetraalkylammonium salt with a non‐coordinating counteranion worked as an effective hydrogen‐bonding catalyst in an aza‐Diels–Alder reaction of imines and a Danishefsky diene. The hydrogen‐bonding interaction between the ammonium salt and an imine was observed as part of a ¹H NMR titration study.     

Solvent Modification in Ion-Pair Extraction: Effect on Sodium Nitrate Transport in Nitrobenzene Using a Crown Ether

A comparative quantitative analysis of the effect of solventmodifiers on the ion-pair extraction of an inorganic salt by a crown ether was conducted with the aim of advancing the understanding of transport of highly hydrophilic metal ions from aqueous salt solutions. Two classes of solvent modifiers that possess electron-pair donor (EPD) or hydrogen-bond donor (HBD) groups were investigated. The equilibrium constants corresponding to the extraction of sodium nitrate into nitrobenzene (NB) employing model neutral host cis-syn-cis-dicyclohexano-18-crown-6 (compound 1) with and without solvent modifier were determined using the SXLSQI computer model. For a series of EPD modifiers—including tri-n-butyl- and tri-phenylphosphate, tri-n-butyl- and tri-phenylphosphine oxide, N,N-di-n-butyl- and N,N-di-phenylacetamide—the enhancement of the NaNO₃ extraction by compound 1 was found to be dependent on the hydrogen-bond acceptance ability of the modifier as quantified by the β solvatochromic parameter. A HBD modifier 3,5-di-t-butylphenol (compound 8), which forms strong hydrogen bonds with nitrate anion in NB, exhibited even greater enhancement of the NaNO₃ extraction by compound 1. The determined extraction constants were correlated with the β- or α-solvatochromic parameters of the solvent modifiers and linear trends were observed. Hydrogen bond interaction between compound 8 and nitrate anion in the presence of the sodium-loaded crown ether in the extraction phaseswas studied by vibrational spectroscopy.     

Structure of Solutions of Polyamido Acid Salts in a Mixed Solvent

The changes in the structural organization of solutions of N,N-dimethylhexadecylammonium salt of the polyamido acid based on 3,3’,4,4’-diphenyltetracarboxylic dianhydride and o-tolidine in a dimethylacetamide-dioxane mixed solvent during 24-h keeping at room temperature were studied by polarized light scattering. The stability of the salt solutions was studied as influenced by the composition of the mixed solvent, chemical nature of its components, and molecular weight of the polymer.__________Translated from Zhurnal Prikladnoi Khimii, Vol. 78, No. 3, 2005, pp. 474–478.Original Russian Text Copyright © 2005 by Kalinina, Silinskaya, Sklizkova, Kozhurnikova, Filippov, Kudryavtsev.