Influence Of Crown Ethers Research Articles

CONDUCTOMETRIC STUDY OF LIGAND STRUCTURE INFLUENCE ON THE Pb(II) COMPLEXATION WITH CROWN ETHERS

The conductometric study of ligand structure influence on the Pb(II) complexation with crown ethers in different solvents has been investigated. In this paper, the complexation reaction of macrocyclic ligand, 18-crown-6 (18C6), dibenzo-18-crown-6 (DB18C6), and Pb(II) cation was studied in different solvents: dichloromethane (DCM) and 1,2- dichloroethane (1,2-DCE). The effects of surfactant structure (Triton X-100 and Triton X-45) on the conductivity of the Pb(II) complex with 18-crown-6 and dibenzo-18-crown-6 ether have been investigated. The conductance data showed that the stoichiometry of the complexes in most cases is 1:1(ML). It is also demonstrated that the influence of crown ethers is deeply affected by the organic solvent used. In the solvents studied, the stability of the resulting complexes showed higher stability in dichloromethane comparing with 1,2- dichloroethane. Macrocyclic ligand 18-crown-6 showed more suitable for complexation of Pb(II) ions compared to dibenzo-18-crown-6. Adding a surfactant affected the higher absolute values of the conductivity of systems, but not the change in the stoichiometric ratio between a metal ion and macrocyclic ligand.

Multiple Enhancement Effects of Crown Ether in Tröger's Base Polymers on the Performance of Anion Exchange Membranes.

The development of anion exchange membranes (AEMs) is hindered by the trade-off of ionic conductivity, alkaline stability, and mechanical properties. Tröger's base polymers (Tb-polymers) are recognized as promising membrane materials to overcome these obstacles. Herein, the AEMs made from Tb-poly(crown ether)s (Tb-PCEs) show good comprehensive performance. The influence of crown ether on the conductivity and alkaline stability of AEMs has been investigated in detail. The formation of hydronium ion-crown ether complexes and an obvious microphase-separated structure formed by the existence of crown ether can enhance the conductivity of the AEMs. The maximum OH- conductivity of 141.5 mS cm-1 is achieved from the Tb-PCEs based AEM (Tb-PCE-1) at 80 °C in ultrapure water. The ion-dipole interaction of the Na+ with crown ether can protect the quaternary ammonium from the attack of OH- to improve the alkaline stability of AEMs. After 675 h of alkaline treatment, the OH- conductivity of Tb-PCE-1 decreases by only 6%. The Tb-PCE-1-based single cell shows a peak power density of 0.202 W cm-2 at 80 °C. The prominent physicochemical properties are attributed to the well-developed microstructure of the Tb-PCEs, as revealed by TEM, AFM, and SAXS observations.

The experimental study on the influence of crown ethers and glycols on the mutual solubility of lithium bromide in water

In this paper, the continuation of our work on searching for anti-crystallization additives to the aqueous solution of lithium bromide is presented. This type of research is important from the viewpoint of absorption refrigeration technology to improve the performance of the efficiency of refrigeration equipment. In this study, three crown ethers: 12-crown-4, 15-crown-5, and 18-crown-6 as well as the glycols: ethylene glycol, diethylene glycol, triethylene glycol and glycerol were investigated as anti-crystallization additives for {LiBr (1) + water (2)} system, conventionally used as a working pair in absorption refrigeration technology. For this purpose, the solubility of lithium bromide in water has been determined in the presence of the organic additive. The solubility measurements have been carried out using a dynamic method at a wide temperature and composition range for different (additive to LiBr) initial mass fraction from w20 = 0.1, 0.2 and 0.3. From experimental SLE data, the comparison range of the liquid state for tested systems at the absorber's working temperature was determined and compared to those from conventional (LiBr + water) system. Further measurements of vapor – liquid equilibria will be performed to select the best anti-crystallization additive.

The Influence of Crown Ether and Alcohol on Unsaturation and Molar Mass of Poly(propylene oxide)s Prepared by Use of Potassium t-Butoxide: Reinvestigation of Chain Transfer Reactions.

Potassium t-butoxide dissolved in tetrahydrofuran effectively initiates homogeneous polymerization of propylene oxide at room temperature. Unsaturation and molar mass (M n) of the polymers prepared depend on the presence of additives, such as macrocyclic ligand 18-crown-6 (L) and t-butanol. Application of the ligand alone results in distinct increase of unsaturation and decrease of M n, whereas use of t-BuOH leads to simultaneous decrease of unsaturation and M n. Activation of t-BuOK/t-BuOH system with the ligand causes further decrease of unsaturation, that is, from 12.0 to 3.5 mol % for OK/OH (1/3) and OK/OH/L (1/3/2) systems, respectively. Unexpectedly, M n of the polymers obtained does not practically change (~4800). This result differs from that reported earlier for neat PO polymerization initiated potassium 1-methoxy-2-propoxide/1-methoxy-2-propanol, in which in the presence of the same ligand M n increases to ~12 400 for the same ratio of reagents. The mechanism of studied processes was discussed.

Study on the influence of crown ether on arsenic behavior during coal pyrolysis

The influence of crown ether on behaviors of arsenic at different temperatures and residence time was investigated during the pyrolysis of Tuanbo (TB) coal. The modes of occurrence of arsenic were determined by sequential chemical extraction, density fractionation and demineralization. The results indicated that at the same temperature and residence time, the arsenic removal adding dibenzo-18-crown-6 was higher than that adding 18-crown-6, and were all higher than that of TB coal during pyrolysis. When temperature was 850 °C and residence time was 30 min, the arsenic removal of TB coal was 30.63%; at the same condition, the arsenic removal while adding 18-crown-6 was 33.21%, higher than that of TB coal; and the arsenic removal while adding dibenzo-18-crown-6 was 67.41%, significantly higher than that of TB coal. From the results, we can see that adding crown ether can improve the arsenic removal during coal pyrolysis, and especially be conducive to the arsenic which is mainly associated with sulfates & monosulfides and that in stable forms.

Ion-selective electrodes for determination of organic ammonium ions: Ways for selectivity control

The influence of the ISE membrane composition on the selectivity for primary, secondary, tertiary, and quaternary alkylammonium cations, as well as for cations of physiologically active amines, has been investigated. Factors studied include the effect of plasticizer (2-nitrophenyl octyl ether, o-NPOE; dibutyl phthalate, DBP; dinonyl adipate, DNA; tris(2-ethylhexyl) phosphate, TEHP) and ion exchanger (potassium tetrakis(4-chlorophenyl)borate, K(TpClPB); potassium tris(nonyloxy)benzenesulfonate, K(TNOBS)), as well as that of the lipophilic cationic additive (tetradecylammonium nitrate, (TDA)NO 3) and neutral carrier (dibenzo-18-crown-6) presence in membrane. It has been established that plasticizer nature affects K i , j pot values both when the target and/or foreign ions have non-ionic polar groups capable of specific interaction with plasticizer, and when the only difference consists in the substitution degree of their ionic groups. K i , j pot values for quaternary alkylammonium cations over primary–tertiary ones change in the following order: TEHP > DBP ≈ DNA > o-NPOE. The highest K i , j pot value change is achieved for the primary–quaternary alkylammonium cation pair, amounting to 3 and 4.7 orders for membranes containing K(TNOBS) and K(TpClPB) as ion exchangers, respectively. In its turn, the ion exchanger influence on the selectivity depends on plasticizer nature, it being maximal for o-NPOE (about 2 orders) and practically non-existent for TEHP. On the whole, as compared to K(TpClPB)-based ISEs, those based on K(TNOBS) show higher selectivity for primary–tertiary alkylammonium cations over quaternary ones. Incorporation of (TDA)NO 3 into membrane causes further improvement of selectivity for primary–tertiary alkylammonium cations in the case of K(TNOBS) only. The maximal total effect of the ion exchanger and lipophilic ionic additive is observed for ISEs with DNA-plasticized membranes and is over 3 orders. The influence of crown ether on the selectivity also depends significantly upon ion exchanger and plasticizer nature. For ISEs with o-NPOE-plasticized membranes the K i , j pot value changes can be as great as 3 (ion exchanger K(TNOBS)) and even 4.5 (ion exchanger K(TpClPB)) orders. On the contrary, for ISEs with TEHP-plasticized membranes the crown ether effect on the selectivity is unessential. The results obtained are explained by peculiarities of organic ammonium cations solvating by plasticizer and association of cations with ion exchangers.

Influence of Crown Ethers on the Electric Percolation of AOT/Isooctane/Water (w/o) Microemulsions

A study was carried out on the influence of different crown ethers on the electric percolation of AOT/isooctane/water microemulsions. The crown ethers used were chosen on the basis of two fundamental criteria: (a) the different sizes of the molecules, where variation is found in the external size as well as the size of the cavity, and (b) the different solubilities of the ethers in water. In all cases we observed a dual behavior of the crown ethers with regard to the percolative phenomenon. At low additive concentrations we observed how the presence of the crown ethers caused an increase in the percolation temperature of the microemulsions, whereas at high additive concentrations there was a reduction in the percolation temperature causing the percolation threshold of the system to move forward. This dual behavior allowed us to define the compensation concentration, which corresponds with the crown ether concentration at which there is no effect on the percolative phenomenon. We observed a correlation between the effect exerted by the crown ethers and the size of the cavity. This shows the importance of the capacity to complexate Na+ and solubilize it in the interface and the continuous medium on the electric percolation. We also observed a correlation between the effect of the crown ethers on the percolation temperature and their external size. This shows the importance of their inclusion in the interface on the percolative phenomenon. Such an inclusion modifies the properties of the AOT film, facilitating the exchange of matter between droplets. We also obtained a satisfactory multiparametric correlation between the logarithm of the compensation concentration, the logarithm of the distribution parameter of the crown ether between water and 1-octanol, and the number of oxygen atoms in the crown ether. This correlation shows that the effect of the crown ethers on the electric percolation is due to its size and capacity to sequester ions, as well as to its solubility in the interface of the microemulsion.

A study of the effect of pH, solvent system, cone potential and the addition of crown ethers on the formation of the monensin protonated parent ion in electrospray mass spectrometry

A systematic investigation has been performed on the effect of pH, cone potential, solvent system and the influence of crown ethers on the relative intensity of the monensin protonated parent ion [M+H]+ in electrospray mass spectrometry (ESI-MS). The analyses were performed on either a triple quadrupole (Quattro-LC) or a quadrupole time-of-flight (Q-tof) mass spectrometer. The pH variation showed that it is possible to detect the protonated ion at low pH. An increase in the cone potential resulted in an increase of the observed signal and the best signal-to-noise was observed at 120 V. The addition of 15-crown-5 and 18-crown-6 ethers to the solution led to an increase of the protonated ion intensity through successful competition of the crown ether for the alkali metal cation. The 15-crown-5 ether, though, showed a distinct disadvantage over the 18-crown-6 ether due to retention of its sodium adduct in the source and detector of the instrument. Acetonitrile–water proved to be the best solvent system for these studies. The best conditions for the 18-crown-6 ether were then applied to an analysis on a Fourier-transform ion cyclotron resonance (BioApex II) mass spectrometer which clearly demonstrated the production of the protonated ion.

Optimization of alkali-alkalkine earth cation separation on weak cation-exchangers

The elution behavior of alkali and earth alkaline cations has been studied on a newly synthesized weak cationexchanger (WCX). Applying this stationary phase isocratic separation of these cations is possible. The dependence of retention on competing ion concentration is described via a simple model. The influence of crown ethers as well as organic eluent additives on selectivity and peak efficiency is demonstrated. With optimized separation such cations can be determined with detection limits of about 0.5 ppm using a conductivity detector without ion suppression.

Influence of crown ethers on thermal decarboxylation of salts of perfluorocarboxylic acids

Thermal decarboxylation of potassium salts of perfluorocarboxylic acids in the presence of 20 mole % of crown ethers takes place in 1 h at 180°C and gives internal perfluoroolefins in yields of up to 95%. The predominance in product mixtures of isomers whose double bond is adjacent to the trifluoromethyl group is associated with kinetic control of the reaction. The kinetic parameters of the process have been found and the relative activities of a series of crown ethers have been established. A scheme is advanced to show their influence on the reaction studied.

The Influence of Crown Ethers and Cryptands on the Kinetics and Mechanism of Elimination Reaction of HCl from 1,1,1 -Trichloro-2,2-Diphenylethane by iso-PrOK in Isopropanol Solvent

The Influence of Crown Ethers and Cryptands on the Kinetics and Mechanism of Elimination Reaction of HCl from 1,1,1 -Trichloro-2,2-Diphenylethane by iso-PrOK in Isopropanol Solvent

The influence of crown ethers on cation migration processes. Part 7.1 The 2‐pivaloyl‐1,4‐benzoquinone radical anion

AbstractThe 2‐pivaloyl‐1,4‐benzoquinone radical anion has been prepared by alkali metal reduction in tetrahydrofuran in the presence of dibenzo‐18‐crown‐6, 15‐crown‐5, or 12‐crown‐4, and its ESR spectrum in each system has been recorded over a range of temperatures. Line‐width variation in the ESR spectrum is observed in some systems. This line‐width variation has been attributed to cation migration between sites adjacent to the non‐equivalent carbonyl groups rather than to restricted rotation of the pivaloyl group. In some systems, however, the rate of migration is too fast and in others too slow, compared with the time‐scale of the ESR experiment, for line‐width variation to be observed. The influence of each crown ether upon the rate of cation migration has been found to depend upon the size of the crown ether ‘cavity’ compared with the size of the alkali metal counter‐ion.

The influence of crown ethers on cation migration processes. Part 6.—The 1,4-naphthoquinone radical anion

The radical anion of 1,4-naphthoquinone has been prepared by alkali-metal reduction, employing Li, Na, K and Cs, in 1,2-dimethoxyethane in the presence of 18-crown-6, 15-crown-5 and 12-crown-4. Linewidth alternation is observed in the e.s.r. spectra of some of these systems owing to intramolecular cation migration between equivalent sites adjacent to the two carbonyl groups. In some systems a species is observed with a much narrower spectrum than that observed in the absence of a crown ether. Remarkably, it is apparent that the crown ethers do not appear to show great specificity towards the counterions which match the size of their cavities.

The influence of crown ethers on cation migration processes. Part 5. The acenaphthene radical anion

The radical anion of acenaphthene has been prepared in tetrahydrofuran and in tetrahydrofuran in the presence of dibenzo-18-crown-6, 18-crown-6, or [2.2.2]cryptand as cation complexing agent, with either Li+, Na+, or K+ as counterion. Linewidth alternation has been observed in the K+/(acenaphthene)–˙/tetrahydrofuran system as a result of cation migration from one side of the molecular plane to the other. The activation energy for this process has been determined (6.6 kJ mol–1) by a procedure which does not require a knowledge of the methylene proton splitting constants in the ‘slow’ exchange limit. Linewidth alternation is not observed in this system upon addition of any one of the foregoing cation complexing agents. However, equilibria between contact (or ‘tight’) and solvent-separated (or ‘loose’) ion pairs have been observed in the Na+/(acenaphthene)–˙/tetrahydrofuran and K+/(acenaphthene)–˙/18-crown-6/tetrahydrofuran systems (ΔH° 10.2 and 14.1 kJ mol–1, respectively). Values of da/dT, determined for each system, appear to be significantly influenced by the counterion. In particular da(4-H)/dT is positive in pure tetrahydrofuran but negative in the presence of the cation complexing agents.

Reagents and synthetic methods 41: oxidations with chromium trioxide under the influence of crown ethers

M. JUARISTI, J. M. AIZPURUA. B. LECEA, and C. PALOMO. Can. J. Chem. 62, 2941 (1984). An unusual new application of crown ethers in organic synthesis is described. Chromium trioxide, in the presence of a crown ether as catalyst, in dichloromethane as solvent, was found an efficient oxidizing system for the chemoselectivc oxidation of mercaptans to disulfidcs. It is also highly efficient for the oxidation of hydroquinoncs to thc corresponding bcnzoquinoncs in almost quantitative yield. A high conversion of polynuclear hydrocarbons into quinones in acetic acid media is also briefly described.

The influence of crown ethers on cation migration processes. Part 4. Some simple benzoquinone radical anions in the presence of dibenzo-18-crown-6

The radical anions of 2,5-di-t-butyl- and 2,5-dimethyl-benzoquinone have been prepared in 1,2-dimethoxyethane with Li+, Na+, or K+ as the counterion in the presence of dibenzo-18-crown-6. Linewidth alternation is observed in the e.s.r. spectra of these radical anions, with the lifetime of the counterion at one of the preferred sites near the radical anion varying in the sequence Li+ > Na+ > K+ at a particular temperature. It is apparent that in the presence of the crown ether ‘contact’ ion pairs are still present in the majority of the systems studied. The relatively minor influence of the crown ether on these systems is probably a result of the similarity in cation solvation environment presented by 1,2-dimethoxyethane and the crown ether.

The influence of crown ethers on cation migration processes. Part 3. The 5,12-dihydrotetracene radical anion with lithium or potassium as counterion

The addition of dibenzo-18-crown-6 to a solution of 5,12-dihydrotetracene in tetrahydrofuran has been found to reduce dramatically the rate of cation migration when Li+ is the counterion. The resulting e.s.r. spectra, which exhibit linewidth alternation, have been computer-simulated employing modified Bloch equations. The activation energy for this process has been found to be 13.5 kJ mol–1. Linewidth alternation is also present when K+ is the counterion, and is accompanied by a significant change in the various splitting constants as the ‘fast exchange’ region is approached.

The influence of crown ethers on the kinetics of E1cB and E2 β-elimination reactions of di(4-nitrophenyl)fluoroethanes with sodium methoxide in methanol

The addition of crown ethers 1,4,7,10,13-pentaoxacyclopentadecane (15C5) and 1,4,7,10,13,16-hexaoxacyclooctadecane (18C6) in quantities equimolar to the base, to β-elimination reactions of 1,1,1-trifluoro-2,2-di(4-nitrophenyl)ethane and 1-fluoro-2,2-di(4-nitrophenyl)ethane promoted by sodium methoxide in methanol, has been investigated. In the E2 reaction of the monofluoro compound, the crown ethers caused no change in the kinetic order and only small changes in the second-order rate constants and activation parameters. The primary deuterium isotope effect was also unaltered by the presence of crown ethers.For the (E1cB)R reaction of the trifluoro compound, no change in kinetic order was found, but slightly larger rate constant changes and an increase in the isotope effect from kH/kD = 1.0 to 1.25 at 25 °C was observed. This is interpreted as an alteration in mechanism from (E1cB)R towards (E1cB)I.

The influence of crown ethers on the rate of intramolecular cation exchange involving the 5,12-dihydrotetracene radical anion

The addition of dibenzo-18-crown-6 to the Na+–5,12-dihydrotetracene radical-anion ion pair has been found dramatically to reduce the rate of cation migration. The resulting e.s.r. spectra (which exhibit linewidth alternation) have been computer-simulated employing modified Bloch equations. The activation energy for the cation migration process has been found to be 19.3 kJ mol–1.

The influence of crown ethers on the activation parameters of the β-elimination reactions of some p-chlorophenylethanes with alkoxide bases

The addition of crown ethers 1,4,7,10,13-pentaoxacyclopentadecane and 1,4,7,10,13,16-hexaoxacyclo-octadecane in quantities equimolar to that of the base, to the dehydrochlorination reactions of the p-chlorophenylethanes, 2,2-bis-(4-chlorophenyl)-1-chloro-, -1,1-dichloro-, and 1,1,1-trichloro-ethane, promoted by sodium ethoxide and sodium t-butoxide, has been investigated. The crown ethers cause no change in the kinetic order of the reaction, but the second-order rate constants and activation parameters are significantly changed to a greater extent than can be accounted for merely by the assumption that the crown ethers bring about complete dissociation of the alkoxide ion-pairs. The nature of the changes are such that different transition state structures within the E2 mechanism must exist in the presence of crown ethers, in addition to different initial state solvation conditions.