Crown ethers determination

In specific applications to phase transfer catalysis, Knbchel and his coworkers compared crown ethers, aminopolyethers, cryptands, octopus molecules ( krakenmole-kiile , see below) and open-chained polyether compounds. They determined yields per unit time for reactions such as that between potassium acetate and benzyl chloride in acetonitrile solution. As expected, the open-chained polyethers were inferior to their cyclic counterparts, although a surprising finding was that certain aminopolyethers were superior to the corresponding crowns. 

Lewis et al. [52,53] have also determined labile chlorines in PVC by a crown ether catalyzed acetoxylation of PVC and the thermal degradation characteristics of the modified polymer. The values were comparable with those obtained by the phenolysis method. 

More accurate information on k3 is obtainable if the equilibrium constant K is also determined at various crown ether concentrations, as shown by Nakazumi et al. (1981, 1983). The results with benzenediazonium tetrafluoroborate and 3- and 4-substituted derivatives demonstrate that k3 is not unmeasurably small, but that ky-values are generally 1-2% of k2 for complexation with 18-crown-6, 0.1-0.5% of k2 with 21-crown-7, and 2-10% of k2 with dicyclohexano-24-crown-8. A dual substituent parameter (DSP) analysis of A 3-values (Nakazumi et al., 1987) showed that the dediazoniation mechanism of the complexed diazonium ions does not differ appreciably from that of the free diazonium ions. 

In the context of Scheme 11-1 we are also interested to know whether the variation of K observed with 18-, 21-, and 24-membered crown ethers is due to changes in the complexation rate (k ), the decomplexation rate (k- ), or both. Krane and Skjetne (1980) carried out dynamic 13C NMR studies of complexes of the 4-toluenediazo-nium ion with 18-crown-6, 21-crown-7, and 24-crown-8 in dichlorofluoromethane. They determined the decomplexation rate (k- ) and the free energy of activation for decomplexation (AG i). From the values of k i obtained by Krane and Skjetne and the equilibrium constants K of Nakazumi et al. (1983), k can be calculated. The results show that the complexation rate (kx) does not change much with the size of the macrocycle, that it is most likely diffusion-controlled, and that the large equilibrium constant K of 21-crown-7 is due to the decomplexation rate constant k i being lower than those for the 18- and 24-membered crown ethers. Izatt et al. (1991) published a comprehensive review of K, k, and k data for crown ethers and related hosts with metal cations, ammonium ions, diazonium ions, and related guest compounds. 

As stated in Section II, the SHG responses to primary cations of ISEs based on several crown ether ionophores could be correlated to the number of primary ion complexes at the phase boundary, which contributed to the membrane potential. We have now incorporated the K+ ionophore bis[(benzo-15-crown-5)-4-methyl]pimelate into 1,2-dichloroethane and nitrobenzene membranes and determined EMF and SHG responses to KCl in presence and absence of ionic sites. 

Two new chromogenic crown ether derivatives based on an identical design principle have been synthesized. The lipophilic KBC-002 shown in Figure 23 is a useful new chromo- ionophor for the highly selective calcium determination with cation exchange type optode53. 

Rate constants for reaction of Ca2+aq with macrocycles and with cryptands (281,282,291) reflect the need for conformational changes, considerably more difficult for cryptands than for crown ethers, which may be considerably slower than formation of the first Ca2+-ligand bond. Ca2+aq reacts with crown ethers such as 18-crown-6 with rate constants of the order of 5 x 107M 1 s, with diaza crown ethers more slowly (286,326). The more demanding cryptands complex Ca2+ more slowly than crown ethers (kfslow reaction for cryptands with benzene rings fused to the macrocycle. The dominance of kA over kt in determining stability constants is well illustrated by the cryptates included in Table X. Whereas for formation of the [2,1,1], [2,2,1], and [2,2,2] cryptates kf values increase in order smoothly and gently, the k( sequence Ca[2,l,l]2+ Ca[2,2,l]2+ Ca[2,2,2]2+ determines the very marked preference of Ca2+ for the cryptand [2,2,1] (290). 

Furthermore, the structures of [Pb(18-crown-6)(CH3CN)3][SbCl6]2 and [Pb(15-crown-5)2][SbCl6]2 were determined. The structure determination of [Pb(18-crown-6)(CH3CN)3][SbCl6]2 is restricted by disorder problems. Both compounds were formed by the reaction of PbCl2, antimony pentachloride and the corresponding crown ether. The... 

Another type of biphase partition comprises equilibration of a crown ether-containing solution with a sparingly soluble solid salt. Assuming that the amount of free salt [M+.X in (1)] at equilibrium equals the solubility—which can be determined separately—one can calculate the association constant ATlp from the amount of solubilized salt. Reinhoudt et al. (1977) applied this technique using Zeise s salts. 

The association constant Ki for several 1 1 complexes of crown ethers [ 1 ]—[23] have been determined in water (Table 3). Compounds having an... 

For monocyclic crown ethers the data presented in Table 4 and the stability constants for glymes [43]—[46] determined by Chaput et al. (1975) can be combined to calculate the macrocyclic effect (Table 7). The data indicate that the gain in binding energy on ring closure shows the same pattern as the ion selectivity of the crown ether, being highest for Na+/15-crown-5, K+/18-... 

The cation affinity of aza-crown ethers depends on the type of substituent attached to the nitrogen. Wester and Vogtle (1978) have determined the cation binding constants for substituted [2.2]-cryptands 138] and [89]—191 ] in... 

Rates of decomplexation (kJ2) of cation complexes can also be determined by nmr spectroscopy on the cation. Rates of complex formation are then calculated from kn and the binding constant. The results for several ligands, cations, and solvents are given in Table 20. Despite the wide variations, the rates of complex formation are all in the range 2 x 107 to 8 x 10 M 1 s 1. In contrast, rates of decomplexation for crown-ether complexes span a much broader range 6.1 x 102 to 2 x 105 s 1. Comparison of crown-ether data with data for [2.2.2]-cryptand [37] and the linear polyether [92] also shows that the... 

The relative order of the catalytic activities of the crown ethers ([20] + [21]) > [9] > [ 11 ] > [ 13] > [8]) is the same as the relative order of their capacities to bind K+ (Table 4). However, the intrinsic reactivities of the ion pairs were also dependent on crown-ether structure, as was shown by experiments in which the alkylation rates were determined at various crown/phenoxide molar ratios. The curve obtained (Fig. 2) is similar to the curves found in titration experiments (Live and Chan, 1976 De Jong et al., 1976b), and shows that the rate constant reaches a maximum (called plateau kinetics in the literature) when all of the salt is complexed. 

Baciocchi and Sebastiani (1979) have recently determined the rates of the elimination of, among other compounds, [187] and [188] promoted by KOt-Bu. The second-order rate constants (M I s l) at 30°C for [187] and [188] were 0.35 and 3.38 x 103 in the absence and 15.6 and >2 x 105 in the presence of 18-crown-6. Hence, the rates of both syn- and anti-elimination increase on complexation of the cation, but the anti-pathway is much more favoured than the syn. The authors conclude that the detrimental effect of the crown ether on the co-ordinating power of the cation is largely outweighed by the increase in basicity of the anion. 

Moss and Pilkiewicz (1974) used crown ethers to determine the freeness of carbenic species by comparing the relative reactivities of various alkenes towards phenylbromocarbene [192], generated either by treatment of benzal bromide [194] with t-BuOK or by photolysis of phenylbromodiazirine [193]. The results (Table 48) show that the reactive species produced by these... 

De Jong et al. (1976c, 1977) have studied the rate of decomplexation of t-BuNHjPFj (AM) complexes of crown ethers. The exchange broadening of the t-butyl nmr signal was used to determine the rate of cation exchange between a kinetically stable (X.AM) and a kinetically unstable (Y.AM)... 

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