Solvent effects crown ether exchange

Of marked interest is the use of organic solvents with crown ether phases [56]. In anion exchange chromatography, organic solvents as mobile phase additives have only a limited effect on selectivity however, the influence of organic solvents on the binding process of cations on crown ethers is well known. In comparison with pure aqueous systems, formation constants in organic solvents... 

The activity of polymer-supported crown ethers depends on solvent. As shown in Fig. 11, rates for Br-I exchange reactions with catalysts 34 and 41 increased with a change in solvent from toluene to chlorobenzene. Since the reaction with catalyst 34 is limited substantially by intrinsic reactivity (Fig. 10), the rate increase must be due to an increase in intrinsic reactivity. The reaction with catalyst 41 is limited by both intrinsic reactivity and intraparticle diffusion (Fig. 10), and the rate increase from toluene to chlorobenzene corresponds with increases in both parameters. Solvent effects on rates with polymer-supported phase transfer catalysts differ from those with soluble phase transfer catalysts60. With the soluble catalysts rates increase (for a limited number of reactions) with decreased polarity of solvent60), while with the polymeric catalysts rates increase with increased polarity of solvent74). Solvents swell polymer-supported catalysts and influence the microenvironment of active sites as well as intraparticle diffusion. The microenvironment, especially hydration... 

Bribre. K.M. Detellier. C. Solvent effects in the metal interchange of crown ether-alkali metal cation complexes. Transition from an associative exchange in nitromethane to a dissociative exchange in acetonitrile studied by Na... 

Halide ions, especially F , are weak bases in aqueous solution. However, in dry aprotic solvents Et4N F effectively induces elimination of HBr from 2-bromoethylbenzene (7). The ammonium chloride and bromide only promote halogen exchange. At 80°, the fluoride decomposes into EtjN, HF, and ethylene, thus demonstrating the high protophilicity of the hard fluoride ion. This same property is also shown by its usefulness for generating dihalo-carbenes. Naked fluoride ion (cation sequestered by a crown ether) promotes facile elimination of haloethylenes to form acetylenes and allenes (8). 

The result with allyllithium 23 a differs only insignificantly from that of an earlier report (10.5 kcal/mol)25>. Complexation of 23 a with TMEDA does not influence the rate of exchange. Hexamethylphosphoric triamide (HMPT), 15-crown-5 ether and [2.1.1]cryptand in tetrahydrofuran (THF) led to rapid decomposition of 23a. Addition of n-butyllithium had essentially no effect on the barrier. Since the 13C NMR chemical shifts of 23 a are independent of the solvent, it is assumed that 23 a exists as a contact ion pair or higher aggregate in the NMR experiments. (The other alkali metals should also form contact ion pairs with the allyl anion because of their well-known tendency to form contact ion pairs even more readily than the lithium cation 26)). 

The kinetic deuterium solvent isotope effects on the acid-catalyzed dissociation of Li(2,l,l) and Ag(2,2,2) have been reported, as have the exchange kinetics " of cryptand (2,2,1) on Tl(2,2,2) and T1(2b,2,2) . The kinetics of complex formation between Li and 18-crown-6 ether in 1,3-dioxalane and 1,2-dimethoxyethane solvents have also been reported. ... 

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