Ion-pair-forming reagents

Several cephalosporin antibiotics have been separated on silica gel 60 HF254 silanized plates (Merck) using ion-pair forming reagents (25). The detection was done under a U V lamp and by iodine vapor. The values can be seen in Table 8 the solvents were as follows ... 

The reactive species is the acylium ion resulting from abstraction of a chloride anion from benzoyl chloride (Eq. 50). This reagent comprises an ion pair formed between two large (soft) ions which are therefore associated as loose ion pairs. According to these assumptions, the absence of a microwave effect should be expected as the polarity evolution is very weak between the GS and TS (two loose ion pairs of similar polarities). 

A remaining critical mechanistic question deals with the mode of product formation from the ion pair formed upon H abstraction. At this point, the reaction can be consummated either by transfer of H- from B to the carbonyl carbon (path a, Scheme 30), or direct abstraction of H- from another R3SiH reagent (path b).248 In this latter scenario, the borane becomes a spectator in the reaction, and the true catalyst is the [R3Si]+ cation. To probe this question, we performed the experiment depicted in Scheme 31. In the case of path b, both pairs of isotopomers should be observed, while if path a is operative, only the unscrambled products should be present. In fact, the product mixture consistent with... 

The diene-Br2 complex is again in equilibrium with the reagents, and nucleophilic attack at carbon can be carried out either by the bromide of the ammonium bromide ion pair, formed at the moment of the electrophilic attack, or by the less nucleophilic pyridine added in excess in the reaction medium. It is noteworthy that this mechanism is characterized by a rate- and product-limiting nucleophilic step which should be quite insensitive to steric hindrance around the double bond. In agreement with a weak influence of the steric effects, pyridinium perbromide reacts in chloroform and tetrahydrofuran with substituted conjugated and non-conjugated dienes to give selectively (>95%) bromine addition to the more alkylated double bond (equation 44). 

Indirect determinations using flame AAS based on the formation of ion-pairs have been used extensively by Valcarcel s group [40,41,42,43,44] for the determination of perchlorate, nitrate and nitrite and surfactants. The extraction system used were similar to that shown in Fig. 3.14, except that the outflow of extractant from the injector loop was controlled by a pump instead of using an impedance coil. The determinations are based on extraction of ion-pairs formed between the analyte and a reagent containing the metal... 

Table 3 lists common ion-pairing (IP) reagents. Formation of the complex depends on factors such as the pH of the aqueous phase, the type of organic solvent, and the nature and concentration of the counter ion. The IP extraction technique is useful for a variety of ionizable compounds which are difficult to extract in the unchanged form such as penicillins, amino acids, and conjugated metabolites. This method allows extraction of quaternary ammonium compounds such as tubocurarine which are ionized at all pH values. 

Tetrakis chelate anions formed when either a silver or potassium P-diketonate is mixed with a lanthanide tris P-diketonate are effective shift reagents for organic-soluble salts. Adding an organo-halide or fluoroborate salt leads to precipitation of silver halide or potassium fluoroborate, resulting in an organic-soluble ion pair. These reagents have been used to enantiomerically resolve the spectra of chiral isothiouronium and sulfonium ions. 

Ion-pairing reagent A compound that forms a complex with an ionic compound to allow its analysis using HPLC. 

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