ANION EXCHANGES

In anion exchange, the negative charge on the sample binds to the positive charge on the resin. Consider the exchange of two anions, X and Y, again between the solution and the resin 

Normal phase chromatography systems use a polar stationary phase with a nonpolar mobile phase. Generally, the packing materials within the normal phase columns are composed of unmodified silica spherical beads (cyano, amine, or diol packing materials can also be used) with the mobile phase consisting of nonpolar organic solvents such as ethanol, chloroform, propanol, or hexane. Table 4.3 outlines the main differences between typical normal phase packing materials. 

Phase Functionality Bond Type Additional Information 

Can be unpredictable because siuTace interactions can take place 

These colmnns can also be used in reverse phase chromatography 

In some cases, the anion exchange can also take place in solid-liquid biphasic conditions either for monomeric or dimeric phosphonium salts, in suspension or by using an anion-exchange resin, or for polymeric phosphonium salts, by washing with solutions of salt MX. Finally, it must be pointed out that the new anion Y can be generated in situ by a classical preparation of such anions (e.g. addition of cyanide anion to CS2 for NCCS2-527 or aromatic substitution with fluoride anion on substituted nitrobenzene derivatives for N02-528). Several recent examples of the various anions exchanges are shown in Table 10. 

The simplest modification occurs on addition of nucleophilic anions X to Lewis acids MX , resulting in the creation of an ate-complex as anionic adduct (Table 11) (reaction 162). The formation of complex anions can also be the result of halogen addition, with the formation of mixed or symmetrical trihalides [YX2], or else of hydracid addition, with formation of anions [XHY]-, which are not very stable and readily dissociated in solution but are characterizable in the solid state (reaction 163). 

Another kind of anion transformation results from chemical reaction, with a reagent introduced in the medium, corresponding to a neutralization of the hydroxide anions 

TABLE 11. Examples of anion modification with formation of ate-complexes 

TABLE 12. Examples of anions modifications with charge transfer 

POMs can he immobilized onto anion-exchange resins and surface-modified metal oxides with quaternary ammonium cation- or amino-functional groups via anion-exchange. Jacobs and coworkers tethered Venturello s catalyst [P04(W0(02)2)4]3-on a commercially available nitrate-form resin with alkylammonium cations and have carried out the epoxidation of allylic alcohols and terpenes with this supported catalyst [166, 167]. The regio- and diastereoselectivity of the parent homogeneous catalysts were preserved in the supported catalyst. For bulky alkenes, the reactivity of the POM catalyst was superior to that of Ti- 3 zeolite with a large pore size. The catalytic activity of the recycled catalyst was maintained completely after several cycles. 

One such organic-inorganic hybrid support has been synthesized by covalently anchoring a N-octyldihydroimidazolium cation fragment onto Si02 (denoted as 

The oxidation was immediately stopped by the removal of the solid catalysts, and POM species could hardly be found in the filtrate after removal of the catalysts. These results could rule out any contribution to the observed catalysis from POM species that leached into the reaction solution, and show that the observed catalysis was truly heterogeneous in nature. In addition, the catalysts could be reused several times without an appreciable loss of their high catalytic performance. 

1 Anastas, P.T. and Warner, J.C. (1998) Green Chemistry- Theory and Practice, Oxford University Press, Oxford. 

4 Special issues on Green Chemistry Accounts of Chemical Research, 35, 685-816 (2002). 

IM-SiOj [SiWioOj Vjln-OHljVlM-SiOj catalyst 

The interlayer anions can easily migrate through the interlayer space of the stacked brucite-like layers, resulting in the ability of these solids for topo-tactic exchange of the anions existing in the interlayer [104]. This ability has been used to prepare new LDHs from other easily prepared LDHs. The use of bulk organic anions as exchanging species leads to exfoliation of the hydroxyl layers [105] to prepare nanocomposites [106]. 

Miyata proposed [34] the following sequence in the selectivity of the layers for monovalent and divalent anions  

The absorption of acid-forming gases (SO2, NO, NO2, HF, HC1) from the air increases with the molecule s water solubility and reactivity and increases with the soil s pH and base status. Weak bases such as NH3 are retained more strongly by acid soils, 

The anions of concern to agriculture include Cl-, HCOJ, NOJ, SO, HPO -, H2PO4, OH-, and F-. In addition, some micronutrients (HjBOij-, MoO -, and HAsO -) and heavy metals (CrO -) exist as anions in soils, as do some pesticides, such as the dissociated phenoxyacetic acids (2,4,5-T and 2,4-D). Molecular species of interest include NH3, undissociated weak acids such as H3BO3 and H4Si04, and the undissociated forms of many pesticides (DDT, 2,4,5-T, ant 2,4-D.) The study of anionic and molecular retention by soils has been the subject of increasing research in recent years. 

The approach in this chapter is to describe various retention mechanisms and to cite examples of their involvement in the retention of specific anionic and molecular species. Several of the mechanisms are general and apply to many of the species listed above. Adsorption isotherms are also discussed because of their widespread use to describe anion and molecular retention by soils. 

If a dilute, neutral solution of KC1 is added to dry montmorillonite, the equilibrium Cl- concentration in the bulk soil solution will be greater than the Cl- concentration in the solution originally added to the day. This phenomenon is observed whenever a salt solution is added to a dry colloid having no adsorbing capacity for 

Preferential substitution of a face-capping chloride ligand over a facecapping chalcogenide ligand can also be accomplished. Reactions between the 

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Pure water for use in the laboratory can be obtained from tap water (hard or soft) by distillation if water of great purity is required, distillation must be carried out in special apparatus, usually made of quartz, not glass or metal precautions must be taken to avoid any spray getting into the distillate. Water which is sufficiently pure for most laboratory purposes can, however, be obtained by passing tap water through cation-exchangers and anion-exchangers as described above, when the water is deionised . 

Anion exchange resins of the quaternary ammonium hydroxide type (e.g., De-Acidlte FF, IRA-400 or Dowex I) are strong bases and are useful cataly s for the cyanoethylatlon of alcohols and possibly of other active hydrogen compounds. 

Anion-exchange resins contain a basic radical, such as —NH and =NH, and are prepared by the condensation of formaldehj de with amines such as m-phenylenediainine and urea. These resins can absorb acids by the formation of salts, —NH3CI and =NHjCl, and are regenerated by treatment with sodium hydroxide or sodium carbonate. 

The exchange resins 6nd application in (i) the purification of water (cation-exchange resin to remove salts, followed by anion-exchange resin to remove free mineral acids and carbonic acid), (ii) removal of inorganic impurities from organic substances, (iii) in the partial separation of amino acids, and (iv) as catalysts in organic reactions (e.g., esterification. Section 111,102, and cyanoethylation. Section VI,22). 

Anion exchange resins—gel type — Strongly basic- —quaternary ammonium functionality... 

Dowex 1-X2 0.6 0.65 Strongly basic anion exchanger with S-DVB matrix for separation of small peptides, nucleotides, and large metal complexes. Molecular weight exclusion is <2700. 

Dowex 2-X8 1.2 0.75 Strongly basic (but less basic than Dowex 1 type) anion exchanger with S-DVB matrix for deionization of carbohydrates and separation of sugars, sugar alcohols, and glycosides. 

Anion exchange resin-gel type— intermediate basicity... 

Bio-Rex 5 2.8 0.70 Intermediate basic anion exchanger with primarily tertiary amines on a polyalkyle-neamine matrix for separation of organic acids. 

Anion exchange resins- —gel type—weakly basic—polyamine functionality ... 

Dowex 4-X4 1.6 0.70 Weakly basic anion exchanger with tertiary amines on an acrylic matrix for the deionization of carbohydrates. Use at pH <7. 

Ion-Exchange Equilibrium. Retention differences among cations with an anion exchanger, or among anions with a cation exchanger, are governed by the physical properties of the solvated ions. The stationary phase will show these preferences ... 

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