Mixed-bed resins

Following a similar strategy, an ingenious mixed resin bed quench and purification strategy was devised for the Dess-Martin periodinane mediated conversion of alcohols to carbonyls. This hypervalent iodine oxidant was viewed as containing an inherent masked carboxylic acid functionality that was revealed at the end of the reaction (Species (11) Scheme 2.30). Therefore purification was easily achieved by treatment of the reaction mixture with a mixed-resin bed containing both a thiosulfate resin and a polymeric base. The thiosulfate polymer was used to reduce excess hypervalent iodine lodine(V) and (III) oxidation states species to 2-iodoben-zoic acid (11), which was in turn scavenged by the polymeric base [51]. 

The nonvolatile oxidant (DDQ) was used to mediate the release of alcohols from solid-phase Wang-type ethers 54.81 Purification of the released alcohols was affected by the use of a mixed-resin bed containing Amberlyst A-26 ascorbate 55 (to reduce excess DDQ to its hydroquinone byproduct) and also Amberlyst A-26 bicarbonate 56 (to sequester the acidic hydroquinone byproducts). Filtration afforded purified alcohols in excellent yields and purities. 

Parlow, J. J. Vazquez, M. L. Flynn, D. L. A Mixed Resin Bed For the Quenching and Purification of Tetrabutylammonium Fluoride Mediated Desilylating Reactions, Bioorg. Med. Chem. Lett. 1998, 8, 2391. 

Hydrolysis during the final rinse and service cycle leads to the prolonged leakage of acid sulfate or chloride from the bottom of the mixed resin bed ... 

Figure 2.9 A single-resin demineraliser vessel (left) and a mixed-bed demineraliser vessel (right). The mixed resin bed separates into lighter anion (top) and heavier cation (bottom) resin beds during backwashing. Acid is used to regenerate the cation resin, and caustic soda is used for regenerating the anion resin. Acid inlet at the bottom of the vessel is not shown.

Percolated water, prepared by percolating distilled water through a mixed resin bed containing Amberlite IR 120 (H) and Amberlite IRA 400 (OH). 

The approximate 1 foot of clear liquid above the resin bed is another troublesome factor. Mixing must occur in this volume when the composition of the influent solution is changed. As there was no visual evidence of gross turbulence in this volume, this effect was not investigated. On this basis these data are conservative perhaps too much so. 

Using a Pasteur pipette equipped with a dropper bulb, begin to add well-mixed slurries of CM-Sephadex in potassium acetate solutions to the appropriate columns. Allow the columns to drip while adding increasing increments of the slurried CM-Sephadex until the settled resin bed reaches a height of 4-5 in. Do not allow the columns to run dry that is, do not allow the top fluid surface to penetrate into the settled resin bed. Instead, add small aliquots of 0.1 M potassium acetate buffer, pH 6.0, or 1.0 M potassium acetate buffer, pH 6.0, to the appropriate columns to keep the fluid surfaces above the resin bed. 

In order to sustain the benefits from fixed bed counterflow regeneration it is imperative that the resin bed remains consolidated and undisturbed at all times during loading and reverse flow regeneration. Some ways this is achieved are discussed later in Chapter 10, but it follows that conventional backwashing of a counterflow designed column is undesirable since the inevitable bed disturbance causes exhausted resin to mix with the lightly loaded resin zone at... 

After the resin bed feed solution has been mixed for at least one half hour, the resin is separated from the liquid phase. The solute concentration remaining in the solution is then determined. The residual concentration is subtracted from the original concentration and the difference is divided by the volume ofthe resin. These numbers and the residual concentration are plotted on log-log paper and frequently give a straight line. 

Repeat nitrogen mix. In Step II, N2 enters the centre of the resin bed through the interface distributor/collector. After 5 min of mixing, water is drained to bed level by opening the vessel drain valve and closing the vent valve. This ensures that the bottom of the bed is well mixed and compacted. 

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