Entrainment

The entrainment method takes its origin in experiments performed by Gernez in 1866, showing that a supersaturated solution of sodium ammonium tar-trate, when seeded with a particle of ( + ) salt, only yielded crystals of that salt. The resolution by entrain-ment is a batch process, which rests on the control of the crystallization rates of the two enantiomers, and implies the utilization of ternary phase diagrams, such as that shown in Figure 3. A solution M, supersaturated with respect of both enantiomers, and containing a small excess (E g%) of one of them (here, l) is seeded with crystals of that enantiomer. Crystallization is then allowed to proceed until the solution has reached composition N. At this point, the rotation of the solution is approximately equal and opposite in sign to that of the starting solution, and the l enantiomer that has crystallized amounts to twice its excess in the original solution M (i.e. + 2E g%). At this stage, the crystals are separated off, and the same z

OH

R = N02: chloramphenicol intermediate R = SCH3: thiamphenicol intermediate

Figure 3 Principle of the entrainment process. In the cycle MNPQ, MN and PQ correspond to the crystallization of enantio-mers /and d, respectively; NPand QMcorrespond the loading of racemic material. In commercial processes, 20-90 crystallizations are commonly performed.

weight of racemic material is added to the filtrate and dissolved by heating. This results in a new supersaturated systems of composition P, symmetrical to M, where the d enantiomer is now in the same excess as was the l in the previous experiment. Seeding with the d form and crystallization up to Q then yields + 2E g% of d, and addition of the same weight of racemate allows the return to M, and so forth.

In practice, the economics of the process depends on the amount of material collected after each crystallization, which should represent at least 10-15% of the solute, and on the number of cycles which can be performed; this number is limited by the build-up of impurities which follows the addition of fresh ra-cemate at each step, and which may eventually disturb the crystallization kinetics. The Roussel-Uclaf and Zambon processes for the manufacture of the chloramphenicol and thiamphenicol intermediates shown in Figure 3 are well-known industrial applications of resolution by entrainment. The method is also of great value for laboratory-scale resolutions, especially at the 100 g to kg scale.

R = N02: chloramphenicol intermediate R = SCH3: thiamphenicol intermediate

Figure 3 Principle of the entrainment process. In the cycle MNPQ, MN and PQ correspond to the crystallization of enantio-mers /and d, respectively; NPand QMcorrespond the loading of racemic material. In commercial processes, 20-90 crystallizations are commonly performed.

For low-melting conglomerates, the entrainment can also be effected without solvent, in a supercooled melt. Such processes are easily understood by means of the melting point phase diagrams, by considering the metastable extension of the liquidus curves below the eutectic temperature.

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Solar Panel Basics

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