Fractional crystallization ratios

For preparative purposes batch fractionation is often employed. Although fractional crystallization may be included in a list of batch fractionation methods, we shall consider only those methods based on the phase separation of polymer solutions fractional precipitation and coacervate extraction. The general principles for these methods were presented in the last section. In this section we shall develop these ideas more fully with the objective of obtaining a more narrow distribution of molecular weights from a polydisperse system. Note that the final product of fractionation still contains a distribution of chain lengths however, the ratio M /M is smaller than for the unfractionated sample. 

Separation Processes. The product of ore digestion contains the rare earths in the same ratio as that in which they were originally present in the ore, with few exceptions, because of the similarity in chemical properties. The various processes for separating individual rare earth from naturally occurring rare-earth mixtures essentially utilize small differences in acidity resulting from the decrease in ionic radius from lanthanum to lutetium. The acidity differences influence the solubiUties of salts, the hydrolysis of cations, and the formation of complex species so as to allow separation by fractional crystallization, fractional precipitation, ion exchange, and solvent extraction. In addition, the existence of tetravalent and divalent species for cerium and europium, respectively, is useful because the chemical behavior of these ions is markedly different from that of the trivalent species. 

The 3-deoxyhexoses were obtained crystalline after separation on a column of cellulose, a rather tedious operation recently separation of these sugars by fractional crystallization has been reported (41). From the major unsaturated ester 27, 3-deoxy-D-ribo- and -arabino-hexose were shown to have been produced respectively in the ratio 66 34, and from ester 28 in the ratio 24 76. Saturation of the enol grouping of the esters 27 and 28 gives, therefore, the 1,2-cis products preferentially. However, some hydrogenolysis of the anomeric acetate group accompanies simple saturation of the double bond and poses another separation problem. 

There is a reasonably good stoichiometric control observed in these aminolysis reactions. Therefore, in principle, the extent of chlorine replacement can be regulated by the ratio of the reactants. If mixtures of products are formed, these are generally amenable to separation by column chromatography. Separation of individual stereoisomers is often accomplished by procedures such as fractional crystallization. 

Tetracyclic keto ester 467, prepared earlier (253), was treated with the anion of diethyl methoxycarbonylmethylphosphonate in dimethylformamide. The reaction supplied the unsaturated ester 492, which was catalytically hydrogenated to diester 493. Dieckmann condensation of 493 yielded two nonenolizable keto esters (494 and 495), which could be separated by fractional crystallization. Sodium borohydride reduction of 18a-methoxyyohimbinone (494) gave two alcohols (496 and 497) in a ratio of about 10 1 at the same time, reduction of 180-methoxyyohimbinone (495) furnished another two stereoisomeric alcohols (498 and 499) in approximately equal amounts. Demethylation of the four stereoisomers (496-499) resulted in the corresponding 18-hydroxyyohimbines (500-503)... 

According to Treuil, the constant concentration ratio of two hygromagmatophile elements is indicative of fractional crystallization. From the Doerner-Hoskins equation ... 

Because of the large demand for p-xylene, another method is now being used by Amoco to increase the percentage of the para isomer in mixed xylenes. They are heated at 300°C with an acidic zeolite catalyst, which equilibrates the three xylenes to an o,m,p ratio of 10 72 18%. The para isomer is separated by fractional crystallization, whereas the o,m mixture is reisomerized with the catalyst to produce more para product. Theoretically, all the xylenes could be transformed into the desired para isomer. The zeolite catalyst has the following structure. 

The nitrosation of 2,5-diphenylpyrrolidine in an ice-cold solution of ethanol and hydrochloric acid with sodium nitrite produced the expected iV-nitroso-2,5-diphenylpyrrolidine. The product could be separated into trans and cis isomers by fractional crystallization using acetone-water mixtures. The ratio of trans to cis isomer was found to be 2.5 1. When the nitrosation was carried out in an acetic acid solution, the ratio of trans to cis isomer remained the same although the yield of the identified products was somewhat lower (71 vs 56 %) [29]. The ratio of the isomers of the starting pyrrolidine was not reported. 

The carbonyl platinum anions, [Pt3(CO)6]2, (n = 1-6,10) were first synthesized and characterized by Chini and coworkers1 3. They obtained these compounds by reaction of Pt(IV) or Pt(II) salts at room temperature with bases such as sodium hydroxide or sodium acetate under a carbon monoxide atmosphere. The product composition is quite sensitive to the Pt-base ratio, reaction time, and reaction conditions. As a consequence of this sensitivity, product mixtures with An = 1 are usually obtained, which are separable only with difficulty by fractional crystallization. Interest in this series of compounds for (a) their unique redox solution chemistry, (b) their use as precursors for higher nuclearity carbonyl platinum anions,4 and (c) their use as precursors for novel supported Pt catalysts5 8 prompted efforts to develop... 

Determination of the ratio of isomers. The polarimetric determination of a- and /3-lactose in solutions is well known. The fraction crystallized in dried products may be easily calculated from the shifts in the relative amounts of a-lactose on rehydration. 

C NMR-spectroscopy indicated a 3/4-ratio of 87 13. Diastereomers 3/4 can be separated by careful medium-pressure silica gel chromatography (petroleum ether/ether 95 5)3b or by fractional crystallization of the diastereomerlc chloroacetates (vide supra). For structural characterization see Note 19. 

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