Eutectic mixtures, simple, preparation

Besides the complex cases of mixed oxides, there exist more simple problems of oxide and scale formation in alloy production. The detrimental effect of oxide shells around metal particles preventing intermixing is well known. The compositional changes resulting from preferential oxidation of one component have also to be taken into account. Instability of the product and/or drastic changes in the thermochemical properties of the material after shell formation (such as massive increases in the required fusion temperature in noble metal eutectic mixtures) are common, in particular in small-scale preparations. These effects still set limits to the availability of catalytically desired alloys for practical purposes (e.g. for compounds with Zr, Si, alkali, Mg). 

Moreover, although the author was unable to isolate CO(NCS) j in a pure form, he did report on many of its reactions with simple organic compounds [538], This preparation of CO(NCS)j was optimized by treating a solution of phosgene in toluene with A[NCS] (A = Na or K) in an autoclave at 100 C [538]. Alternatively, COClj reacts with K[NCS] in a LiCl-KCl eutectic mixture at 400 "C to give CO(NCS)2 in 84% yield [1018] ... 

The attainment of high levels of enantiopurity is not always possible by enzymatic or diaste-reomeric resolutions or by asymmetric syntheses alone. It is however frequently possible to prepare a pure enantiomer from a partially resolved sample by simple recrystallization. For this process to proceed successfully it is necessary that the initial enantiopurity of the mixture is greater than that of the eutectic point in the phase diagram. By utilization of the phase diagram, the optimal quantity of solvent required can be calculated. It is also possible to calculate the maximum expected yield. 

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