Occurrence and Preparation

Sorbitol, D-mannitol, L-iditol and dulcitol are found in plant sources. Synthetic preparation is simpler than isolation. The six other isomers 

Sorbitol can be made by the reduction of three naturally occurring hexoses, D-glucose, D-fructose and L-sorbose. D-Mannitol and L-iditol, respectively, are concurrently produced from the ketoses. However, D-glucose, because of its greater availability, is the only practical source. 

The first recorded reduction of D-glucose to sorbitol was in 1890 by Meunier who used sodium amalgam. Ipatieff reported the first catalytic hydrogenation of D-glucose to sorbitol. Since then there have been 

The superiority of catalytic hydrogenation of the readily available D-glucose as a laboratory method and the commercial availability of sorbitol has rendered the earlier reductions by sodium amalgam of 

D-mannitol/ although normal molasses contains only a negligible amount. It apparently arises from anaerobic fermentation. D-Manni-tol is also present in silage and sauerkraut. 

Shellac is the refined form of lac, the secretion of the lac insect parasitic on certain trees in India, Burma, Thailand and to a minor extent in other Asian countries. 

In commercial practice the crop is taken from the tree shortly before emergence of the new brood. Some of these twigs are then tied to new trees to provide future sources of lac but the rest, sticklac, is subjected to further processing. The average yield per tree is about 20 lb per annum, usually one crop being allowed per tree per year. 

Subsequent treatment of the sticklac carried out by hand or by mechanical methods first involves removal of woody matter and washing to remove the associated lac dye to produce seedlac, containing 3-8% of impurities. This may be further refined by various methods to produce the shellac flakes of commerce. 

The next stage may best be described as a primitive hot-filtration process. Two members of the village sit across the front of a simple fire resembling a Dutch oven, holding between them a bag about 30 feet long and about two inches in diameter. The lac inside the bag melts and, through one of the operators twisting the end of the bag, the lac is squeezed out. The lac is then removed from the outside of the bag and collected into a molten lump which is then stretched out 

In the factory processes the sticklac is first passed through crushing rollers and sieved. The lac passes through the sieve but retains the bulk of the woody matter. The sieved lac is then washed by a stream of water and dried by a current of hot air. A second mechanical cleaning process removes small sticks which have not been removed in the earlier roller process. The product, seedlac, now contains 3—8% of impurities. 

---

Occurrence and preparation Chemical composition Properties Applications... 

Lanthanide elements, 411, 389 contraction, 413 electron configurations, 415 occurrence and preparation, 413 oxidation numbers, 414 properties, 412 Lanthanum... 

Just as, in Group VB, niobium, so, in this Group, molybdenum provides most of the examples of the chalcogenide halides. The occurrence and preparation of such compounds are described in numerous publications. In most cases, they have been obtained as powders, with the composition based on chemical analyses only. The presence of defined, homogeneous phases is, therefore, in many cases doubtful. In addition, some published results are contradictory. A decision is possible where a complete structure analysis has been made. As will be shown later, the formation of metal-metal bonds (so-called clusters), as in the case of niobium, is the most characteristic building-principle. Such clusters... 

Actinides occurrence and preparation. With the exception of U and Th, the availability of the actinides of the first half of the series ranges from the g to kg scale that of the elements of the second half of the series from the mg scale for Cf to the sub-mg scale for Es. Isotopes of Np, Pu, Am, Cm can be available as byproducts of nuclear fuel processing other elements such as Ac, Cf, Bk, Es can be obtained by irradiation of selected isotopes in high flux reactors, or by reprocessing large quantities of ore (Pa). 

---