Precipitate formation from alkoxides

Solubility of LiCl in MeOH, EtOH, and"BuOH is 30.4, 19.6, and 13.9%, respectively. That is why after refluxing of the reaction mixture and washing off the precipitate with alcohol, alkoxides free from LiCl are obtained. However, this reaction in many cases is also complicated by formation of bimetallic complexes. Formation of stable intermediate complexes is especially characteristic when LiOR is applied for alkoxylation. Thus Li4Y40(0Bu )12Cl2 was isolated in reaction of YC13 with 2 mols of LiOBu (i.e., on lack of OR-ligands) [553]. 

There are a number of claims4 40 51,69,150,247,268-274 for the isolation of bimolecular ethers from various other heterocyclic cations although the structures of these products have rarely been unambiguously established. The reaction mechanism outlined for the formation of 114 probably does occur in other heterocyclic systems, particularly in those cases in which alkoxide ion formation from the pseudobase readily occurs. Solubility considerations may dictate the precipitation of the bimolecular ether rather than the pseudobase from basic aqueous solutions containing relatively high concentrations of the heterocycle. However, such bimolecular ether formation will usually be in direct competition with the pseudobase disproportionation reaction (Section V,D) which shows the same pH dependence. 

Zukoski et al. Formation of Uniform Precipitates from Alkoxides 453... 

Zhu J., Liu S., Palchik O., Koltypin Y., Gedanken A. Shape-controlled synthesis ofsUver nanoparticles by pulse sonoelectrochemical methods. Langmuir 2000 16 6396-6399 Zukoski C.F., Look J.L., Bogush G.H. Formation of uniform precipitates from alkoxides. Adv. Chem. Series 1994 234 451-465... 

In 1956 Gerhard Kolbe (1) published the first results that showed that spherical silica particles could be precipitated from tetraethoxysilane in alcohol solutions when ammonia was present as the catalyzing base. Several years later, in 1968, StOber, Fink, and Bohn (2) continued in this research area and published the frequently cited original article for the preparation of monodispersed silica particles form alkoxide solutions. StOber et al. improved the precipitation process and described the formation of exceptionally monodispersed silica particles. The final particle size could be controlled over a wide range from about 50 nm to 1 1/2 p,m. Variations of the particle size could be achieved by different means, e.g., temperature, water and ammonia concentration, type of alcohol (solvent), TEOS (tetraethoxysilane) concentration, or mixing conditions. 

The most interesting result is the formation of a transparent colloidal solution of ceria with 2 nm particles. Cerium metal tips with the superficial layers of oxide are allowed to react in 2-methoxyethanol at 250 to 300°C, and removal of coarse ceria particles originating from the superficial layers yields the colloidal solution. Addition of water to the solution does not cause any change except dilution of the color of the solution, but addition of a drop of a solution of any kind of salt immediately causes precipitation of ceria particles. - The reaction mechanism is as follows The solvent slowly dissolves the superficial layers, and when the solvent reaches the metal, rapid reaction occurs, yielding an alkoxide solution. The concentration of the ceria precursor becomes so high that a burst of nucleation occurs, yielding the colloidal solution. The reaction of cerium acetylacetonate in the same solvent yields ceria particles but does not give a colloidal solution. 

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