Silicon tetra-alkoxides

Polymers with composition SiP206X2 (where X = OH, OMe, Me, Ph, Cl) which contain an eight-membered ring, can be obtained from silicon tetra alkoxides and phosphoryl chloride (9.271a). Corresponding discrete ring compounds are also known (9.271b) [46],... 

Amorphous silica (natural and synthetic) may exist in several different forms including silica gels, nonporous precipitates, and hydrogenic silicas. These can all be regarded as polycondensation products of orthosilicic acid. The methods of preparation are either by acid precipitation from silicate solutions, or by the hydrolysis of silicon derivatives, silicon tetra-halides, or alkoxides (Mitchell [1966]). To follow the development of the properties of silica gel, it is necessary to consider polymerization of silicic acid from the sol stage. 

Finally, the / er butyldimethylsilyl (TBS) ether is cleaved with tetra- -butylammonium fluoride (TBAF) to provide alcohol 22. TBAF is suitable for this deprotection step because silicon has a high affinity for fluorine. Thus, fluoride attacks a d-orbital of the silicon atom in 46 providing the negatively charged pentacoordinate intermediate 47, which then breaks down with loss of the alkoxide 49 to give 48. 

In view of an interesting gradation observed (Section 2.4) in the reactivity of tetra-halides of silicon, titanium, zirconium, and thorium with ethyl and isopropyl alcohols, the reverse reactions of alkoxides of these elements with hydrogen chloride (bromide) were investigated by Mehrotra who demonstrated that the reaction products of... 

The alkoxides, Si(OR) such as tetraethyl orthosilicate (TEOS), Si(OC2Hj) or tetra-methyl orthosilicate (TMOS), Si(OCHj) are the well started precursors in the sol-gel formation. Alkoxides are ideal chemical precursors for sol-gel synthesis because they react readily with water, in which the hydrolysis of Si atom takes place to form a hydroxytriethyl orthosilicate or a hydroxytrimethyl orthosilicate.The reaction is called hydrolysis, because a hydroxyl ion becomes attached to the silicon atom as follows ... 

Currently, microelectronics relies on bulk sihca as an important dielectric material that is often used as an insulating template for further reactivity. On the nanoscale, sUica can be synthesized by polymerizing silicic acid in an aqueous system, or through hydrolysis and condensation of silicon alkoxides in the Stober synthesis [51]. The mechanism of these two methods is unique. The first method is dominated by monomers and tetra-functionalized species, such that the resultant sihcate sols are uniform, which means that they are fully hydrolyzed and grow by monomer addition. In contrast, for the second method, di- and tri-functionalized species are dominant for alkoxides. Regardless of the synthesis used, these particles induce a fractal interior with minimal morphological control due to their common template, ammonium hydroxide [51]. 

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