ROLE OF PENTACOORDINATE INTERMEDIATES IN THE HYDROLYSIS REACTION OF ORGANIC SILICATES

The alcoholysis and hydrolysis reactions of XSi(OR)3, XSi(OR)3 + 10% KOR (X = H, OR), and K[XSi(OR)4] (X = H) or [K,18-crown-6][XSi(OR)4] (X = OR) were studied to approach the mechanism of the reaction. We found the following: (i) The first step of the process in the case of XSi(OR)3 (X = H, OR) was the formation of a pentacoordinate intermediate by nucleophilic activation at silicon. (ii) The rate-determining step was the coordination of R'OH (R/ = H, R) to the pentacoordinate silicon leading to a hexacoordinate intermediate (or transition state), which gave the product. (iii) A step by step nucleophilic substitution occurred on a pentacoordinate silicon derivative, K[H2Si(OR)3] and K[HSi(OR)4], giving another pentacoordinate compound, K[HSi(OR)4] and [K,18-crown-6][Si(OR)5], respectively.