Kinetics and mechanism of the hydrolysis and alcoholysis of alkoxysilanes

As part of our work with the use of silane coupling agents, we have been investigating the transformations of alkoxysilancs into siloxanes. The influence of a para-substituted phenyl group attached to the silicon was investigated and the rates of acid catalyzed hydrolysis and alcoholysis of these para-substituted phenylalkoxysilanes have been determined. The kinetics and mechanism of the reactions are presented. These reactions are of interest because of their role in the use of silane coupling agents as adhesion promoters, in the preparation of zinc-rich silicate coatings, in the sol-gel process and in the preparation of silicones in general. The hydrolysis reaction was found to be first order in acid, zero order in water and to have a Hammett p of -1.42 when catalyzed by sulfuric acid. These results are consistent with current opinion that the reaction mechanism is SN1, and involves a positive intermediate, possibly a siliconium intermediate. The alcoholysis reaction was found to be first order in both the silane and the catalysts, and of intermediate order in the alcohol, when catalyzed by carboxylic acids. When catalyzed by dichloroacetic acid, the reaction has a Hammet p value of +0.43. This is consistent with a concerted displacement reaction between the alkoxysilane and the complex involving the alcohol and the carboxylate anion. The intermediate is a negatively charged intermediate, probably a penta-substituted silicon species.