Mechanism of the Acid-Catalyzed Si−O Bond Cleavage in Siloxanes and Siloxanols. A Theoretical Study

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Theoretical ab initio calculations were carried out to simulate the hydrolysis of the siloxane bond under neutral and acidic conditions. The most important factors reducing the energy barrier of the reaction are the protonation of the siloxane oxygen (acid catalysis) and the basic assistance to the nucleophile (responsible for withdrawal of the proton from a nucleophile). The basic assistance may be effectively accomplished by hydrogen bond complexes, which help to transfer a proton from the nucleophile to the leaving group. Such complexes may consist of water, silanol, and acid. Solvation lowers the barrier of acid-catalyzed hydrolysis, due to better stabilization of the charged transition states. The enhanced reactivity of the terminal siloxane bond in siloxanols is connected partly with the ability of the SiOH group to participate in hydrogen bond structures facilitating the intramolecular proton transfer to the siloxane oxygen.