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

Summary: Mechanism of the Acid-Catalyzed Si-O Bond Cleavage in
Siloxanes and Siloxanols. A Theoretical Study
Marek Cypryk*, and Yitzhak Apeloig
Center of Molecular and Macromolecular Studies of Polish Academy of Sciences,
90-363 Loīdzī, Sienkiewicza 112, Poland, and Department of Chemistry and the Lisa
Meitner-Minerva Center for Computational Quantum Chemistry, Technion-Israel
Institute of Technology, 32000 Haifa, Israel
Received December 13, 2001
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.
Introduction

  

Source: Apeloig, Yitzhak - Department of Chemistry, Technion, Israel Institute of Technology

 

Collections: Chemistry