Kinetics and Mechanism of Hydrolysis of a Silicate Triester, Tris(2-methoxyethoxy)phenylsilane

Article abstract of DOI:10.1021/ja00526a015

The kinetics of hydrolysis in dilute aqueous solution of tris(2-methoxyethoxy)phenylsilane to phenylsilanetriol have been studied.The hydrolysis exibits specific acid and general base catalysis, the latter with a Broensted β value of 0.7.The specific acid catalysis mechanism is probably A-2 (k_H3O+/D3O+ = 1.24, ΔS^<*> = -39 cal deg^-1 mol^-1).At high pH (>10) the rate of appearance of the triol is limited by the rate of hydrolysis of one of the intermediates in the hydrolysis sequence, bis(2-methoxyethoxy)phenylsilanol, which, under these conditions, forms an inert anion.At lower pH the hydrolysis of bis(2-methoxyethoxy)phenylsilanol is several times faster than that of tris(2-methoxyethoxy)phenylsilane while that of the second intermediate, 2-methoxyethoxyphenylsilanediol, is probably faster than the above two hydrolyses at all pHs.It is argued that the form of general base catalysis observed suggests that the base-catalyzed reactions involve either an S_N2^**-Si or S_N2^*-Si mechanism with formation of a pentacoordinate intermediate.Generalization of the argument used here is explored.