REACTION-RATES OF THE CONSECUTIVE TRANSESTERIFICATION OF TETRAETHOXYSILANE WITH METHOXIDE IONS AND DISTRIBUTION REACTION-RATES OF ETHOXYTRIMETHYLSILANE AND METHOXYTRIMETHYLSILANE IN ALKALINE ALCOHOL-SOLUTIONS

The reaction kinetics of Si(OC2H5)4 and methoxide ions, Si(OC2H5)4 + 4 CH3O- --> Si(OCH3)4 + 4C2H5O-, were investigated in methanol solutions with Fourier transform infrared spectroscopy. The net reaction was as sumed to consist of four consecutive reaction steps. The formation of ethanol was followed and the rate constants were established as k1 = 0 .43+/-0.09 M-1 s-1, k2 = 0.39 +/- 0.10 M-1 s-1, k3 = 0.29+/-0.05 M-1 s -1 and k4 = 0.17+/-0.06 M-1 s-1 at 295 K. The reverse of the reaction sequence was neglected since the concentration of produced ethoxide io ns was assumed to be low in methanol solution compared with methoxide due to the fast equilibrium reaction, C2H5O- + CH3OH half arrow right over half arrow left C2H5OH + CH3O-, for which the equilibrium constan t was estimated to 1.52+/-0.35. The reaction kinetics of the equilibri um reaction, (CH3)3SiOC2H5 + CH3O- half arrow right over half arrow le ft (CH3)3SiOCH3 + C2H5O-, were also studied with FTIR spectroscopy. Th e changes in concentrations of (CH3)3SiOC2H5 and (CH3)3SiOCH3 were mea sured in solvents with different molar ratios of ethanol and methanol. The rate constants for the forward and reverse reactions were graphic ally established as 2.49+/-0.26 M-1 s-1 and 3.08+/-0.47 M-1 s-1 at 295 K. The equilibrium constant for the reaction, (CH3)3SiOC2H5 + CH3OH = (CH3)3SiOCH3 + C2H5OH, was determined to 1.37+/-0.31 at 295 K.