ON THE SYNTHESIS OF SILOXANES XXIV. VERIFICATION OF INDUCTIVE AND STERIC SUBSTITUENT CONSTANTS FOR SILOXY GROUPS BY REACTION OF DI- AND TRICHLOROSILANES AND -SILOXANES WITH LITHIUMT-BUTOXIDE

Abstract

Abstract The reactions of dichloromethylorganylsilanes (RSiMeCl2) and dichlorosiloxanes ([tbnd]SiOSiMeCl2) as well as of trichloroorganylsilanes (RSiCl3) and trichlorosiloxanes ([tbnd]SiOSiCl3) with lithium t-butoxide in diethyl ether were investigated kinetically by turbidimetric measurements in order to verify the inductive substituent constants σ∗ for siloxy groups determined earlier by the 29Si NMR shifts of compounds (Me3SiO)3Si—Y (Y = siloxy group) and the steric substituent constants Es for siloxy groups which were obtained by the rate constants of the acetolysis reactions of compounds ClMe2Si—Y (Y = siloxy group). The reaction constants were determined using chloroorganylsilanes to be: p∗ (RSiMeCl2) = 0.85; δ (RSiMeCl2) = 0.85; p∗ (RSiCl3) = 1.02; δ (RSiCl3) = 1.28. The correlation of lg krel – p∗ · σ∗ values of the corresponding chlorosiloxanes with Es values using σ∗- and Es-constants of siloxy groups determined earlier gave correlation straight lines with the slope of 0.81 ([tbnd]SiOSiMeCl2) or 1.12 ([tbnd]SiOSiCl3), respectively, that means the δ-values of the silanes and siloxanes are nearly identical. But the reactivity of the di- and trichlorosiloxanes is only about one tenth of the values expected by the reaction constants and the σ∗- and Es-constants of siloxy groups previously determined. We suggest that this behavior is caused by an electron donating effect of the siloxy groups connected with silicon atoms bearing two or three chlorine atoms. As this influence is contrary to the electron attracting effect of siloxy groups at silicon atoms with only one chlorine atom, we assume that the inductive effect of siloxy groups is extremely sensitive to changes in electron density at the silicon atom with which they are connected. As the correlation straight lines are only parallel shifted, obviously the electron density at a silicon atom is mainly determined by the first generation of substituents.