Synthesis of siloxanes

1H NMR spectroscopy has been used in a kinetic study of the cleavage of siloxane bonds by hydrogen chloride in dioxane. The cleavages show an induction period which is associated with the autocatalytic effect of the water formed during the reaction. The kinetic behavior can be expressed in terms of a rate law that includes rate contants for cleavage by both dioxane. HCl (k1) and H2O·HCl(k2). The k′1 (k′1  k1 · [HCl]−4) and k′2 (k′2  k2 · [HCl]−3) values correlated with σ* values; thus ϱ* values of 1.4 and 0.7 were obtained for k′1 and k′2, respectively. The σ* values of 0.35 that we previously derived for the Me3SiO group applies in this reaction. The reaction of 1,3-bis(p-methoxyphenyl)tetramethyldisiloxane with hydrogen chloride involves cleavage of silicon-aryl rather than SiO bonds.