Rapid chloride-exchange reactions of organosilicon and organogermanium chlorides

Abstract

The kinetics of the ³⁶Cl^–-exchange reaction of a number of labile organo-silicon and -germanium chlorides of the type R₃MCl have been studied in acetone–dioxan mixture with the aid of an automated flow–quench system. The reactivity of all substrates decreases with increasing length of the alkyl chain; steric effects are more important in the reactions with silicon chlorides than with those of germanium. Benzyl- and 2-phenylethyl-substituents show an unexpectedly high reactivity. The kinetic order in the carrier salt, Li³⁶Cl, is one half for R = n-butyl and cyclohexyl (M = Si) and R = phenyl (M = Ge) but for 2-phenylethyl and benzyl the order is unity: these orders are accounted for in terms of the relative reactivities of ion pairs and free ions. Activation parameters indicate a lower activation enthalpy but more negative entropy for germanium as compared with silicon.

Racemization and exchange studies in this solvent strongly support an associative mechanism for the chloride-exchange reaction.