Mechanism of reaction of magnesium alkyl halides with β-silicoalkyl halides

Summary

• 1. Contrary to the views of american workers, it has been shown that β-silicoalkyl halides may be used for the synthesis of silicohydrocarbons by the Grignard-Wurtz reaction; this is illustrated by the synthesis of 12 silicohydrocarbons, in yields of 20–50% (not counting a large number of syntheses with smaller yields).

• 2. A mechanism is advanced for the so-called β-decomposition of β-silicoalkyl halides (R₂Si-C-C-X). This consists of liberation of an olefin molecule, with attachment of the β-halogen atom to the silicon. The reaction may proceed spontaneously, at temperatures of from +50° to −50°, according to the structure of silicoalkyl halide, or it may be activated by the Grignard reagents. The Grignard reagents R°MgX react simultaneously both with the product of β-decomposition R₂SiX and with the β-silicoalkyl halides, the yields of silicohydrocarbons (products of β-decomposition or of synthesis) varying widely according to the nature of R and R°.

• 3. It is shown that the amount of β-decomposition fails in the series primary → secondary → tertiary silicoalkyl halides, and is greater for bromides that for chlorides. The radical R also affects the results, the yield of decomposition products falling, and of synthesis products rising, in the order C₂H₃, CH₃, C₂H₆, C₂H₇, C₄H₂, i.e., in order of diminishing electronegativity of the radicals. The extent of β-decomposition could in some cases be reduced by conducting the syntheses at low temperature.