Explosion during distillation of crude dichlorotetramethyldisilane

Jean L. Vincent
Ronald W. Skinner
Air Products & Chemicals
Allentown, Pa.

Chemical & Engineering News (11 June 2001) Vol. 79, No. 24, pp. 2.

The procedure described by Makoto Kumada and colleagues [J. Org. Chem., 21, 1264 (1956)] for the preparation of substituted disilanes through protonation of hexamethyldisilane with sulfuric acid is still widely referenced. Jonathan D. Rich (C&EN, March 26, 1990, page 2) and Keith A. Horn (C&EN, June 4, 1990, page 2) previously reported violent exotherms that occurred during the distillation of dichlorotetramethyldisilane (DCTMDS) and chloropentamethyldisilane. However, the causes were unclear.

We have recently experienced an explosion during the distillation of crude DCTMDS prepared by the Kumada method. Our investigation revealed that the crude DCTMDS contained significant amounts of silylsulfates or dissolved sulfuric acid. Increased reaction time with sulfuric acid may have contributed to oversulfation. An ARC test on the crude DCTMDS under an N2 atmosphere showed an exotherm with an onset at 145 ºC that totaled over 600 J/g. A substantial pressure rise with evolution of sulfur dioxide occurred with the exotherm. A mixture of 10% concentration H2SO4 and DCTMDS showed a similar exotherm. Pure DCTMDS was found to be thermally stable. The thermal decomposition of crude DCTMDS is autocatalytic, requiring 50 minutes at 150 ºC, 10 minutes at 175 ºC, or just seconds at 190 ºC to reach explosive conditions. This occurs in the absence of air or water.

Winton Patnode and F. C. Schmidt reported the instability of silylsulfates during distillation of bis(trimethylsilyl)sulfate [J. Am. Chem. Soc., 67, 2272 (1945)], which is currently available for use as a trimethylsilylating agent. We performed DSC analysis on this commercial material and observed an exotherm with an onset at 225 ºC and releasing 202 J/g.

We would like to reiterate the use of caution when following the Kumada procedure or similar procedures using sulfuric acid or trifluoromethylsulfonic acid to protonate organosilanes, as well as when using silylsulfate reagents.

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