Thermal and Photochemical Decomposition of Gaseous Aluminum Trimethyl

Abstract

The thermal decomposition of gaseous aluminum trimethyl, in the absence of hydrogen, is nearly 94 percent homogeneous and of kinetic order three‐halves over the pressure range 10 to 85 mm measured at room temperature. The reaction products consist largely of methane and to a lesser extent ethane, ethylene, and hydrogen and a solid deposit on the walls of the reaction vessel. A reaction mechanism involving methyl radicals is proposed. The apparent energy of activation, calculated on this basis, is found to be 45 kcal., in agreement with the experimentally determined figure. The high ratio of methane produced to alkyl decomposed suggests that aluminum trimethyl in the vapor state at room temperatures must consist at least of tetrameric molecular complexes. In the presence of hydrogen, both the decomposition rate and the energy of activation are reduced as compared to the decomposition of the pure alkyl, a puzzling effect for which no satisfactory explanation can at present be offered. Photochemically, the presence of hydrogen seems to have little effect on the over‐all decomposition but enhances the relative amount of methane slightly. In this, aluminum trimethyl appears to differ from mercury dimethyl which is known to decompose faster thermally and photochemically in the presence of hydrogen than in its absence.