Termination Products and Processes in the Pyrolysis of Dimethyl Ether

Abstract

Termination processes in the pyrolysis of dimethyl ether studied by trace product analysis using gas—liquid chromatography revealed the presence of methyl ethyl ether, 1,2-dimethoxyethane, and methanol. These ethers had been predicted in the work of Benson and Jain by including CH3OCH2 as a chain terminator, and their identification is evidence for the reaction CH3OCH2→CH3+CH2O being an energy transfer process in its second-order region. Methanol was also predicted from the reactions of CH3O or CH2OH. Quantitative measurement of the trace termination products were made to obtain yields for MeOEt, Me2O, and C2H6, which could be used to calculate the rate constant ratio k2′/k2 for the attack by CH3 on MeOEt compared to Me2O. When the steady state is approached from both sides, k2′/k2 ranges from 4–12 and is greater than unity, as expected. A reasonable difference in activation energy of about 2–4 kcal is estimated for these reactions. However, the MeOEt gas chromatography peak was broad when resolved from Me2O resulting in loss of precision and together with the required estimates in the steady-state equation (7) severely limit the usefulness of trace termination product analysis in determining new rate constants. Ethane was measured and compared to a calculated steady-state value. Because of attack on the ethane and the disappearance of radicals by other termination processes the ratio of the Me2O decomposed to the C2H6 found is not as good a measure of the chain length as those calculated from the rate law.