KINETICS AND MECHANISMS OF THE PYROLYSIS OF DIMETHYL ETHER: I. THE UNINHIBITED REACTION

Abstract

An experimental study has been made of the thermal decomposition of dimethyl ether, the temperature range being 500 to 550 °C and the pressure range 100 to 700 mm Hg. A considerable surface effect was noted, and the results were not very reproducible. The reaction was of the three-halves order and the rate constant could be expressed as 2.98 × 1014e−54,900/RT cc1/2 mole−1/2 sec−1. On the basis of the results obtained in the presence of hydrogen sulphide (D. J. McKenney and K. J. Laidler. Can. J. Chem. 41, 2009 (1963)) it is concluded that the initiating step, the dissociation of CH3OCH3 into CH3O and CH3, is in its second-order region. In order for the overall order to be three-halves the main terminating step must be either of the type ββM or βμ. The concentrations of the various radicals, and the rates of the various chain-ending steps, are calculated from known or estimated kinetic parameters for the elementary processes. It is concluded that the predominant chain-ending step is probably CH3 + CH3 + M → C2H6 + N, but that there may be a significant contribution from CH3 + CHO + M → CH3CHO + M and from CH3 + CH2OCH3 + M → C2H5OCH3 + M.