KINETICS AND MECHANISMS OF THE THERMAL DECOMPOSITION OF ACETALDEHYDE: I. THE UNINHIBITED REACTION

Abstract

The kinetics of the uninhibited decomposition of acetaldehyde have been reexamined. The initial rates of the decomposition of pure acetaldehyde show strict three-halves-order dependence at temperatures from 480 to 525 °C, and the activation energy is 47.6 kcal per mole. Foreign gases, which decrease the rate of reaction, cause a significant increase in order. The rate of ethane formation is second order in acetaldehyde, and Trenwith has found the hydrogen formation to be second order in acetaldehyde. The results are shown to be consistent only with a mechanism involving second-order initiation and the third-order reaction 2CH3 + M → C2H6 + M as the terminating step. The rate of the initiation process is increased only to a small extent by the addition of inert foreign gases; it is suggested that the initial process may be CH3CHO + CH3CHO → CH3CHOH + CH3CO, with a subsequent breakdown of CH3CHOH into CH3CHO + H. The mechanism is shown to account for the overall kinetic behavior and for the formation of the minor products.