The pyrolysis of acetaldehyde in the presence of nitric oxide

The kinetics of the acetaldehyde pyrolysis have been studied at temperatures from 450° to 525°C, at an acetaldehyde pressure of 176 torr and at 0 to 40 torr of added nitric oxide. The following products were identified and their rates of formation measured: CH 4 , H 2 , CO, CO 2 , C 2 H 4 , C 2 H 6 , H 2 O, C 3 H 6 , C 2 H 5 CHO, CH 3 COCH 3 , CH 3 COOCHCH 2 , N 2 , N 2 O, HCN, CH 3 NCO, and C 2 H 5 NCO. Acetaldehyde vapor was found to react with nitric oxide slowly in the dark at room temperature, the products being H 2 O, CH 3 COOCH 3 , CO, CO 2 , N 2 , NO 2 , HCN, CH 3 NO 2 , and CH 3 ONO 2 . The rates of formation of N 2 and C 2 H 5 NCO depend on how long the CH 3 CHO‐NO mixture is kept at room temperature before pyrolysis; the rates of formation of the other products depend only slightly on the mixing period. The pyrolysis of “clean” CH 3 CHO–NO mixtures (i.e., the results extrapolated to zero mixing time, which are independent of products formed in the cold reaction) are interpreted as follows: (1) There are two chain carriers, CH 3 and CH 2 CHO, their concentrations being interdependent and influenced by NO in different ways: the CH 3 radical is both generated and removed by reactions directly involving NO, whereas CH 2 CHO is generated only indirectly from CH 3 but is also removed by direct reaction with NO. (2) An important mode of initiation by NO is its addition to the carbonyl group with the formation ofwhich is converted into; this splits off OH with the formation of CH 3 NCO or CH 3 + OCN. (3) Important modes of termination are\documentclass{article}\pagestyle{empty}\begin{document}$$ \begin{array}{*{20}c} {{\rm CH}_3 + {\rm NO} \to {\rm CH}_3 {\rm NO} \to {\rm CH}_2 {\rm NOH} \to {\rm HCN} + {\rm H}_2 {\rm O}} \\ {{\rm CH}_{\rm 2} {\rm CHO} + {\rm NO} \to {\rm CH}_2 ({\rm NO}){\rm CHO} \to {\rm CH}({\rm NOH}){\rm CHO} \to {\rm HCN} + {\rm H}_2 {\rm O} + {\rm CO}} \\ \end{array} $$\end{document}The steady‐state equations derived from the mechanism are shown to give a good fit to the experimental rate versus [NO] curves and, in particular, explain why there is enhancement of rate by NO at higher CH 3 CHO pressures and, at lower CH 3 CHO pressures, inhibition at low [NO] followed by enhancement at higher [NO]. The cold reaction is explained in terms of chain‐propagating and chain‐branching steps resulting from the addition of several NO molecules to CH 3 CHO and the CH 3 CO radical. In the “unclean” reaction it is found that the rates of N 2 and C 2 N 5 NCO formation are increased by CH 3 NO 2 , CH 3 ONO, and CH 3 ONO 2 formed during the cold reaction. A mechanism is proposed, involving the participation of α‐nitrosoethyl nitrite, CH 3 CH(NO)ONO. It is suggested that there are two modes of behavior in pyrolyses in the presence of NO: (1) In the paraffins, ethers, and ketones, the effects are attributed to the addition of NO to a radical with the formation of an oxime‐like compound. (2) In the aldehydes and alkenes, where there is a hydrogen atom attached to a double‐bonded carbon atom, the behavior is explained in terms of addition of NO to the double bond followed by the formation of an oxime‐like species.