The Chain Photolysis of Acetaldehyde in Intermittent Light

Abstract

(1) By comparison of the data on the photochemical decomposition and the azomethane‐induced decomposition of acetaldehyde vapor, it has been concluded that the chain‐carrying steps in the two cases are different. For the photolysis the following mechanism is suggested: $$\begin{array}{cc}\text{(1)}& {\mathrm{CH}}_3\mathrm{CHO}\mathrm{+h\nu \to }{\mathrm{CH}}_3+\mathrm{CHO}\mathrm{,\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}}\\ \text{(2)}& \mathrm{CHO}+{\mathrm{CH}}_3\mathrm{CHO}\to {\mathrm{CH}}_4+\mathrm{CO}+\mathrm{CHO}\mathrm{,\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}}\\ \text{(3)}& \mathrm{CHO}+{\mathrm{CH}}_3\to \mathrm{stable\; products}\mathrm{\hspace{0.17em}(}\mathrm{chain-ending}\mathrm{),}\\ \text{(4)}& \mathrm{CHO}\to \mathrm{stable\; products}\mathrm{\hspace{0.17em}(}\mathrm{chain-ending}\mathrm{).}\end{array}$$ (There are possibly other chain‐ending steps similar to reaction (3)). (2) Experiments have been performed using intermittent light at 200° and 300°. As a result of these experiments, estimates have been made for the individual rate constants, k₂, k₃, and k₄, and it has been found that the activation energies are 2.6, 11.3, and 7.8 kcal., respectively. (3) It has been found that k₂ and k₃ check closely with the collision number multiplied by the appropriate Arrhenius factor. There is little or no evidence of necessity for mutual orientation of the molecules. The significance of this has been discussed. Reaction (4) probably takes place at the walls, and the value of k₄ is of the same order of magnitude as the fraction of the molecules hitting the walls per second, times the appropriate Arrhenius factor. (4) The extent of and effect of diffusion of radicals beyond the illuminated region have been considered.