Question 6.WP.7: The gas phase decomposition of ethanal occurs according to C......
The gas phase decomposition of ethanal occurs according to
{\mathrm{CH}}_{3}{\mathrm{CHO}}(\mathrm{g})\to{\mathrm{CH}}_{4}(\mathrm{g})+{\mathrm{CO}}(\mathrm{g})
The major products are \mathrm{CH}_{4} and CO, with \mathrm{C}_{2}\mathrm{H}_{6},\,\mathrm{CH}_{3}\mathrm{COCOCH}_{3}{\mathrm{~and~CH}_{3}C O C H}_{3} being minor products. The radicals {\mathrm{CH}}_{3}^{•},{\mathrm{CH}}_{3}{\mathrm{CO}}^{•}{\mathrm{~and~}}\mathrm{CHO}^{•} are present. Deduce a mechanism which fits these facts.
- Initiation produces radicals by homolytic splitting of a bond. A rule of thumb (Problem 6.2) suggests that C-C splits more readily than C H, which in turn splits more readily than C-O or C=O, and a good first guess is
{\mathrm{CH}}_{3}{\mathrm{CHO}}\to{\mathrm{CH}}_{3}^{•}+{\mathrm{CHO}}^{•}
confirmed by the presence of the two radicals.
- Propagation produces the main products,\mathrm{CH}_{4} and CO, often by H abstraction reaction from reactant. There are two H which can be abstracted, and the presence of {\mathrm{CH}}_{3}\mathrm{CO}^{•} suggests that
{\mathrm{CH}}_{3}^{•}+{\mathrm{CH}}_{3}{\mathrm{CHO}}\to{\mathrm{CH}}_{4}+{\mathrm{CH}}_{3}{\mathrm{CO}}^{•}
is more likely than
{\mathrm{CH}}_{3}^{•}+{\mathrm{CH}}_{3}{\mathrm{CHO}}\to{\mathrm{CH}}_{4}+~^{•}{\mathrm{CH}}_{2}{\mathrm{CHO}}
- The second propagation step must regenerate the first chain carrier, here {\mathrm{CH}}_{3}^{•}, and produce a major product, suggesting
{\mathrm{CH}}_{3}{\mathrm{CO}}^{•}\to{\mathrm{CH}}_{3}^{•}+{\mathrm{CO}}
- The minor products are produced by reactions of the chain carriers. Here the nature of the products indicates recombination reactions.
{\mathrm{CH}}_{3}^{•}+{\mathrm{CH}}_{3}^{•}\to{\mathrm{C_{2}H}}_{6}
\mathrm{CH}_{3}^{•}+\mathrm{{CH}}_{3}\mathrm{CO}^{•}\to\mathrm{CH}_{3}\mathrm{COCH}_{3}
\mathrm{CH}_{3}\mathrm{CO}^{•}+\mathrm{CH}_{3}\mathrm{CO}^{•}\to\mathrm{CH}_{3}\mathrm{COCOCH}_{3}
The last two minor products confirm that H abstraction occurs on the CHO group of {\mathrm{CH}}_{3}{\mathrm{CHO}}.
- This leaves the non-chain carrier {\mathrm{CHO}}^{•}, produced in initiation, unaccounted for. It might only be present in trace amounts, as initiation occurs very infrequently compared to propagation, and if so it will be removed in a subsequent reaction to produce a trace product. No evidence is given as to what this will be.
A possible mechanism which fits the experimental facts:
\mathrm{CH}_{3}\mathrm{CHO}\stackrel{k_{1}}{\rightarrow}C H_{3}^{•}+\mathrm{CHO}^{•}
\mathrm{CH}_{3}^{•}+\mathrm{CH}_{3}\mathrm{CHO}\stackrel{k_{2}}{\longrightarrow}\mathrm{CH}_{4}+\mathrm{CH}_{3}\mathrm{CO}^{•}
\mathrm{CH}_{3}\mathrm{CO}^{•}\stackrel{k_{3}}{\longrightarrow}\mathrm{CH}_{3}^{•}+\mathrm{CO}
\mathrm{CH}_{3}^{•}+\mathrm{CH}_{3}^{•}\stackrel{k_{4}}{\rightarrow}C_{2}\mathrm{H}_{6}
\mathrm{CH}_{3}^{•}+\mathrm{CH}_{3}\mathrm{CO}^{•}\stackrel{k_{5}}{\rightarrow}\mathrm{CH}_{3}\mathrm{COCH}_{3}
\mathrm{CH}_{3}\mathrm{CO}^{•}+\mathrm{CH}_{3}\mathrm{CO}^{•}\stackrel{k_{6}}{\rightarrow}\mathrm{CH}_{3}\mathrm{COCOCH}_{3}