Thomas W. Davis

Determination of Free Radicals in Acetone Photolysis

Acetone has been photolyzed in a flow system at several temperatures in the pressure range 0.5 to 11 mm with exciting radiation of wave‐length 2650–2900A and 2537A. The free methyl radicals formed in the decomposition have been determined as lead tetramethyl. Approximately one methyl radical is formed for every four molecules of carbon monoxide, the actual proportion depending on temperature and pressure. Relatively more free radicals are formed at 100°C than at lower temperatures and more are formed at lower pressure than at high. A process involving the existence of a long lived metastable state for photoactivated acetone has been suggested to explain the results. The proposed scheme is found consistent with most of the published work on acetone photolysis.

Chain Length and Chain‐Ending Processes in Acetaldehyde Decomposition

The C–C bond strength in acetaldehyde is probably less than a previously postulated value of 93 kcal. and may be as low as the 75 kcal. value calculated by Grahame. The chain‐ending processes in the pyrolysis, in the azomethane‐induced decomposition, and in the photolysis of acetaldehyde are all bimolecular but it is clear that the processes in the second and the third decompositions cannot be identical. The uncertainty of the process in the pyrolysis introduces an uncertainty into Grahames calculation. In any event, the process is not the combination of methyl radicals in the gas phase to yield ethane. In the photolysis (and perhaps in the pyrolysis) HCO seems to be involved in the chain‐ending reaction. In the induced decomposition CH3CO seems to be concerned. The recombination of methyl radicals as a wall reaction may occur in packed vessels. Activation energies of the various reactions are discussed.

The Potential Energy Relationships in Normal and Excited Acetaldehyde

By a simple modification and extension of the system of potential energy hypersurfaces used to correlate the observations in the thermal and the photochemical decomposition of acetaldehyde it is shown that the following effects are explained: (1) the dependence on temperature and wave‐length of relative probabilities of free‐radical and ultimate molecule production in the primary photochemical act; (2) the disappearance of fluorescence at shorter wave‐lengths or at higher temperatures; (3) the separate maxima for band and continuous absorption; (4) the high quantum yield reported at 3340A.

Free Radical Determination in Biacetyl Photolysis

Biacetyl vapors have been decomposed by radiation of wave‐length 2537A in a flow system. The total amount of decomposition was determined by collection and analysis of the gaseous products. The amount of methyl radical formed was determined by allowing the products to pass over and react with lead mirrors. The amount of lead transported establishes the maximum quantity of methyl formed assuming a ratio of CH3 to Pb of 4:1. The data show that approximately one methyl group is released for every two or three molecules of carbon monoxide in the products and consequently that about half of the total reaction proceeds without the primary formation of free radicals. A Spence and Wild type of reaction scheme will explain both the analytical data and observations on fluorescence.