I t is confirmed t h a t the yields of all products in the photooxidation of azomethane a t relatively high oxygen pressure depend on conversion in a manner which would be explained if a reactive hydrogen donor were produced in the early stages of the reaction. Evidence is presented which indicates t h a t formaldehyde cannot be active as a n inhibitor in the system a t 162' C. I t is suggested that methyl radicals react with oxygen in two ways. The third order formation of methyl peroxy radicals leads to methoxy radicals and, eventually, to methanol, while the bimolecular reaction between methyl radicals and oxygen leads to a vibrationally excited state of formaldehyde. Thc latter is thought to undergo oxidization to performic acid, which acts as the inhibitor in the system. Yields of formaldehyde, of nitrogen in excess of that formed in the primary process, and of nitrous oxide are linearly related regardless of the conversion up to about 4%. Methanol is a major product of the oxidation and, if account is taken of its yield, a carbon balance of the order of 90% is obtained.

In a further investigation of the photooxidation of azomethane it was found t h a t the yields of all products depend on the fractional conversioll in a manner which suggests that some inhibitor is produced. Products from experiments a t lower pressures of azolnethane are obtained in yields appropriate to the higher conversions. Only very slnall changes in yields are observed for relatively large variations in light intensity. T h e implications of these facts t o the mechanism of the photooxidation are discussed.In an earlier i~~vestigation of the photooxidation of azomethane ( I ) it was postulated that formaldehyde should be a major product of the reaction. I t proved impossible to detect this product, however, presumably because of the well-known tendency of the material to polymerize a t temperatures below 100" C (2).Preliminary work, iildicatiilg that formaldehyde was indeed an important product, was done as follows. The mixture of reaction products, unreactecl azomethane, and oxygen was condensed a t -196" C in a small side arm attached to a grease-free reaction system. After the side arm was allowed to warm to room temperature, the vapor phase was removed by pumping for several minutes and the side arm sealed off. The invisible residue in the side arm was dissolved in dilute H2S04 and the chromotropic acid test (3) applied to the resulti~lg solution. Positive identification was achieved, which was not due to hydrolysis of residual azomethane as shown by the absence of a test on the unphotolyzed mixture.A reaction system was constructecl based 011 this technique for the ailalysis of formaldehyde. E X P E R I M E N T A LThe reaction system consisted of a fused quartz reaction cell (4.8 cin i.d., 10 cm long, -180 cm3 volume), a glass-enclosed, magnetically driven stirrer, a U-tube through which the cell contents were circulatecl, and conilectiilg tubing. Reactants were introduced from storage and manometers through a stainless steel diaphragm valve (Hoke, No. 413) and led to the analytical system through a similar valve. The entire system, including the valves but excluding the U-tube, was housed in a thermostatted air oven. Gradients and fluctuations in the region of the cell were less than 1" C. Other parts of the oven had larger gradients so that temperature differences of as much as 5" C existed between the cell and other units in the oven. The U-tube, which extended through the walls of the oven, could be kept a t oven temperature with heating tape or cooled to any temperature desired.Gases were led from the reaction system through heated tubing to a removable U-tube (Dow Corning Silicone greased joints), and thence to a Hg cutoff leading to LeRoy stills (4) and a grease-free gas burette. 'Manz~script received M a y 8, 1969.

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