No abstract

The ratio of the rate constants for disproportionation and combination for ethyl radicals, k d / k kd 2C2Hs + C2H4 + C2Hs has been measured over the temperature range 298-173 K in the gas phase. Ethyl radicals were produced by direct photolysis of ethylenefollowed by addition of hydrogen atoms to ethylene. At low temperatures the only important reactions of the radicals were combination and disproportionation. The ratio kd/kc was obtained from measurements of the rates of formation of ethane, butane, and butene. No change in the ratio kd/k, was observed over the temperature range studied, leading to the conclusion that Ed -E, = 0, within the experimental error. The significance of this result is discussed in relation to other measurements in both gas and liquid phase and to the nature of the transition state for this reaction.

Trifluoroacetaldehyde has been used extensively in numerous photolysis and pyrolysis experiments in which the CF3 radical is generated. However, almost invariably the CF3CHO is prepared through the dehydration of CF3CHO-• 2H2O by P2O5. We have found this method tedious and expensive. Alternatively, CF3CHO can be generated from trifluoroacetaldehyde ethyl hemiacetal or methyl hemiacetal by refluxing with an excess amount of concentrated sulfuric acid (yield ~50%). The CF3CHO is passed through a drying agent and stored at -78°C. The methyl hemiacetal can be purchased from Halocarbon Products Corporation, 82 Burlews Court, Hackensack, N.J. 07601.We acknowledge the help of Dr. N. Ogamachi of the Halocarbon Products Corporation in confirming the method outlined above.

The thermal decomposition of 95:5 mixtures of trifluoroacetaldehyde (TFA) and di-tert-butyl peroxide (DTBP) has been studied at 100 Torr over the temperature range of 390 to 440 K. The major decomposition products included CO, CF3H, CH3COCH3, and CH4 while C2F6, CF3CHOHCH3, CF3CH3, CF3COCH3, C2H6, (CF3)2CHOH, and H2 were also found. In addition to the usual reactions for TFA thermal decomposition, reactions of methyl radicals with TFA to form isopropoxyl radicals were found. The alcohol products result from H atom abstraction reactions of the isopropoxyl radicals while CF3COCH3 is a decomposition product. Arrhenius parameters for several reactions were determined: for DTBP decomposition, log k = 15.82 − 37.73/2.303RT; for H abstraction from TFA by CH3, log k = 8.30 − 7.37/2.303RT; for H abstraction from TFA by CF3, log k = 8.98 − 8.61/2.303RT. Consideration has also been given to several rate constant ratios for the formation and decomposition of isopropoxyl radicals.A study of the reaction order for the formation of CF3H, C2F6, and CH4 showed that the orders were 3/2, 1, and 1 respectively for these three products. A reaction mechanism involving 14 individual steps is proposed to explain the reaction products and the observed orders of reaction.

The thermal decomposition of trifluoroacetaldehyde a t temperatures of 471 to 519°C has been studied by measuring the rates of formation of CFIH and C2Fs. It is concluded that the high-pressure reaction mechanism involves a Rice-Herzfeld reaction scheme with firstorder initiation and second-order termination via CF, combination. However, a falloff in reaction rates is observed at pressures below 100 mmHg. The Arrhenius parameters of the three rate constants corresponding to the overall reaction, the initiation reaction, and an abstraction reaction have been evaluated.