No abstract

The quantum yield of the gas phase photochlorination of C2Cl4 and C2Cl3H have been measured. The light intensity of a Hg vapour lamp transmitted by a filter has been measured with a potassium ferrioxalate actinometer. The spectral intensity distribution of the light was measured and corrections were applied for intensity loss and multiple reflexions. The quantum yield is given by Φ = Ia−1/2k(Cl2) with log k = –(1065 ± 8)/T + 3.941 ± 0.021 for the photochlorination of C2HCl3. Conditions are described which permit one to use this reaction as an actinometer for other photochlorinations, so as to avoid the corrections mentioned above and permitting one to use the approximation Ia = 2,303. Ioϵl(Cl2) even at high absorption (up to about 180 mm Cl2 with light of about 4300 A). The photochlorination of C2Cl4 has a somewhat more complexe mechanism and is therefore not very adequate for actinometry.

La spCcificitC des lialoy Cnatioiis atomiques V. -Etude des conditions de statioiinaritC et du comportement non st alionnairc Par J. ADAM (l), S. DUSOLEIL et P . GOLDFINGER (Bruxelles) SUMMARY: 1. It is shown that for complex reactions of the type of the atomic halogenation reactions the real rate cannot be distinguished from the rate calculated by the mwa1 steady state treatment)) if ]c,/kt < 2 &/0,693 (A),;kP and kt are the rate constants of the chain propagation and termination and E / ( A )~ the relative accuracy of measuring initial concentrations (A),. This means that the time required for a measurable amount of reaction is large compared to the lifetime of the chain. The ratio of the concentrations of two chain carriers is practically equal to the steady state ratio after a time which is large compared to the Zifetime of one chain cawier. The conditions are discussed under which the concentration of the chain carriers must be low and the chains long. -2. These considerations permit one to describe the conditions which me necessary for integrating the equations representing the rate of formation and disappearance of the intermediates and which are necessary for the calculation of the ratios of reaction rates in steady and intermittent light. -3. It is shown that the photochlorinations of C2C1, and C,HCl, previously studied satisfy the above conditions. Preliminary results with intermittent light yield values for the frequency factors of elementary reactions in agreement with previous estimates.

IT has been shown recently' that it is useful to consider a general mechanism for atomic halogenation reactions, which, in the case of the photochlorination of C2Cl4 and C2HC1 6 , is given by : hv Cl2 --+ 2Cl(1)(5)