PHOTOCHEMISTRY 289 



square root of the number of photons absorbed by the system. The 

 quantum yield ranged from 300 to 500 molecules reacting for each 

 quantum absorbed. The reaction in carbon tetrachloride solution has 

 very nearly the same characteristics, and the quantum yields are about 

 the same, slightly higher in certain cases. According to one proposed 

 mechanism, the large quantum yield can be explained on the basis of a 

 chain reaction involving the radical C2CI5. 



The photochlorination of tetrachloroethylene is strongly inhibited 

 by oxygen, even in small amounts, but in place of the chlorination reac- 

 tion an oxidation reaction takes place. This chlorine-sensitized photo- 

 oxidation of tetrachloroethylene gives chiefly trichloroacetylchloride, 

 together with small amounts of phosgene. The rate of this reaction is 

 proportional to the number of photons absorbed — not to the square root 

 of the number, as in the chlorination reaction. It is proportional also 

 to the first power of the chlorine concentration, is independent of the 

 amount of oxygen, and is only slightly dependent upon the concentration 

 of the tetrachloroethylene. 



The quantum yield is about 300 molecules per photon in the gas phase. 

 In carbon tetrachloride solution the quantum yield for this reaction is 

 much less, approximately 2 molecules per photon. It is independent 

 of concentration or light intensity in the gas reaction, but in carbon 

 tetrachloride solution it depends on the concentration of the chlorine. 

 An increase of 10°C. causes the reaction to go 1.2 times as fast. In 

 solution, after the oxygen is used up, the reaction goes much faster, as 

 might be expected from the fact that the quantum yield is 300 in the 

 absence of oxygen and 2 in the presence of oxygen. In comparing the 

 experiments in gas phase and in solution, it may be pointed out that 

 the solvent seems to stop the oxidation chain but not the chlorination 

 chain. 



Chlorine-hydrogen Combination. — This reaction has been studied many 

 years, and more has been published on it than on any other photochemical 

 reaction. It has been a most baffling reaction, and many apparently 

 contradictory results have been reported. 



The quantum yield is enormous, running up as high as a million mole- 

 cules per photon absorbed. This fact led to the first proposal for a 

 chain-reaction mechanism, and historically the reaction is responsible 

 for a number of interesting developments in chemical kinetics and in 

 photochemistry. The reaction is complicated by this long chain mecha- 

 nism, by an induction period, and by the fact that small amounts of air 

 inhibit the reaction. The reaction is inhibited not only by oxygen but 

 by traces of various impurities. 



The most recent and perhaps the most exhaustive research on this 

 reaction is due to Ritchie and Norrish (44, 40). The gases, chlorine, 

 hydrogen, and, in some of the experiments, oxygen and hydrogen chloride 



