288 BIOLOGICAL EFFECTS OF RADIATION 



of this reaction has been carried out by Noyes and his associates (48). 

 The reaction is quite complex and involves both addition of chlorine to 

 benzene and substitution of hydrogen by chlorine. A short chain is 

 involved in the reaction. The rate of the reaction is proportional to the 

 square root of the light intensity and directly proportional to the pressure 

 of the chlorine and the pressure of the benzene. The fact that the rate 

 of reaction depends upon the square root of the light intensity indicates 

 that chlorine atoms are involved, and the primary process is doubtless 

 the dissociation of the chlorine molecule into chlorine atoms. The large 

 quantum yield can be satisfactorily accounted for by a series of uiter- 

 mediate reaction steps involving hydrogen atoms or chlorine atoms. 



The experiments were carried out in all-glass apparatus, and the total 

 pressure of the gases was measured with a glass diaphragm. The chlorine 

 remaining after illumination was determined by treatment with potassium 

 iodide, and the hydrochloric acid formed was obtained by direct analysis. 

 Experiments were carried out at 3660 and 3130 A. The interpretation 

 of the reaction is seriously complicated by the fact that solid and liquid 

 products condense out on the walls of the reaction chamber. These 

 products are then subject to further chlorination. Substitution and 

 addition take place simultaneously, particularly after the benzene has 

 been partially chlorinated, but at low benzene pressures almost the entire 

 initial photochemical reaction is that of addition. The final reaction 

 products, after long periods of illumination, always approach the composi- 

 tion of the dodecachlorocyclohexane (C6CI12). 



The chlorination of chlorobenzene was studied in a similar manner 

 (22) in an attempt to decide the correct mechanism for the photochlorina- 

 tion of the benzene. The ultimate product is the same as for the benzene. 

 The relative amounts of substitution and addition products were carefully 

 determined, and several mechanisms were suggested and critically 



discussed. 



Chlorination of Tetrachloroethylene.— This is an interesting reaction 

 because there is no opportunity for chlorine to substitute for hydrogen, 

 and the only product would appear to be a simple one, — hexachloro 

 ethane (C2CI6). However, as in many other photochemical reactions, 

 and thermal reactions as well, the reaction is seriously influenced by 

 the presence of oxygen, even in small amounts. The reactions have been 

 studied in detail by Dickinson and his associates, both in the gas phase 

 (14) and in a solution of carbon tetrachloride (15, 31). 



The blue line, 4358 A of the mercury arc was isolated by means of 

 filters, and the light was passed through a pyrex cylindrical cell with 

 polished ends. The light passing through the cell was measured by a 

 thermopile and galvanometer. 



In the absence of oxygen, the rate of the chlorination was directly 

 proportional to the concentration of chlorine and proportional to the 



