PHOTOCHEMISTR Y 29 1 



It is seen that this reaction is extremely compUcated, and it is no 

 wonder that different investigators, working under different conditions 

 of oxygen pressure, hght intensity, and size of reaction chamber, and 

 without special apparatus to remove impurities, have failed to agree. 



The Chlorination of Trichlorohromomethane. — This photochemical 

 reaction is a fairly simple one to interpret. The overall reaction goes 

 according to the following equation: 



Clo + hv -{- 2CCl3Br = 2CCI4 + Bra 



The chlorine atom drives out the bromine atom from the compound, 

 and there is very little chance for any other side reaction. There seems 

 to be no chance for the complication of a chain reaction. A quantum 

 yield of 0.9, slightly less than unity, was obtained in the pure liquid and 

 in solutions, even when highly diluted with carbon tetrachloride or 

 silicon tetrachloride (19). If these solvents are not pure, but contain 

 dissolved oxygen, the reaction is inhibited. 



Eder's Reaction. — The photochemical reaction between mercuric 

 chloride and ammonium oxalate produced by visible light in solution 

 has been the subject of many investigations. Mercurous chloride is 

 precipitated and carbon dioxide is evolved, as given by the following 

 reaction : 



2HgCl2 + (NH4)2C204 = Hg2Cl2 + 2NH4CI + 2CO2 



The reaction can be followed either by the evolution of the gas or by 

 weighing the amount of the precipitate formed. The reaction involves 

 a long chain, so that large effects are produced for small amounts of light. 

 The reaction, however, is very complex. It involves an induction period 

 and is very sensitive to numerous inhibitors and catalysts. It has been 

 used sometimes as an actinometer, but it is too erratic and too subject to 

 catalytic effects to be recommended for this purpose. 



The kinetics of a similar reaction between potassium oxalate and 

 mercuric chloride has been thoroughly investigated by Roseveare and 

 Olson (45). The reactions can be sensitized to the longer wave-lengths 

 of light by the addition of dyes, such as eosin and various ions. 



Fulgides. — Certain substances possess the property of changing 

 temporarily their color when exposed to light. This phenomenon is 

 called phototropy. Prominent in this group of substances are the 

 fulgides, which are derivatives of the compound: 



CHi=C— COv 



I >o 



CH2=C— CO/ 



When three phenyl groups are substituted for hydrogen, a yellow powder 

 is produced, which turns brown on exposure to blue light and reverts to 

 yellow when placed in the dark (49). The changes are reversible, but 



