PHOTOCHEMISTRY 287 



In this case the oxygen prevents the easy addition of bromine to cinnamic 

 acid; only when the bromine molecules are excited by the absorption of 

 light or possibly when they are dissociated into atoms by the light, is it 

 possible for bromine to add easily to the double bond of cinnamic acid. 

 Bromine-hydrogen Reaction. — Bromine vapor shows discontinuous 

 absorption above 5107 A, and at wave-lengths shorter than this it shows 

 continuous absorption. In this region bromine molecules are dissociated 

 into bromine atoms, one of which exists in the excited state. When 

 mixed with hydrogen at room temperature, no reaction takes place, but 

 at temperatures above 150° there is direct union of the bromine with 

 hydrogen when illimiinated, giving hydrobromic acid. The following 

 reaction probably represents the course of the reaction: 



1. Bro + /ii/ = Br + Br 



2. Br + Hs = HBr + H 



3. H + Br2 = HBr + Br, etc. 



4. Br + Br = Bra 



The kinetics of this reaction was worked out by Bodenstein and Liitke- 

 meyer (8). 



At room temperature the bromine atoms recombine (probably 

 through collisions with a third body), regenerating the original bromine 

 molecules, as in equation 4. At higher temperatures the bromine 

 atoms obtain sufficient energy to react with hydrogen molecules, giving 

 hydrobromic acid and hydrogen atoms, as shown in equation 2. When 

 this reaction starts in, it is possible for the hydrogen atoms to continue 

 a chain, as given by equation 3, followed again by a bromine atom com- 

 bining with another hydrogen molecule, etc. When hydrobromic acid 

 becomes quite concentrated, the reverse of equation 2 may take place to a 

 considerable extent. These facts, and several others, are nicely sum- 

 marized by the following equation: 



^[HBr] _ rj [H2] 



where k and k' are constants and / is the intensity of light absorbed. 



The dependence of the photochemical reaction on the square root 

 of the light intensity is frequently found when dissociation of a molecule 

 into two atoms is the primary process. It follows directly from the mass 

 law. This photochemical reaction and the thermal reaction are dis- 

 cussed in detail by Kistiakowsky (26). 



Chlorination of Benzene. — When a mixture of chlorine and benzene 

 is illuminated with ultra-violet light or blue light, a variety of chlorinated 

 benzene products is produced. The reaction has been studied by several 

 different investigators, in solution and in the gas phase. A careful study 



