2(» 1{ \I)I AlION lUOLOGY 



certain gciuMal rules iiiid aiialojiiics are helpful in fi;ui(linjj; the inluition of 

 the kineti('i.sl. It is rarely true that a reaetion mechanism c-an ho 

 re^rarded as true or even as very pr()l)ahly true. Usually all that can be 

 claimed for a mechanism is that it is consistent with all pertinent infor- 

 mation. This does not pieclude the possibility that some other mecha- 

 nism, or even many other mechanisms, may likewise be compatible with 

 the data. A mimber of reaction mechanisms which were at one time 

 tentatively accepted had to be discarded later when more information 

 became available. In spite of these iinsatisfactor\' characteristics of the 

 mechanisms of complex reactions, there ajjpears to be no way of obtaining 

 information about complex reactions other than by postulating and test- 

 ing mechanisms. 



EXAMPLES OF THE PRINCIPAL TYPES OF PHOTOCHEMICAL REACTIONS 



The types of photochemical reactions which have been studied most 

 extensively are decompositions, oxidations, polymerizations, hydrolyses, 

 and internal rearrangements. A great part of the early photochemical 

 literature is devoted to oxidations, especiall}^ chlorinations. Much of the 

 classical information about chain reactions was derived from these latter 

 measurements. From a biological point of view the details of these inves- 

 tigations are of little immediate interest, and for this reason they are 

 omitted from the following discussion. 



DECOMPOSITION REACTIONS 



The photochemical decomposition of a wide \'ariety of compounds, 

 ranging in complexity from hydrogen iodide to complex azo dyes, has been 

 investigated, in many cases with considerable care. The decomposition 

 of hydrogen sulfide is a good example of a carefully studied reaction of a 

 simple molecule. The al)sorption of hydrogen sulfide is continuous, 

 becoming appreciable at about 2800 A and reaching a maximum near 

 1900 A. The corresponding primary act is presumably one of direct 

 ojitical dissociation, yielding a hydrogen atom and a hydrosulfide radical. 

 Photochemical measurements (Forbes et al., 1938) made with radiation 

 wave length of 2080 A demonstrate that one molecule of hydrogen sulfide 

 is decomposed for each photon absorbed, over a wide range of pressures 

 and of light intensities. The following mechanism is consistent with all 

 available reliable information and, for this simple system, is probably 

 correct : 



H,S + /^j' ^ IIS + H (primary step), 



H -h HoS -> H, + HS j 

 2HS -> HoS + S 



(secondarj^ steps) . 



The photolysis of a variety of aldehydes and ketones has been investi- 

 gated extensively and in detail (Xoyes and Leighton, 1941). The results 



