SVNTHKSIS OF ORGANIC COMPOUNDS m IONIZING RADIATION 



qucnt introduction of the active nucleide into the complex molecule. Such 

 labelled products are, in general, very difTicult to synthesize by orthodox 

 methods and the products arc expensive and in hit^h demand in the biological 

 sciences. 



The irradiation of an air-saturated solution of cholesterol in acetic acid 

 gives a very interesting example of specificity of a radiation synthesis*'*. 

 Although there are many possible sites of attack in the sterol molecule, 

 acetylation occurs at a specific site with about 34 per cent yield of the 

 cholesteryl acetate: 



Irradiation in 

 aq. acetic acid 



CH3COO 



Certain polymerizations are uniquely caused by ionizing radiations. The 

 y irradiation of perfluoropropylene, perfluorobutadienc, perfluoroacrylo- 

 nitrile, perfluoroisobutylene and perfluoroamylpropylene gave liquid or solid 

 polymers of low chain length, while 1,1-dihydroperfluorobutylacrylate gave 

 a rubbery polymer which is highly cross-linked*^. It seems that polymeriza- 

 tion at very low temperatures or in the solid state is specific to ionizing 

 radiation. An example is the polymerization of tetraethylene glycol 

 dimethacrylate at — SS'^C *^ or of isobutene in the liquid jjhase at — 80°C ^^. 



The irradiation of oleic acid gives rise, among other polymerized acids, to 

 the decarboxylation product 8-heptadecene. This compound can easily be 

 separated in a very pure form*'. 



Although none of the examples given above refer to processes which are 

 likely to be taken up by the chemical industry, it is clear that there are many 

 reactions yet to be found which will be developed in time. The possibilities 

 are enormous and so is the developmental research required. 



(b) In cases where chain reactions occur with the conversion of a large 

 mass of material after the initial ionizing event and where chain initiators 

 are normally a more expensive catalyst or source of radiation (u.v. or electric 

 discharge) — 



There are many reactions which proceed by chain processes to yield 

 products of commercial importance. Perhaps the most striking technical 

 example is the halogenation of benzene. In the presence of u.v. or y radiation 

 a rapid reaction occurs between chlorine and benzene to form an isomeric 

 mixture of 1,2,3,4,5,6,-hexachlorocyclohexane and the temperature cannot 

 be controlled easily. Greater control is possible with the reaction of a solution 

 of benzene (20 per cent) in carbon tetrachloride. (In this case, Gy(C6HgCl6) 

 = 85,000). In general, the yields of the y-isomer are about 12 per cent for 

 each type of initiation. This suggests that the primary process in each case 



is CI2 >2C1 and, apart from reasons of convenience, there is no advantage 



of one type of radiation over the other. 



The chlorination of toluene*^ is, however, quite a different reaction when 

 it is initiated by y radiation from that initiated with u.v. In the latter case 



106 



