K. H. NAPIER AND J. H. GREEN 



is unlikely \\ ith iodination. However, the fact that there are hydrogen atoms 

 present in the irradiation mixture does not mean that all molecular hydrogen 

 comes from hydrogen atoms reacting as follows : 



H- + C^H.^ > H, + C,H;, (12) 



Experiments by Dewhurst^ and Dorfman^" on the effect of scavengers on 

 the hydrogen yield, have shown that small amounts of scavengers cause the 

 hydrogen yield to drop to about 65 per cent of that from irradiations without 

 scavengers, and then to remain constant at this level with increased scavenger 

 concentration. These results are interpreted to mean that 65 per cent of 

 the hydrogen is from a molecular detachment process, and that the remaining 

 35 per cent is from hydrogen atoms. 



As the total dose increases, the proportion of products eluted through the 

 columns becomes at first higher until most of the iodine has become organi- 

 cally combined, but then this proportion decreases slowly. This indicates 

 that for the investigation of primary products, the experimental results 

 should be extrapolated to zero dose. 



A further complicating factor is the scavenging for free radicals by the 

 alkyl iodides themselves. The smaller alkyl iodides seem to be more efficient 

 in this respect than the larger iodides. If a solution of labelled ethyl iodide 

 in pentane is irradiated and then put through the chromatographic column, 

 products corresponding to secondary and primary amyl iodides are formed. 

 The ratio of the concentrations of these is the same as in the original iodine 

 scavenging irradiations. This is a secondary effect, but complicates the 

 primary effects, as irradiation must be sufficient to cause reaction of at least 

 50 per cent of the total iodine, to give convenient solutions for analysis. 



Of the products formed by the iodine scavenging reactions, most may be 

 explained by the scavenging of free radicals formed by simple, bond scission. 

 However, the presence of secondary propyl, butyl and amyl iodides is 

 difficult to explain. One idea^ is that, when charged C5H+2 decomposes, it 

 does so according to equations (5), (6) and (7). The charged ions have all 

 the excess energy left after splitting, and the methyl and ethyl free radicals 

 have only thermal energies. It seems that the ions rearrange, and it may be 

 calculated that the secondary structure is more stable than the primary 

 structure^^ When these ions are neutralized and lose their energy by col- 

 lision with other molecules, they then may combine with iodine. The pres- 

 ence of these iodides in the products indicates that a process of this kind is 

 likely. 



Organic iodides, that are not eluted through the chromatographic 

 columns, are either iodides higher than C5 or, more probably, polyiodides. 

 The polyiodides could be formed by addition of iodine to the double bond 

 left by molecular abstraction of hydrogen. Dewhurst^ has quoted results 

 that the amount of double bonding formed by the irradiation of hexane 

 decreases from a G- value of 1 -3 in pure hexane to 0-6 when there is iodine 

 in the solution. This agrees with the theory that iodine adds to these double 

 bonds. Neither Dewhurst nor ourselves have found any alkyl iodides with 

 carbon chains longer than the parent hydrocarbon. 



In interpreting results from scavenging techniques one is faced with the 

 question of the efficiency of these scavengers. It is realized that iodine as a 



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