Third Report of the Committee on Photochemistry Reprint and Circular Series of the National Research Council (1938) / Chapter Skim
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Free Radicals
Free Radicals
Pages 65-74
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From page 65... ...
From such studies are slowly accumulating a series of data, first qualitative and later quantitative, with the aid of which a more certain interpretation of secondary reaction paths can be deduced. The photoprocesses leading, in the primary process of absorption, to the production of free radicals include the photolysis of alkyl iodides, the photodecomposition of metal alkyls, the mercury-photosensitized hydrogenations of unsaturated hydrocarbons, and the mercury-photosensitized decompositions of saturated hydrocarbons.
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From page 66... ...
These continuous spectra point to dissociation in the primary absorption act, with radicals or saturated molecules as possible products in addition to the metal atoms or metal radical complexes. In the mercury-photosensitized hydrogenation of unsaturated hydrocarbons, when hydrogen is present in any marked amount, the primary process occurs between excited mercury and molecular hydrogen generaing atomic hydrogen.
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From page 67... ...
In the short wave banded region, West and Ginsburg assume the production of optically excited molecules and their interaction with normal iodide molecules. With lead tetramethyl vapor Leighton and Mortensen ;~11)
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From page 68... ...
THE SECONDARY PROCESSES The primary processes in the photolysis of methyl iodide, lead tetramethyl, mercury dimethyl and, according to Steacie and Phillips, in the photosensitized decomposition of ethane, all lead to the production of methyl radicals. There are, however, conspicuous differences in the products finally obtained.
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From page 69... ...
When, however, the iodine atoms are trapped by silver foil, reaction 9 appears to compete favorably with the methane-producing reaction (reaction 4~. A sequence of reactions to account for the photolyses of mercury and lead methyls is suggested by the following scheme of Thompson and Linnett: Hg(CH3~2 + hv = Hg(CH3)
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From page 70... ...
for the chain-propagating reactions is in accord with the quantum yield of unity at ordinary temperatures and of 2.2 at 200°C. The presence of only traces of methane in the room temperature product indicates that, owing to the weakness of the Hg C bond, this is broken in preference to the C H bond in the mercury alkyls struck by methyl radicals.
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From page 71... ...
Here, as West has shown, the products are predominantly ethane and ethylene, with no butane and with minor amounts of hydrogen and methane. As in the case of the methyl radicals from methyl iodide it is the secondary processes which must account for the non-formation of butane.
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From page 72... ...
The reactions must occur more readily than the methyl reactions, since even in presence of silver foil no butane was observed. The increased yield in the photolyses of the alkyl iodides in the liquid or dissolved state over that in the gaseous state is ascribed by West, at any rate in part, to the action of the solvent molecules in providing third bodies for the yield-increasing radical associations.
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From page 73... ...
There is no doubt that the majority of these results are consistent with a primary act producing free methyl radicals with minor, if any, intramolecular rearrangement to form stable molecules. The change in the character of the hydrocarbon products with temperature is consistent with the data on the reactions already discussed.
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From page 74... ...
The overall reaction becomes correspondingly more complex, the authors being of the opinion that the ethane formed results, not from the recombination of the methyl radicals, but frown decomposition of the more complex radicals and molecules. In view of the preceding discussion this conclusion should be accepted with great reserve.
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