Alcohols mercury-photosensitized

Similar mercury-photosensitized alcohol dehydrodimerizations leading to glycols have been documented (Equation (17)).26... 

The possibility of mercury-photosensitized reactions was first predicted in 1922 by Franck (45) and experimentally verified in the sensitized decomposition of H2 by Cario and Franck (46). They found that free H atoms were produced when a mixture of Hg vapor and H2 was irradiated with the 254 nm Hg resonance line at room temperature. Bates and Taylor (23) studied the Hg-sensitized decomposition of methanol, ethanol, and ethylamine, and showed that with these compounds the rate of the sensitized reaction was faster than the direct photolysis by about two orders of magnitude. Again hydrogen was the major product. Aldehydes were formed from the alcohols. 

It therefore seems likely that C3H7OH2+ will be the major positive ion remaining in pure isopropyl alcohol after ion-molecule reactions, though it will be of the clustered form C3H7OH2+(C3H7OH)y, and that neutralization will proceed via Reaction 6. The mercury photosensitized decomposition of isopropyl alcohol shows a high quantum yield of H atoms (21) and a yield of H atoms from dissociation of excited isopropyl alcohol molecules might also be expected in the radiolysis. Hence, as for water vapor, the yield of H atoms from pure isopropyl alcohol will have... 

Another technique involving photolysis which is often effective involves exposure of a solid containing a trace of some photosensitive compound which absorbs in a region of lower energy than the onset of absorption by the solvent . Typically, dilute solid solutions of hydrogen peroxide in a wide range of solvents can be photolysed with a medium-pressure mercury arc. Early studies of this sort, described in Section III, involved the use of alcohols and it was shown that radicals characteristic of the alcohols were trapped. No evidence for trapped hydroxyl radicals was forthcoming and it was concluded that a secondary reaction occurred such as ... 

Holland and Cooper and Wells - comment on a side reaction which leads to the destruction of the photosensitizer and the formation of colored products at low concentrations of alcohol. This reaction may well be the same as that which was observed by Mooney and Stonehill and which was interpreted in terms of photosensitized decomposition of water, with subsequent hydroxylation of the anthraquinone salt. A similar type of reaction was noted by van Beek and Heertjes during their investigation into the chemical and photochemical degradation of anthraquinone mordant dyes in methanol. Irradiation of 3-nitroalizarin in methanol in Pyrex vessels, with visible or ultraviolet light from mercury-vapor lamps, leads to the formation of 4-methoxy-3-nitroalizarin. The methoxylation encountered in this instance would appear to be analogous to the hydroxylation of the sensitizer noted by Mooney and Stonehill, although, it must be pointed out, no such conclusion was drawn by the authors. 

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