Photobehavior of Alkyl Halides

Kropp, P.J. (1984), Photobehavior of Alkyl Halides in Solution Radical, Carbocation, and Carbene Intermediates, Acc, Chem. Res. 17, 131. 

Alkyl iodides afford mixtures of radical- and ion-derived photoproducts in solution, with the latter usually predominating. Indeed, this is a powerful method for generating carbocations, including many that cannot be readily prepared by other methods. Alkyl bromides display similar photobehavior, but with a lower proportion of ionic products. Analogous behavior has also been observed for phenyl thioethers and selenoethers, as well as some organosilicon iodides. In a process related to the formation of ionic intermediates, irradiation of dihalomethanes in the presence of alkenes results in cyclopropanation, a synthetically useful procedure that complements traditional methods. This chapter, which is concerned with alkyl halides, is a major expansion of an earlier review. The solution-phase photobehavior of aryl, benzylic, and homobenzylic hahdes has been reviewed, along with that of alkyl systems." The photobehavior of alkyl halides in the gas phase has also been reviewed. ... 

The photobehavior of alkyl halides varies with structure, depending on the ease of P-ehmination. Bridgehead and Related Halides Nucleophilic Trapping vs. Reduction... 

Kropp and his co-workers reported competing ionic and radical photobehavior for a number of alkyl halides. Photolysis of a-halo ketones (ROCCX) is understood to proceed via competing homolysis of CC and CX bonds as the primary processes. Tomioka et al. also proposed that irradiation of a-halo ketones in alcohols brings about competition between ionic and radical reaction paths with the formation of photoalcoholysis and photoreduction products. During the course of our studies, it was found that irradiation of a-iodocycloalkanones in the nonpolar solvents (e.g., hexane) afforded mainly the elimination product, an a,(3-unsaturated cyclic ketone, accompanied by a small amount of photoreduction product. " Therefore, it is considered that this photochemical behavior can be explained as shown in Figure 56.1 on the basis of the results mentioned above. 

McNeely, S.A., The Photobehavior of Unsaturated Alkyl Halides, Ph.D. dissertation. University of North Carolina, Chapel Hill, 1976. 

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