|MadSci Network: Chemistry|
Acyl radicals can be generated in a few different ways. The problem is that they decarbonylate rapidly unless they are in the presence of an efficient trapping agent. An example of this is in the auto-oxidation of aldehydes. The aldehyde C-H bond is relatively weak and therefore the hydrogen is abstracted by an oxygen radical. The resulting acyl radical is immediately trapped by a molecule of oxygen to generate a peroxyl radical that then abstracts another hydrogen from another aldehyde C-H to continue the auto-oxidative cycle. In the Norrish II reaction, photolytic cleavage of the C-C bond to a carbonyl carbon generates an acyl radical and a carbon centered radical (usually in the same molecule) these then rearrange and recombine to form a new compound. This reaction tends to be low yielding and unpredictable due to the high energy of the intermediate radical species and the many processes that radicals can undergo. I think the fenton reaction, where an Fe(II) salt promotes the formation of an oxygen centered radical by homolysis of an O-O bond, if the oxygen centered radical was allowed to abstract a hydrogen from an aldehyde and the acyl radical that was formed could be trapped efficiently. It would be tough to find a trapping agent that didn't readily trap the oxygen radical directly though! In my research as an undergraduate I made a series of azodicarboxylates. These compounds can, under certain conditions, decompose to release dinitrogen with concomitant formation of two acyl radicals. But again, the radicals are short lived and go on to do other chemistry in a relatively uncontrolled fashion. I was not studying this decomposition in my work so I cannot say anything more about the utility of this method to make acyl radicals. One other way to form an acyl radical is by the thermal decomposition of the Barton ester of a beta-keto acid. In this method, the acid fuctionality decarboxylates to give the acyl radical that will decarbonylate if it is not efficiently trapped.
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