Substitution mechanism operational classification

The interest in photoreactions that involve chemical change in an aromatic ring continues at a high level. Aromatic photosubstitution reactions have assumed a greater importance than they once had, but the classification of these reactions on a mechanistic basis is not easy because one of several different mechanisms may operate, and published information may not be sufficient to distinguish between the likely possibilities. However, it is clear that straightforward photochemical electrophilic substitution is as yet of very limited importance. 

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