Acid-Catalyzed Nucleophilic Aromatic Photosubstitution. A Reconsideration of Protonation in Excited States of Nitrobenzenes

Article abstract of DOI:10.1021/ja00216a028

3-Bromonitrobenzene is photosubstituted inefficiently but cleanly by chloride ion to give 3-chloronitrobenzene.The photosubstitution is catalyzed by hydronium ion; the limiting quantum yield of 0.021 at infinite +> and 3.0 M -> is only 2-fold higher than the quantum yield (0.011) of the uncatalyzed photosubstitution at 3.0 M ->.The limiting quantum yield at infinite chloride ion concentration with no added acid is also 0.021.That 2-propanol does not intervene on the photosubstitution pathway to cause photoreduction indicates that radical intermediates are not involved, and the absence of deuterium incorporation on the aromatic ring accompanying photosubstitution indicates that the catalysis does not involve formation of a dihydrobenzene intermediate.The reaction appears to be an example of the so-called <*> mechanism and the catalysis to be a manifestation of protonation of the ³?,?^* state, which is the higher energy triplet state in 3-bromonitrobenzene.