The photochemistry of nitrobenzene in isopropyl alcohol has been studied at 366 mp. Evidence for hydrogen abstraction from isopropyl alcohol, following n + a* excitation of nitrobenzene, is presented. Results are consistent with the triplet state of nitrobenzene being the reactive species. Phenylhydroxylamine is generated as the initial product, which oxidizes in air to nitrosobenzene and couples with the latter to form azoxybenzene. The quantum yield for the disappearance of nitrobenzene in degassed solutions was determined to be 1.14 * 0.08 X lo-*. The over-all photochemical reduction of nitrobenzene involves four hydrogen atom abstractions, as evidenced by the ratio of acetone formation to nitrobenzene disappearance.xygen atom transfer has been reported by Hastings 0 and Matsen' to explain the vapor phase photochemistry of nitrobenzene, which results in nitrosobenzene and p-nitrophenol. Chapman2 has reported the photoisomerization of the aromatic nitro group to nitrite ester. Other photochemical studies2+ of aromatic nitro compounds have been reported with emphasis on addition reactions to olefins; however, it is surprising that quantum yield studies for such compounds are lacking. Rebbert and Slagg6 have reported the photochemistry of nitroalkanes, and the quantum yields for these molecules were found to be small compared to unity. With the aim of understanding the photochemistry of aromatic nitro compounds, a quantum yield study of nitrobenzene was undertaken.
Quantum yield studies at 366 µ of nitrobenzene in isopropyl alcohol-water mixtures containing hydrochloric acid indicate that protonation of nitrobenzene in the excited state (triplet) enhances the photochemical disappearance of nitrobenzene, which occurs by two competing reactions: (1) hydrogen abstraction from isopropyl alcohol and (2) chloride ion attack on the aromatic ring. The quantum yield results for the hydrogen abstraction reaction in 50% isopropyl alcohol-water solutions suggest that the nitrobenzene triplet begins to protonate when [H+] «3 M. This unexpected behavior indicates that nitrobenzene is a stronger base in the excited state relative to the ground state. Ultraviolet spectra indicate that protonation in the ground state is not important for the concentration range of acid studied (0-12 M HC1). The quantum yield for nitrobenzene disappearance in 12 M HC1 is = 0.30.
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