Nitrobenzene photochemistry. II. Protonation in the excited state

Abstract

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.

The Influence of the Nitro Group on the Dissociations of Some Aromatic Acids and Bases in Their Lowest Excited Triplet States*

“…It would seem reasonable, however, by inference from the results of pK,* measurements on the nitroquinolines and the nitrophenols, that the nitro group in nitrobenzene would not be more than 1-2 units more basic in the triplet state than in the ground state. This would yield an approximate value of -9.3 for the p K,* of nitrobenzene, considerably more acidic than the range of Ho=-l.O to -4.0 (3M-12'M) HCI, reported by Hurley and Testa [3] for the enhanced rate of photochemical formation of phenylhydroxylamine from nitrobenzene. If, however, the lowest excited singlet state is .considered as being responsible for the enhanced rate of photoreduction of nitrobenzene, more reasonable results are obtained.…”

“…Dissociation constants of these compounds in their lowest excited triplet states are of considerable interest to the photochemist, especially in the evaluation of photochemical reactions involving acid or base catalysis. For example, the increased rate of photoreduction of nitrobenzene in 3 M HCI (in isopropanol-water) to phenylhydroxylamine, has been attributed to protonation of the nitro group in the triplet state which is presumably more basic than in the ground state, because the protonated ground state is not present appreciably at acidities lower than a Hammett acidity value of H, = -10 (-95% H2S04) [3].…”

Triplet excited states of nitronaphthalenes. III. 1,4‐dinitronaphthalene

“…Since the proton is known to attach itself to one of the oxygens of the nitrogroup [4,10,21,22], the low reactivity of the triplet 1,4-DN02N toward the proton seems to indicate that the oxygen of the nitro group becomes electron deficient during electronic excitation, which in turn is consistent with the assignment of the 1,4-DNOzN triplet to a predominantly n -+ x* state and furthermore with the high reactivity of this state toward the electron [23,24].…”

Charge-transfer complexes and photochemistry

“…Nitrobenzene undergoes photoreduetion to phenylhydroxylamine in the presence of iso-propanol (39). This photoreduction reaction also takes place in acidic solution (40).…”