Nitrobenzene, photoreduction

The efficiency of nitrobenzene photoreduction may be increased remarkably in 2-propanol/hydrochloric acid mixtures. In 50% 2-propanol/water containing 6 moles l i HCl, acetone and a complex mixture of chlorinated reduction products are formed i ). Both HCl and 2-propanol (as hydrogen source) are needed. When sulfuric acid is substituted for HCl, enhanced photoreduction does not occtu . When using mixtures of HCl and LiCl to maintain a constant chloride concentration (6 M) and vary [H+], a constant disappearance quantum yield 366 =0.15 is found within the [H+]-range 0.05—6 moles l i. This strongly suggests that chloride ions play an essential role, probably via electron transfer to 3(n, tt )-nitrobenzene i > [Eq. (1)], but it is also evident from the data presented that the presence of add is probably important in subsequent steps, [Eq. (3)]. 

Protonation of (n, 7t )-nitrobenzene had been suggested earlier and later questioned I8) on account of an estimated extremely weak basicity of 3(n, tt )-nitrobenzene. Enhanced basicity of the lowest excited singlet state compared to ground and lowest excited triplet state has been derived from shifts in the phosphorescence and absorption spectra of nitrophenols ). On this basis, the increased rate of nitrobenzene photoreduction in acidic solution is formd to be thermodynamically unfeasible in the lowest excited triplet state >. Although it might be thermodynamically feasible in the excited singlet state, the short lifetime of the latter state may make this possibility unlikely. 

Both CIDNP and ESR techniques were used to study the mechanism for the photoreduction of 4-cyano-l-nitrobenzene in 2-propanol5. Evidence was obtained for hydrogen abstractions by triplet excited nitrobenzene moieties and for the existence of ArNHO, Ai N( )211 and hydroxyl amines. Time-resolved ESR experiments have also been carried out to elucidate the initial process in the photochemical reduction of aromatic nitro compounds6. CIDEP (chemically induced dynamic electron polarization) effects were observed for nitrobenzene anion radicals in the presence of triethylamine and the triplet mechanism was confirmed. 

The photoreduction of nitrobenzene had already been observed by Ciamician and Silber More recently, quantitative studies have been carried out by Testa and his group n-is) and other authors ts-is), Jt jg generally accepted n-is, 17,18) that the long wavelength ( -> 340 nm) absorption of nitrobenzene must be... 

Details of nitrobenzene photochemistry reported by Testa are consistent with the proposal that the lowest triplet excited state is the reactive species. Photoreduction, as measured by disappearance quantum yields of nitrobenzene in 2-propanol is not very efficient = (1.14 0.08) 10 2 iD. On the other hand, the triplet yield of nitro benzene in benzene, as determined by the triplet-counting method of Lamola and Hammond 28) is 0.67 0.10 2). This raises the question of the cause of inefficiency in photoreduction. Whereas Lewis and Kasha 29) report the observation of nitrobenzene phosphorescence, no long-lived emission from carefully purified nitrobenzene could be detected by other authors i4,3o). Unfortunately, the hterature value of Et for nitrobenzene (60 kcal mole i) is thus based on an impurity emission and at best a value between 60 and 66 kcal mole can be envisaged from energy-transfer experiments... 

From Ah = 0.8x10 1 mole i s i and straightforward kinetics the rate constant for radiationless decay of triplet nitrobenzene could be derived Aat = 0.6 X 10 s i. Thus it is easily unterstood why oxygen quenching (assumed to be diffusion controlled does not affect photoreduction in 2-propanol to a large extent upon irradiation in air-saturated ([O2] 10 M) solutions, the quantum... 

With good hydrogen donors photoreduction of nitrobenzene becomes more efficient the rate constant for hydrogen abstraction from tributylstannane by (n, 7i )-nitrobenzene has been determined as 4 x 10 1 mole s-i... 

The reduction of nitrobenzene may also be accomplished by triplet excited pyrochlorophyll Comparison of the quantum 5delds of such photoreductions by hydrazobenzene in ethanol-pyridine solutions with the polarographic quarter... 

The photoreduction of nitrobenzene using p5o ex filtered light from a medium pressure mercury arc was studied in petroleum, toluene, ether, 2-propanol, tert-butyl alcohol, diethylamine, triethylamine, aqueous solutions of 2-propanol and diethylamine and also in aqueous t-butylalcohol containing sodium boro-hydiide 3 >. Varying amounts of aniline, azo- and azoxybenzene were obtained. In the presence of a fourty-fold excess of benzophenone, a six-fold increase in the rate of aniline formation in ethereal solution was observed, and aniline formation was completely suppressed by addition of biacetyl or octafluomaphthalene Since unreacted nitrobenzene could be recovered in these experiments, it is demonstrated that the triplet state of nitrobenzene was quenched. 

It should be noted at tins point that the mechanism of photoreduction in amine solvents is highly likely to be quite different from that in hydrogen donor solvents. In the former class of solvents, electron transfer seems to prevail. StericaJly hindered nitrobenzenes are not capable of hydrogen abstraction from hydrogen donors as ethers or 2-propanol (see Section A. 1.4) but are efficiently photoreduced in di- or triethylamine ). 

The photoreduction of eight electron-acceptor- and four electron-donor-substituted nitrobenzenes has been studied and quantum yields for either starting material disappearance or product formation have been reported 7). Photolysis of 4-nitrobenzonitrile and 4-nitrotoluene in air-saturated solutions was completely quenched and thus a triplet multiplicity of the reacting excited state was derived 7). 

Group I comprises nitroaromatics with lowest (n, tt )- [or (7t,n )- with considerable (n, 7T )-contribution] excited states, the triplets of which behave electro-philiccilly and tend to abstract hydrogen atoms or electrons and thus typically undergo photoreduction. Compounds which fit this correlation were Nitrobenzene 2-nitro-, 4-nitro- and 4,4 -dinitrobiphenyl, 2-nitrofluorene, 3-nitroaceto-phenone, 1,8- and 1,5-dinitronaphthalene. 

An example of radical coupling foUowing hydrogen abstraction by excited nitro-ethane from cyclohexane or diethyl ether in solution has also been reported Formation of -methyl-N-arylnitrones is observed during photoreduction (via electron transfer) of sterically hindered nitrobenzenes in triethylamine 39) ... 

In aliphatic amines (diethylamine or triethylamine) the intramolecular hydrogen abstraction is quenched almost completely. Instead, smooth photoreduction of the nitro group without participation of the side chain is observed with 1,3,5-tri-fezf-butyl-2-nitrobenzene (5) and 14, R = C(CH3)3 ). Products derived from the respective phenylhydroxylamines were isolated in both cases. Again, an electron transfer, which does not seem to suffer from steric restrictions, is operative (see also Section A. 1.3). 

The proposed electron transfer mechanism for this photoreduction parallels that given earlier for nitrobenzene and is supported by the observation of two first order transient absorptions (t 1 ms) in flashed acidified 2-propanol solutions of 7. These absorptions are assigned to the radical 4 and its conjugate acid 5. 

In contrast to nitrobenzene, as significant inefficiency of photoreduction is observed at high (6 moles l t) HCl concentrations, which has not been explained yet. 

Photoexcited nitrobenzene may be used for benzylic hydroxylation (at C-9) of 17/3-acetoxy-3-methoxyoestra-l,3,5(10)-triene. The photochemistry of the 17/3-nitro-steroid (217) is markedly solvent dependent, the major products being in ether the 17-desnitro-compound (218), in propan-2-ol the hydroxylamine (219), and in EtOH-NaOEt the hydroxamic acid (220) and the cyclopropane (221). The hydroxamic acid (220) is probably formed through the oxaziridine (Scheme 7). Although there are analogies to this in the photochemistry of nitrones and oximes, the photoreduction of a nitroalkane in propan-2-ol to an alkyl-hydroxylamine appears to have no precedent. Further studies of photochemistry of conjugated... 

Figure 3. Photoreduction on p-Si or redox couples with redox potentials lying above the conduction band edge as determined by dark flat-band potential measurements. (a) Photoreduction of 1,3 dimethoxy-4-nitrobenzene in methanol (E0 = —1.0V vs. SCE) (b) photoreduction of anthraquinone in acetonitrile (E0 = —0.95 V vs. SCE) Ec for p-Si in both cases in the dark is —0.85 V vs. SCE (------------------------------) dark (---) light (5).

Figure 6. Wavelength dependence of the photoreduction current of 1,3 dimethoxy-4-nitrobenzene (20mMj in methanol (1M TMAC)...

Nitroarenes display strong absorption in the near-UV region and are efficiently photoreduced.1187 An intermolecular version of this reaction is depicted in Scheme 6.187. The photoreduction is initiated by hydrogen atom abstraction. Substituted nitrobenzenes 393, where X is an electron-donating group (X = p-Me, /)-OMe) or nitrobenzene itself (X = H) are photoreduced in the presence of propan-2-ol to the corresponding... 

PROBABLE FATE photolysis photoreduction could occur if nitrobenzene is adsorbed onto humus particles, atmospheric and aqueous photolytic half-life 67-200 days oxidation only important as method of destroying photoreduction products, photooxidation half-life in water 125 days-22 yrs, photooxidation half-life in air 0.544-5.44 hrs hydrolysis not important volatilization not fast enough to be important, if released to water, some volatilization is expected sorption adsorbed by humus and probably by clay biological processes no bioaccumulation of any significance, biodegradation is relatively slow... 

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