Nitration Mechanisms of Anisole during Gamma ... - ACS Publications

Downloaded by NORTH CAROLINA STATE UNIV on December 8, 2012 | http://pubs.acs.org Publication Date (Web): October 15, 2010 | doi: 10.1021/bk-2010-1046.ch017

Chapter 17

Nitration Mechanisms of Anisole during Gamma Irradiation of Aqueous Nitrite and Nitrate Solutions Gracy Elias,*,1 Bruce J. Mincher,1 Stephen P. Mezyk,2 Thomas D. Cullen,2 and Leigh R. Martin1 1Idaho

National Laboratory, P.O. Box 1625, Idaho Falls, ID 83415, U.S.A. State University-Long Beach, 1250 Bellflower Blvd., Long Beach, CA 90840, U.S.A. *Corresponding Author. Phone: (208) 526-0979. Fax: (208) 526-0603. E-mail [email protected]. 2California

The nitration of aromatic compounds in the condensed phase is of interest to nuclear waste treatment applications. This chapter discusses our investigation of radiolytic aromatic nitration mechanisms in the condensed phase toward understanding the nitration products created during nuclear fuel reprocessing. The nitration reactions of anisole, a model aromatic compound, were studied in γ-irradiated acidic nitrate, neutral nitrate, and neutral nitrite solutions. The nitrated anisole product distributions were the same with and without radiation in acidic solution, although more products were formed with radiation. In the irradiated acidic condensed phase, radiation-enhanced nitrous acid-catalyzed nitrosonium ion electrophilic aromatic substitution followed by oxidation reactions dominated over radical addition reactions. Neutral nitrate anisole solutions were dominated by mixed nitrosonium/nitronium ion electrophilic aromatic substitution reactions, but with lower product yields. Irradiation of neutral nitrite anisole solution resulted in a statistical substitution pattern for nitroanisole products, suggesting non-electrophilic free radical reactions involving the •NO2 radical.

© 2010 American Chemical Society In Nuclear Energy and the Environment; Wai, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Introduction The ligands proposed for use in nuclear fuel reprocessing including some that contain aromatic functional groups will be exposed to highly radioactive aqueous nitric acid solutions, and many have been shown to undergo nitration in irradiated acidic solution (1–3). Some nitration products of the modifier, Cs-7SB, were found in the FPEX (Fission Product Extraction) formulation used for Cs extraction in the presence of nitric acid (1). The selectivity of ligands for the complexation of certain actinides and lanthanides may be affected by nitration products found in the nitric acid system. The radiolytic degradation of a ligand can reduce the ligand concentration, thus depressing the forward extraction distribution ratios. The degradation products may also act as complexing agents that increase forward extraction distribution ratios, or complex undesirable elements to reduce the selectivity of the solvent extraction system (4). Degradation can also affect ligand solubility, viscosity and other solvent properties. In irradiated nitric acid systems, some of these reactions presumably occur due to reactions with radiolytically-produced nitrogen-centered radicals like •NO, •NO2 and •NO3. In concentrated nitric acid solution, substitution of a nitro group for a hydrogen atom on the aromatic ring occurs in electrophilic or free radical reactions. Conventional “thermal” nitration occurs with high positional selectivity through electrophilic aromatic substitution via the nitronium ion, shown in Equations 1-2 (5) for a concentrated nitric acid solution.

Electrophilic nitration in nitric acid solution is also catalyzed by nitrous acid under conditions of high acidity. Nitrous acid protonates and dissociates to produce the nitrosonium ion NO+, as shown in Equation 3. The nitrosonium ion is electrophilic and reacts with aromatic compounds as shown in Equations 4 and 5 to produce a nitroso product (6):

Nitroso species are readily oxidized to form the corresponding nitro products (7):

Nitrous acid is continually generated in nitric acid solution via nitric acid oxidation of solutes. The ortho/para ratio for the nitrated products for this type of reaction is < 1, and it depends on the type of aromatic substrate and the acidity of the solution (8, 9). Thus, in acidic solution, nitration may occur by a combination of NO2+ and NO+ ion electrophilic aromatic substitution reactions, with low yields of meta products. 206 In Nuclear Energy and the Environment; Wai, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

Titov (10) discussed the mechanism of radical aromatic nitration. The direct reaction with aromatic compounds is shown in Equation 7 (11):


The hydrogen atom abstraction reaction to produce a stable nitrated aromatic product is shown below (5):

The nitration of aromatic compounds by the less electrophilic •NO2 radical is reported to occur without positional selectivity, resulting in a statistical distribution of 40% ortho, 40% meta and 20% para-substituted products (12) except when steric considerations reduce the abundance of the ortho-isomer. The •NO3 radical is more electrophilic and more reactive than the •NO2 radical and its reactions with organic compounds are generally reported to occur via hydrogen atom abstraction and electron transfer to produce carbon-centered radical products (13). In this chapter, we report experiments designed to elucidate the mechanism of the nitration of anisole in γ-irradiated aqueous nitric acid. Anisole (C6H5-OCH3) is highly reactive towards nitration and thus large amounts of products may be generated at moderate absorbed radiation doses. Since a suite of radical and ionic reactive species are produced in this system, solutions of nitric acid, neutral nitrate and neutral nitrite were irradiated in separate experiments to isolate selected reactive species. Following irradiation, the stable products were measured using high performance liquid chromatography (HPLC). Pulse radiolysis techniques were used to measure reactions of the •NO2 and •NO3 radicals with anisole and related compounds to elucidate these mechanisms kinetically.

Experimental Methods Saturated and known concentrations of reagent-grade neat compounds of anisole were prepared in 6 M HNO3, 6 M NaNO3, and NaNO2 (0.05, 0.1, 0.5 and 1 M) solutions. The acid, neutral nitrate and neutral nitrite solutions were chosen to maximize production of the •NO3 radical or •NO2 radical. These were then sparged with N2 to remove oxygen and to provide adequate mixing. Oxygen was eliminated to minimize peroxyl radical formation and subsequent loss of products to ring-opening reactions which would lead to decreased concentrations of the nitro aromatic products that were the object of this study. Aliquots of the prepared acidic and neutral solutions of anisole were then transferred to 1 mL glass vials for irradiation. Hydrazine, a nitrous acid scavenger, was used in unirradiated solutions of anisole and 6 M HNO3 to investigate nitrous acid catalyzed nitration of anisole in nitric acid. Irradiations were performed using a Nordion Gammacell 220E (Ottawa, Canada) 60Co γ-source with a nominal dose rate of 12 kGy h-1. The samples were exposed in a series of absorbed doses, including an unirradiated control, by varying the duration of exposure to the γ-ray source. Dosimetry was performed using GafChromic (Goleta, CA, USA) radiochromic films, traceable to Fricke dosimetry. The temperature in the irradiator sample chamber was 50 °C. 207 In Nuclear Energy and the Environment; Wai, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


The linear accelerator (LINAC) electron pulse radiolysis system at the Radiation Laboratory, University of Notre Dame, was used for the reaction rate constant determinations in this study. During rate constant measurements, the solution vessels were sparged with a minimum amount of nitrogen gas necessary to prevent air ingress. Solution flow rates in these experiments were adjusted so that each irradiation was performed on a fresh sample. Dosimetry was performed using N2O-saturated, 1.00 x 10-2 M KSCN solutions at λ = 475 nm, (Gε = 5.2 x 10-4 m2 J-1) with average doses of 3-5 Gy per 2-3 ns pulse. All measurements were conducted at a room temperature of 18 ± 1 °C. For HPLC analyses, a 10 µL injection volume was analyzed using a Shimadzu HPLC (Shimadzu LC-10 AD VP pumps, CTO-10 AC VP column oven, SILHTc autosampler and a SPD-M10A VP photo-diode array detector and Class VP software) and a Supelco 25 cm x 4.6 mm, 5 µm C-18 column at 55 °C. Methanol and water (55:45) at a flow rate of 0.5 mL min-1 was used isocratically with a run time of 30 minutes and the analytes were detected at a wavelength of 254 nm. The approximate retention time for anisole was 15 minutes. Products were identified by comparison of the retention times and UV spectra to standard compounds. All results are the means of the triplicate analyses. For GC-MS analyses, a Shimadzu GC system (GC-MS QP 2010, AOC-5000 auto injector, and GCMS solution software) coupled to a quadrupole mass spectrometer operating in electron impact ionization (EI) mode was used. The GC separation was performed using a Restek XTI-5 capillary column with a length of 30.0 m x 0.25 mm id and a film thickness of 0.25 µm. The oven temperature was programmed as follows: 40 °C (3 min.); 5 °C/min to 100 °C; 50 °C/min to 250 °C (2 min.). 1 µL injection of sample with a split ratio of 15.0 was used for anisole products analysis in the extracted methylene chloride solution.

Results and Discussion Kinetic Study Direct radiolysis of HNO3 gives radiolysis products of both HNO3 and water as shown in Equations 9101112.

The aqueous electrons (eaq-) will immediately be converted to •H atoms (14):

Both •NO3 and •NO2 transient absorption spectra were measured (the peak absorption maximum •NO3, 640 nm; •NO2, 400 nm) here and the production of •NO3 is favored over •NO2 in the nitric acid medium. 208 In Nuclear Energy and the Environment; Wai, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

The transient absorption spectrum of the •NO2 radical was measured in N2Osparged 0.10 M sodium nitrite solution. Under these conditions, •NO2 radicals are produced through the reaction of •OH radicals with nitrite ion as shown in Equation 13:


The relatively high concentration of nitrite in this system means that reactions with the formed eaq- and •H atoms also occur (eq. 141516):

For rate constant (k) determinations, the rate of change of the decay kinetics at the peak absorption maximum (•NO3, 640 nm; •NO2, 400 nm) was observed. Typical kinetic data are shown for the •NO3 radical reaction with anisole in Table 1. By plotting these fitted pseudo-first-order rate constants against anisole concentration, a second-order rate constant of (4.42 ± 0.05) x 109 M-1 s-1 was obtained. For the •NO2 radical measurements, no significant change in the rate of decay of transient absorbance was determined, even at the highest solute concentrations used. The k value for anisole was found to be < 2 x 105 M-1 s-1. Steady-State Radiolysis: Acidic Nitrate, Neutral Nitrate and Neutral Nitrite Solutions Acid-catalyzed nitration occurs typically by electrophilic aromatic substitution where the isomer distributions show very low concentration of the meta isomers (15). Figure 1 shows the results of anisole nitration measured using HPLC. Both ortho- and para-nitroanisoles were produced, but the lack of meta-nitroanisole suggests that nitration occurred via the electrophilic substitution mechanism (12). The HPLC analysis also showed the ingrowth of para-nitrophenol, dinitrophenols, and nitrous acid. The main nitration products, nitroanisoles and nitrophenols, were confirmed using GC-MS. The overall ortho/para ratio of nitroanisoles in all anisole samples was ~0.35, which is characteristic of nitrosonium ion (NO+) dominated electrophilic substitution catalyzed by nitrous acid (7, 16). The nitrous acid scavenger hydrazine reduced the nitroanisole concentration by over two orders of magnitude (see Figure 2), and the ortho/para ratio for the mono-nitrated products changed from 0.35 to >1. This change in product ratios indicated the absence of nitrous acid catalyzed nitrosonium electrophilic substitution. In addition, the absence of p-nitrophenol, dinitrophenols, and color formation in these solutions supports this cconclusion (16). Under these conditions the nitration mechanism is due to nitronium ion (NO2+) electrophilic aromatic substitution (5). In the presence of radiation, the 209 In Nuclear Energy and the Environment; Wai, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

amounts of nitrated products and the rate of these reactions increased due to the radiolytically-enhanced production of nitrous acid (17) as shown in Figure 3.


Table 1. Kinetic data for •NO3 radical reaction with anisole Anisole Concentration (µM)

k (s-1)

93

(4.46 ± 0.05) x 105

203

(9.5 ± 0.2) x 105

338

(1.46 ± 0.02) x 106

661

(3.01± 0.05) x 106

Figure 1. The loss of anisole (A) and production of 2-nitroanisole (2-NA) and 4-nitroanisole (4-NA) for irradiated anisole in 6 M nitric acid. Reported errors are the standard deviation of triplicate measurements.

210 In Nuclear Energy and the Environment; Wai, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Figure 2. The trend (fitted lines) for the ingrowth of 2-nitroanisole (2-NA) and 4-nitroanisole (4-NA) for thermal anisole reaction with 6 M HNO3 in the presence and absence of the nitrous acid scavenger hydrazine (H). Errors shown are the calculated method errors applied to each individual measurement, (13.9 % for 2-NA and 12.2 % for 4-NA).

Figure 3. The production of nitrous acid with irradiation of nitric acid of different concentrations. Reported errors are the standard deviation of triplicate measurements.



Figure 4. The production of 2-nitroanisole (2-NA), 3-nitroanisole (3-NA) and 4-nitroanisole (4-NA) as a function of absorbed dose in irradiated anisole in 0.5 M sodium nitrite solution. Errors shown are the calculated method errors applied to each individual measurement, (13.9 % for 2-NA 11.0% for 3-NA and 12.2 % for 4-NA). Anisole in irradiated neutral nitrate solution produced 2-nitroanisole, 4-nitroanisole, 2-nitrophenol, and 4-nitrophenol. Production of 4-nitroanisole was predominant over 2-nitroanisole showing a prevalence of nitrous acid catalyzed electrophilic substitution reaction. The concentrations of the nitroisomers were a factor of 10-100 less than those produced in the acidic media. Formation of 2-nitrophenol is characteristic of free radical reactions (18), whereas the 4-nitrophenol formation is characteristic of nitrous acid catalyzed electrophilic reaction (17). This product distribution suggests that a combination of electrophilic nitration with NO+, NO2+, and free radical reactions with •OH and •NO2 occurs in irradiated neutral sodium nitrate solution. In irradiated sodium nitrite solution, only the •NO2 radical is present, in equilibrium with its addition product N2O4. The UV/Vis spectrum of NO2 in irradiated solution confirmed the dominance of NO2 radical under these conditions. The decomposition rate of anisole irradiated in neutral nitrite solution was slow. The concentration of all three nitroanisole isomers produced in irradiated 0.5 M sodium nitrite solution at different irradiation doses is shown in Figure 4. It can be seen that the overall concentration of meta-nitro isomer (concentration at lower doses not given in Figure 4 because it co-eluted with anisole at lower concentrations) was higher compared to those of the ortho and para-nitro isomers. The nitroanisole distribution was 30-50% for 2- and 3-nitroanisoles and 20% for 4-nitroanisole. This is in qualitative agreement 212 In Nuclear Energy and the Environment; Wai, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

with the statistically random distribution resulting from •NO2 radical reactions with toluene, reported by Olah et al. (12). The product 2-nitrophenol was also detected in 0.1 M sodium nitrite solution at 346.9 kGy, characteristic of free radical reaction mechanisms in the presence of •OH and •NO2 (18).


Conclusion The reaction mechanisms of anisole, the simplest aryl alkyl ether and a strong ortho-para director, in nitric acid, neutral nitrate, and neutral nitrite solutions under γ- and pulse radiolysis were investigated in this research. HPLC with UV detection was primarily used to assess the reaction products. The distribution of nitrated derivatives of anisole in acidic solution with the predominance of para-nitroanisole was the same with and without irradiation, although the concentration of products was higher in the irradiated solution. For anisole in the irradiated acidic condensed phase, radiation-enhanced nitrous acid-catalyzed electrophilic aromatic substitution dominated over radical addition reactions. Experiments with neutral nitrate/anisole solutions showed a mixture of nitronium and nitrosonium ion reactions, and small amounts of free-radical reaction products, but with lower product yields than for the nitrous acid catalyzed reactions in acidic solution. Nitroanisole products in neutral nitrite/anisole solution occurred in a statistical fashion, suggesting a non-electrophilic free radical reaction involving the •NO2 radical. However, the •NO2 radical reaction is unlikely to be an important pathway in the acidic condensed phase due to its low reaction rate constants. Thus, the predominant products expected in irradiated acid solution are due to the enhanced nitrous acid catalyzed reaction resulting from radiolytic generation of nitrous acid.

Acknowledgments This research was funded by the INL-LDRD program, sponsored by the U.S. Department of Energy (DOE), under DOE Idaho Operations Office contract DEAC07-99ID13727. Kinetics experiments were performed at the DOE Radiation Laboratory, University of Notre Dame.

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Nitration Mechanisms of Anisole during Gamma ... - ACS Publications

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