Synthesis of nitromethane from acetic acid by radiation-induced nitration in aqueous solution

Article abstract of DOI:10.1016/j.mencom.2007.09.014

The γ-irradiation of solutions containing acetic acid, nitric acid and/or their salts produces nitromethane.

Full text of DOI:10.1016/j.mencom.2007.09.014

Mendeleev
Communications
Mendeleev Commun., 2007, 17, 289–290
Synthesis of nitromethane from acetic acid
by radiation-induced nitration in aqueous solution
Boris G. Ershov,* Andrei V. Gordeev, Gennady L. Bykov, Andrei A. Zubkov and Inessa M. Kosareva
A. N. Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences,
119991 Moscow, Russian Federation. Fax: +7 495 335 1778; e-mail: ershov@ipc.rssi.ru
DOI: 10.1016/j.mencom.2007.09.014
The γ-irradiation of solutions containing acetic acid, nitric acid and/or their salts produces nitromethane.
It is known1,2 that nitration, that is, the incorporation of nitro
groups into organic molecules, occurs on treatment with nitrating
mixtures. The process involves various mechanisms, in particular,
the radical mechanism. In the latter case, it is the ·NO2 radical
formed from nitric acid and nitrogen oxides that serves as the
active nitrating agent. The reaction preferably occurs at elevated
temperatures (above 180 °C) and high pressures.
The composition of the liquid phase was analysed by UV
spectroscopy using an M-40 spectrophotometer with an upgraded
control and signal detection system.
Nitromethane was determined using the reaction with p-diazo-
benzenesulfanilic acid as reported elsewhere.3 In addition, it
was also analysed using IR spectroscopy and gas chromato-
graphy–mass spectrometry (GC-MS).
In this study, we have discovered that ionizing radiation of
aqueous solutions of acetic acid containing nitric acid and nitrates
at room temperature results in nitration of acetic acid. The process
occurs due to reactions of radicals resulting from radiolysis of
the solution. The phenomenon discovered is practically important
because treatment of spent nuclear fuel produces considerable
amounts of liquid radioactive waste containing organic com-
pounds, nitric acid and its salts. The fact that nitro compounds
are formed upon irradiation of liquid radioactive waste may be
important in determination of the safety of radioactive waste
storage and treatment.
Nitrate–acetate mixtures (~10 ml) were placed in sealed glass
tubes about 30 ml in volume and evacuated under irradiation.
Solutions of nitric and acetic acids and their salts (reagent
grade) were prepared using distilled water.
Solutions used for nitromethane analysis were prepared
using methanol, sulfanilic acid, sodium nitrite (reagent grade),
Na2HPO4·7H2O (reagent grade) and NaH2PO4·2H2O (reagent
grade).
The results of this study confirm unambiguously that nitro
compounds are formed upon irradiation of solutions containing
nitric and acetic acids and their salts. The IR spectra of chloro-
form extracts from γ-irradiated solutions with various con-
centrations of AcOH, HNO3 and NaNO3 (absorbed dose of
0.2–0.5 MGy) showed, in comparison with a nonirradiated
solution, additional bands at 1605, 1560 and 1380 cm–1 charac-
teristic of the stretching vibrations of the N=O group in organic
nitro compounds. The intensity of these bands increased if
small amounts of nitromethane were added to the solution. On
the other hand, the intensities of the other peaks remained
almost unchanged. The formation of nitromethane is confirmed
by GC-MS. The mass spectrum of the dichloromethane extract
from irradiated solution of 0.4 M AcOH and 0.3 M NaNO3
(absorbed dose of 1.5 MGy) revealed peaks with m/z 61 and 46,
whose positions and intensities coincided with those of the
peaks observed for nitromethane. These peaks belong to the
MeNO+2 and NO+2 ions, respectively.
Figure 1 shows the optical absorption spectrum of a γ-irradiated
aqueous solution containing 0.4 M AcOH and 0.6 M NaNO3, after
chemical analysis for nitromethane.3 According to this technique,
the reaction of nitromethane with p-diazobenzenesulfonic acid
for 5–10 min produces an intense absorption band at 440 nm
(e = 9×103 dm3 mol–1 cm–1) owing to the formation of 1-nitro-
formaldehyde-p-sulfophenylhydrazone. Figure 1 shows that
absorption of this kind actually appears. The analytical reaction
is selective;3 other derivatives of p-sulfophenylhydrazone formed
in reactions, e.g., with nitroethane or nitropropane, and perhaps
with other nitro compounds, show absorption at £395 nm.
In order to study the dose dependence of nitromethane accu-
mulation, we used 0.4 M acetic acid and 0.3 M NaNO3 solu-
tions. Irradiation was carried out until the absorbed dose was
5 MGy, at which point the nitrate initially present in the system
decomposed almost completely. The formal first-order decom-
position rate of nitrate was calculated to be 5.4×10–7 dm3 mol–1 s–1
(at a dose rate of 4.5 kGy h–1). As nitrate was consumed, forma-
tion of nitromethane was detected in the solution. One can see
(Figure 2) that the concentration of nitromethane in the solution
increases with absorbed dose to reach a maximum (1.65×10–2 M)
at about 3.5 MGy, after which its concentration decreases. Unlike
Irradiation was carried out on a GUG-120 setup with the
60Co radioactive isotope. The dose rate was 4.5 kGy h–1.
The composition and amount of gases were analysed with a
Tsvet-5 chromatograph using a 2 m column packed with
0.4–0.8 mm grade silica gel. IR spectra were recorded using an
M-80 spectrometer with an upgraded control and signal detec-
tion system.
1.5
1.0
0.5
0.0
350
400
450
/nm
500
550
Figure 1 Optical absorption spectrum recorded during nitromethane analysis3
in a γ-irradiated air-free solution containing 0.4 M AcOH and 0.6 M NaNO3.
The optical cell thickness is 0.5 cm. The absorbed dose is 0.5 MGy.
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Mendeleev Commun., 2007, 17, 289–290
their contributions to the radiation-chemical reactions of the
solution components. Under the experimental conditions, the
2.0
1.5
1.0
0.5
0.0
·
OH radicals are preferably (by more than 95%) involved in
·
reactions with acetic acid to give CH2COOH radicals, whereas
–
·
eaq and the H atoms are consumed (by almost 100%) for the
·
reduction of nitric acid and the nitrite ion to give the NO2 radical.
The radical and ion radical species generated in the primary
reactions react with each other and with dissolved compounds
to give new radicals and the products.
0
1
2
3
4
5
6
An important role in the formation of nitromethane in nitrate–
Dose/MGy
·
acetate solutions apparently belongs to the NO2 radical, which
Figure 2 Nitromethane concentration as a function of absorbed dose
during γ-irradiation of an air-free solution containing 0.4 M AcOH and
0.3 M NaNO3.
·
has strong oxidizing properties. It reacts with the CH2COOH
radical to give nitroacetic acid:
NO2 + CH2COOH ® CH2(NO2)COOH,
2.0×108 dm3 mol–1 s–1
.
·
·
nitrate decomposition of which can be tentatively described by
a first-order rate equation, the kinetic curve of nitromethane
formation is more complex. The initial section of the accumula-
tion curve has a pronounced s-shape (Figure 2). This suggests
that the formation of nitromethane is preceded by generation of
an unstable intermediate, which is consumed during subsequent
irradiation.
Nitromethane is formed in a concentration range from 0.2 to
1.6 M of each of the components, i.e., acetic acid (or sodium
acetate) and nitric acid (or sodium nitrate), both in acidic and
neutral solutions.
Radiation-chemical transformations of dilute aqueous solu-
tions result from reactions with ionic and radical species that
are formed due to water decomposition under ionizing radiation.
In general, this process can be expressed by the following
equation:
According to our calculations based on the diffusion-kinetic model
of radiolysis of water and aqueous solutions,6 the rate constant
of this reaction can be estimated as 2.0×108 dm3 mol–1 s–1.
Nitroacetic acid is an unstable compound. The double salt
KO2NCHCOOK can be isolated from a solution in a pure
state.7 In neutral solutions, it undergoes decarboxylation to give
more stable products, nitromethane and carbon dioxide:8
CH2(NO2)COOH ® MeNO2 + CO2,
2.5×10–2 s–1
.
Presumably, acidic and alkaline solutions contain two tauto-
·
meric forms, O2NCH2COOH and HO2NCHCOOH, dissociation
of which gives one- and two-charged ions, respectively. It has
been found8 that only the dissociated form of nitroacetic acid,
that is, O2NCH2CO–2, is unstable. As a result, the observed
decomposition rate of nitroacetic acid directly depends on its
concentration in solution, which in turn depends on the pH
value. It has been found8 that the pK values for the first and
second dissociation constants of nitroacetic acid are ~1.68
and 8.9, respectively. Therefore, nitroacetic acid is rather stable
in acidic and alkaline solutions due to the low concentration of
the dissociated form O2NCH2CO–2. Before the irradiation, the
solutions studied had pH 2.5. Thus, nitroacetic acid formed as a
result of radiation-chemical reactions in nitrate–acetate solutions
decomposes to give nitromethane.
It was noted previously4,5 that radiolysis of aqueous solutions
of nitro compounds results in their decomposition. This study
shows that they can also be formed due to the radiation-induced
nitration of organic compounds. This fact is important for the
ecological assessment of the safety of treatment and storage of
liquid radioactive waste containing organic compounds and
nitric acid.
–
+
· ·
H2O
eaq (2.7), H (0.5), OH (2.9), H2 (0.5), H2O2 (0.7), H (2.7)
The resulting hydrated electron (e–aq) and hydrogen atom (H)
have high reduction potentials (–2.7 and –2.3 V, respectively),
whereas the hydroxyl radical (OH) has a high oxidation capacity
(2.9 V).4 The numbers in parentheses indicate the primary
radiation-chemical yields of radiolysis products in bulk solution
per 100 eV of absorbed energy for soft radiation types (γ-rays
or fast electrons) with low linear energy transfer. A specific
feature of acidic solutions that we studied is that the major part
of hydrated electrons (e–aq) in the reaction
+
e–aq + H ® H,
2.3×1010 dm3 mol–1 s–1
·
·
are transformed to H atoms.
The radiation-chemical processes that occur upon irradia-
tion of aqueous solutions of both nitric acid and nitrates and,
separately, acetic acid and acetates have been studied in con-
siderable detail.4,5 The ion radical products of water radiolysis
References
–
·
·
(hydrated electron eaq, H atom and hydroxyl radical OH) react
with dissolved compounds. In sufficiently general form, the
radiation-chemical reactions in acidic nitrate–acetate solutions
can be described by the set of reactions listed below.4,5 In the
1 S. S. Novikov, G. A. Shvekhgeimer, V. V. Sevost’yanova and V. A.
Shlyapochnikov, Khimiya alifaticheskikh i alitsiklicheskikh nitrosoedi-
nenii (Chemistry of Aliphatic and Alicyclic Nitro Compounds), Khimiya,
Moscow, 1974 (in Russian).
2 The Chemistry of the Nitro and Nitroso Groups, ed. H. Feuer, Wiley,
New York–London–Sydney–Toronto, 1970.
3 I. R. Cohen and A. P. Altshuller, Anal. Chem., 1959, 31, 1638.
4 A. K. Pikaev, Sovremennaya radiatsionnaya khimiya. Radioliz gazov i
zhidkostei (Modern Radiation Chemistry. Radiolysis of Gases and
Liquids), Nauka, Moscow, 1986, p. 178 (in Russian).
5 G. V. Buxton, C. L. Greenstock, W. P. Helman and A. B. Ross, J. Phys.
Chem., Ref. Data, 1988, 17, 513.
6 B. G. Ershov and A. V. Gordeev, in Sovremennye problemy fizicheskoi
khimii (Modern Problems of Physical Chemistry), Granitsa, Moscow,
2005, pp. 520–541 (in Russian).
7 H. Feuer, H. S. Hass and K. S. Warren, J. Am. Chem. Soc., 1949, 71,
3078.
·
case of acetic acid, these reactions generate the CH2COOH
( CH2COO ) and MeCO radicals:
–
·
·
·
·
MeCOOH + OH ® CH2COOH,
1.4×108 dm3 mol–1 s–1
3.5×105 dm3 mol–1 s–1
1.8×108 dm3 mol–1 s–1
;
;
.
·
MeCOOH + H ® ·CH2COOH + H2,
MeCOOH + eaq ® MeCO + OH ,
–
–
·
·
·
In nitrate solutions, the reactive species include NO2 and NO3:
·
·
HNO3 + OH ® NO3 + H2O,
5.0×105 dm3 mol–1 s–1
6.0×106 dm3 mol–1 s–1
7.8×109 dm3 mol–1 s–1
;
;
.
·
·
HNO3 + H ® NO2 + H2O,
HNO3 + eaq ® NO2 + OH ,
–
–
·
8 K. J. Pedersen, J. Phys. Chem., 1934, 38, 559.
Acetic and nitric acids and their salts in solutions compete
in reactions with eaq, H and OH. The reaction rate constants
presented above4,5 allow us to give a quantitative estimate of
–
·
·
Received: 18th January 2007; Com. 07/2862
– 290 –