Effect of medium acidity on the efficiency of oxidation of 2,4,6-trinitrotoluene to 2,4,6-trinitrobenzoic acid

Article abstract of DOI:10.1007/s11172-016-1571-0

An effect of boric acid additives on oxidation of 2,4,6-trinitrotoluene (TNT) to 2,4,6-trinitrobenzoic acid (TNBA) with chromic anhydride in concentrated (96—100%) H₂SO₄ has been studied. In the presence of tetrahydrosulfatoboric acid HB(HSO₄)₄ formed in situ (up to 5 mol.%) or added as a preliminary prepared solution (up to 1 mol. %), TNT is selectively oxidized to TNBA in the yields up to 95—99%. The mechanism including formation of TNT dication as a key step of its oxidation at the methyl group has been suggested.

Full text of DOI:10.1007/s11172-016-1571-0

2216
Russian Chemical Bulletin, International Edition, Vol. 65, No. 9, pp. 2216—2219, September, 2016
Effect of medium acidity on the efficiency of oxidation of
2
,4,6ꢀtrinitrotoluene to 2,4,6ꢀtrinitrobenzoic acid
L. V. Мikhal´chenko, V. N. Leibzon, М. Yu. Leonova, and V. P. Gultyai
N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences,
4
7 Leninsky prosp., 119991 Moscow, Russian Federation.
Fax: +7 (495) 135 8824. Eꢀmail: guvp@ioc.ac.ru
An effect of boric acid additives on oxidation of 2,4,6ꢀtrinitrotoluene (TNT) to 2,4,6ꢀ
trinitrobenzoic acid (TNBA) with chromic anhydride in concentrated (96—100%) H SO has
2
4
been studied. In the presence of tetrahydrosulfatoboric acid HB(HSO ) formed in situ (up to
4
4
5
mol.%) or added as a preliminary prepared solution (up to 1 mol. %), TNT is selectively
oxidized to TNBA in the yields up to 95—99%. The mechanism including formation of TNT
dication as a key step of its oxidation at the methyl group has been suggested.
Key words: 2,4,6ꢀtrinitrotoluene, tetrahydrosulfatoboric acid, 2,4,6ꢀtrinitrobenzoic acid,
oxidation, chromic anhydride, cationꢀradicals, dications.
Oxidation of the methyl group of the substituted toluꢀ
drastic conditions. For example, a procedure to obtain
TNBA through oxidation of TNT by nitric acid at
enes is one of important synthetic approaches to practiꢀ
1
9
cally valuable products. If the aromatic ring contains elecꢀ
180—190 °С under pressure of 35—40 atm is known.
10
tronꢀdonating substituents activating the molecule, the
oxidation depth can be controlled through the choice of
an approariate oxidant. Introduction of one or especially
more electronꢀwithdrawing substitutents in the aromatic
ring deactivates the molecule to oxidation. In this case,
the oxidation potential of a compound is considerably
increased while the reaction selectivity decreases: under
drastic conditions the oxidation of the methyl group is
accompanied with the undesired exhaustive oxidation to
carbon dioxide and water.
Oxidation of TNT with sodium dichromate or chromic
anhydride11 in concentrated sulfuric acid at 40—55 °С has
long and widely been used as a laboratory method to obꢀ
tain TNBA. Over many years one managed to improve
1
2,13
this method,
in particular, by adding acetic acid to
1
3
a reaction mixture. However, no explanations of a change
in the TNBA yield depending on the synthetic conditions
were given in these studies.
From our point of view, the problems connected with
rising the TNBA yield are due mainly to the fact that
oxidation of TNT to TNBA proceeds effectively only at
very high potentials of the redox system. Potential of
In the present study, oxidation of 2,4,6ꢀtrinitrotoluene
TNT) to 2,4,6ꢀtrinitrobenzoic acid (TNBA) is considꢀ
(
2
—
3+
ered. This process is important both from practical and
theoretical points of view. Based on 2,4,6ꢀtrinitrobenzoic
acids, a variety of heterocyclic systems has been syntheꢀ
sized2 being halfꢀproducts in the synthesis of biologically
active molecules, dyes, and aromatic condensation polyꢀ
mers. Decarboxylation of TNBA under mild conditions
in turn smoothly affords practically valuable 1,3,5ꢀtriꢀ
a Cr O7 /Cr system should increase with growth of the
2
medium acidity. Therefore, it may be suggested that oxiꢀ
dation of TNT with chromic anhydride should be carried
out not only in concentrated acid but probably in the presꢀ
ence of superacids. To confirm this proposal, we performed
oxidation of TNT with chromic anhydride in concentratꢀ
ed sulfuric acid in the presence of boric acid.
,3
4
,5
nitrobenzene.
Oxidation of polynitrotoluenes requires high potenꢀ
tials of an oxidation system. Direct oxidation of polynitroꢀ
toluenes on an electrode (anode) found no practical appliꢀ
Results and Discussion
As underlined above, for effective oxidation of TNT to
TNBA high oxidation potential of the system Еох is necesꢀ
sary. Approximate evaluation using the Nernst equation
6
cation because of low efficiency. In our previous studies,
we have developed an electrochemical method for tervalent
cobaltꢀmediated oxidation of polynitrotoluenes in concenꢀ
with account of the Hammett acidity function (H ) shows
o
7
,8
2–
3+
trated nitric acid at temperatures of 35—40 °С. In conꢀ
trary to the electrochemical oxidation of TNT to TNBA,
chemical oxidation of TNT to TNBA proceeds under rather
that for the redox system Cr O7 /Cr the value of Еох
2
grows from 2.3 to 2.7 V upon changing the sulfuric acid
1
4—16
concentration from 75 to 95%.
We have found experꢀ
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 2216—2219, September, 2016.
066ꢀ5285/16/6509ꢀ2216 © 2016 Springer Science+Business Media, Inc.
1

2
,4,6ꢀTrinitrobenzoic acid
Russ.Chem.Bull., Int.Ed., Vol. 65, No. 9, September, 2016 2217
imentally that on performing the oxidation of TNT in
5% sulfuric acid the yield of TNBA is 45%, while in 98%
H SO it rises up to 70%. Thus, to raise the yield of TNBA
sis. According to the literature data,17,18 mixing boric
acid with concentrated H SO affords a very strong acid,
7
2
4
namely, tetrahydrosulfatoboric acid HB(HSO ) , which
2
4
4 4
one should maximize Еох of the redox system by raising
the medium acidity.
provides the significant growth of the acidity of the
medium in our case. From the literature data, it follows
that an increase in the acidity of the medium due to the
presence of this superacid results in acceleration of deꢀ
Acidity of sulfuric acid can be significantly increased
by adding boric acid. The Н value for the H BO soluꢀ
о
3
3
1
7,18
20
tions in concentrated H SO4 may reach –13.62,
while for 98% H SO it is only –10.41. To determine
sulfation of some aromatic amino sulfonic acids as well
2
1
5
as in an increase in the effective reaction rate of their
2
4
2
1
experimentally the effect of H BO on the yield of TNBA,
isomerization.
The graph of dependency of the acidity H of the H SO
4
3
3
TNT was oxidized with chromic anhydride at various conꢀ
centrations of H SO and H BO . In all cases, chromic
o
2
solutions in the presence of boric acid versus its concenꢀ
2
4
3
3
1
8
anhydride was used in a small excess (20 mol. %). Known
tration has a sharp rise just in the area corresponding to
100% H SO . Experimental data presented in Table 1 corꢀ
procedures were applied to perform oxidation of TNT,
2
4
isolation, and purification of TNBA.1
0,19
Results are shown
relates with the proposal that at high concentration of
H SO even minor additives of boric acid considerably
in Table 1. Yields of TNBA and picric acid (PA) were
calculated based on the reacted TNT.
2
4
increase the TNBA yield. Usage of the preprepared soluꢀ
tion of HB(HSO ) in 100% H SO proved to be most
From the data presented in Table 1, it follows that an
increase in the acidity of the medium due only to the rise
in the concentration of H SO results in the noticeable
4
4
2
4
effective, i.e., under conditions of a nearly complete abꢀ
sence of water in the system.
2
4
growth of the TNBA yield, namely, from 65 up to 81%,
even per se. Addition of boric acid at any of the examined
concentration of H SO increases considerably the amount
In all the experiments performed, picric acid (PA) was
the side product (up to 10%). Its appearance among the
reaction products results probably from the subsequent
oxidative transformations of the formed TNBA under
2
4
of the TNBA formed, being as high as 95—99% in some
experiments. In all cases, the TNT conversion remains
sufficiently high (74—80%), and in the presence of H BO
2
2
"drastic" conditions. Interestingly, the PA content in the
reaction products decreases with an increase in the mediꢀ
um acidity. It should be emphasized that no other side
products of TNT oxidation were detected. In this case,
the losses during isolation of the oxidation products were
minimal for high concentrations of H SO and also
3
3
it reaches 90%. The addition of boric acid is more effecꢀ
tive, the more concentrated H SO is used for the syntheꢀ
2
4
2
4
Table 1. TNT oxidation with chromic anhydride in sulfuric
acid in the presence of boric acida
decrease as the concentration of boric acid increased. This
result testifies that when Еох is not high enough to effecꢀ
tively oxidaize the TNT methyl group, side reactions can
take place, e.g., the cleavage of the aromatic ring followed
by the decomposition to СО and Н О. Therefore, it is
b
Entry [H SO ] [H BO ] Conversion
Yield (%)
2
4
3
3
(
wt. %) (mol. %)
(%)
TNBA
PA
2
2
necessary to take into account the influence of Еох on the
direction and the depth of conversion of compounds resisꢀ
tant to oxidation.
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
196
196
198
198
198
198
198
100
100
100
100
100
100
100
100
0
7.3
0
80
74
80
79
85
90
90
80
89
89
89
88
86
89
84
65
88
70
74
74
95
88
81
86
88
88
88
91
99
92
10
12
—
1.3
2.4
5.8
8.5
0
1.3
2.8
5.8
0.3
0.6
0.9
1.1
15
13
11
15
18
15
14
14
14
12
0.6
0.8
Considering the oxidation mechanism of TNT in terms
1
,23
of the single electron transfer,
can be presented by Scheme 1.
the oxidation reaction
c
c
The first step of oxidation of aromatic compounds in
1
most cases presents the formation of a cation radical
transforming into the corresponding benzyl radical in the
presence of the methyl group capable of eliminating the
proton. For TNT, polynitrobenzyl radical should be
formed, whose stability is apparently low. The basis for
0
1
2
3
4
5
d
d
d
d
such a hypothesis is provided by quantum chemical calcuꢀ
24,25
laitons
performed for benzylic radicals bearing elecꢀ
tronꢀwithdrawing substituents in paraꢀposition. These calꢀ
culations allow one to draw a conclusion that such subꢀ
stituents in a ring do not favour the stabilization of benzyl
radicals. This fact is also supported by the experimental
a
b
c
2
,4,6ꢀTrinitrotoluene (2 g), Cr O (2.1 g).
2 3
Unreacted TNT.
Reaction mixture was kept for 20 h at room temperature
after the reaction was accomplished.
d
26
Addition of a preliminary prepared solution of HB(HSO4)4
data. For instance, in a study oxidative dimerization of
in 100% H SO .
4ꢀnitroꢀ, 2,4ꢀdinitroꢀ, and 2,4,6ꢀtrinitrotoluenes with
2
4

2
218
Russ.Chem.Bull., Int.Ed., Vol. 65, No. 9, September, 2016
Мikhal´chenko et al.
Scheme 1
mation of TNT dication, which is capable of oxidizing
smoothly to TNBA, rises. For the methyl group of TNT
to be effectively oxidized, the value of the potential of the
second electron transfer should be reached, i.e., the preꢀ
requisite of Еох ≥ Е is to be met. Thus, the results obꢀ
2
tained on the TNT oxidation provide background to explain
the mechanism of the TNT oxidation via the formation of
a dication rather than a cation radical. Reactivity of dicaꢀ
2
8,29
tions in electrophilic reactions is known
to be suffiꢀ
ciently higher than that of monocations. Evidently, it is
just this fact that accounts for practically quantitative and
selective synthesis of TNBA under conditions favoring
the formation of TNT dications. The plausible mechaꢀ
nism of TNT oxidation comprising the formation of
a dication as a key reaction species at a specific reaction
centre takes place probably also on oxidation of other comꢀ
pounds resistant to oxidation.
Тo conclude, we have studied the effect of the medium
acidity on the yield of TNBA on the TNT oxidation with
chromic anhydride in concentrated H SO in the presence
2
4
of boric acid. The yield and selectivity of the TNBA forꢀ
mation increases in the presence of H BO , as explained
3
3
by an increase in acidity of the medium due to the formaꢀ
tion of HB(HSO ) contributing to the rise of the oxidaꢀ
polynitroalkanes in the presence of bases has been investiꢀ
gated. Being electron acceptors, polynitroalkanes convert
anions of polynitroaromatic compounds into the correꢀ
sponding polynitrobenzyl radicals. The latter are dimerꢀ
ized at the methyl group forming 1,2ꢀsubstituted ethanes
and stilbenes. The yield of dimeric products for 2,4ꢀdiꢀ
nitrotoluene is sufficiently lower than for 4ꢀnitrotoluene.
In contrary to 4ꢀnitrotoluene and 2,4ꢀdinitrotoluene, in the
presence of sodium tertꢀbutoxide TNT decomposes yieldꢀ
ing resinous unidentifiable products. Trinitrobenzyl radiꢀ
cal formed during the oxidation of the TNT anion under
the above conditions is propably insufficiently stable to
participate in the dimerization reaction.
4
4
Scheme 2
In the study27 on oxidation of polymethylbenzene nitro
derivatives by cyclic voltammetry in superacidic media, it
was concluded that fluorosulfonation at the methyl group
proceeds following the EECꢀmechanism, i.e., the subꢀ
strate dication reacts with fluorosulfonic acid (a nucleoꢀ
phile) rather than the cation radical.
Considering these data as well as our results on signifiꢀ
cant increase in TNBA yield under conditions wherein the
redox potential of the the oxidation system should be inꢀ
creased, one might propose the mechanism of TNT oxiꢀ
dation (Scheme 2) differing from that depicted on Scheme 1.
Such mechanism (see Scheme 2) explains the role of
the redox potential upon oxidation of TNT to TNBA. If the
oxidation potential Eox is not high enough for the second
electron to be transferred from the cation radical to
give the TNT dication (Eox < Е ), the labile trinitrobenzyl
2
radical (or cation radical) apparently undergoes a cleavꢀ
age of the aromatic ring followed by oxidation of intermeꢀ
diates to СО and Н О. As E increases, the rate of forꢀ
2
2
ox

2
,4,6ꢀTrinitrobenzoic acid
Russ.Chem.Bull., Int.Ed., Vol. 65, No. 9, September, 2016 2219
tion potential of the Cr O72–/Cr3+ system. The effect of
4. S. A. Shevelev, V. A. Tartakovsky, A. L. Rusanov, in Comꢀ
bustion of Energetic Materials, Eds K. K. Kuo, L. T. DeLuca,
Begell House, Inc., New York, 2002, p. 62.
2
the H BO additives is observed, when using concentrated
3
3
(
96—100%) solutions of H SO . Oxidation of TNT procꢀ
2 4
5
. Е. Yu. Оrlova, Khimiya i technologiya brizantnykh vzryvꢀ
chatykh veshchestv [Chemistry and Technology of High Exploꢀ
sive Agents], Khimiya, Leningrad, 1973, p. 180 (in Russian).
. F. Sachs, R. Kempf, Ber., 1902, 35, 2704.
eeds most effectively in solutions practically free of water.
Experimental
6
7
. L. V. Мikhal´chenko, M. Yu. Leonova, V. N. Leibzon, V. P.
Gultyai, V. А. Tartakovskyi, Pat. RF 2485093, Bull. of Invenꢀ
tions, 2013, No. 17; http://www1.fips.ru/wps/portal/IPS_Ru
(in Russian).
Solutions of sulfuric acid of 98 and 100% concentrations
were prepared by adding oleum to commercially available sulfuric
acid (chemically pure grade). Concentration of H SO and oleum
2
4
was determined by titration with 0.1 M NaOH. Solution of tetraꢀ
8. L. V. Mikhal´chenko, M. Yu. Leonova, V. K. Leusenkova,
V. T. Novikov, V. P. Gultyai, Russ. J. Electrochem. (Engl. Transl.),
2015, 51, 1054—1060 [Electrokhimiya, 2015, 51, 1190].
hydrosulfatoboric acid was prepared by dissolving boric acid in
1
8
oleum of the known concentration according to the reaction
9
. A. Astrat´ev, V. Marchukov, V. Suschev, A. Aleksandrov,
Abstr. of 218th ACS National Meeting (USA, New Orleans, LA,
August 22—26), 1999, Abstract No. 50.
B(OH) + 3 H S O = HB(HSO ) + 2 H SO .
3
2
2
7
4 4
2
4
2,4,6ꢀTrinitrotoluene was recrystallized from ethanol; H BO3
3
1
0. H. T. Clarke, W. W. Hartman, Org. Synth., 1922, Vol. 2,
p. 95 [Org. Synth., 1941, Coll. Vol. 1, p. 543].
1. DE Pat. 127325, 1901 (kl. 12o).
was dried over conc. H SO .
2
4
Oxidation of TNT was carried out on fourꢀnecked flask
equipped with a stirrer, a thermometer and a drying tube. To
1
1
2. А. B. Tronov, N. V. Chernobrovin, N. V. Pupova, Trudy
Altaiskogo politekhnicheskogo instituta [Proceedings of the
Altai Polytechnical University], 1968, No. 2, p. 68 (in Russian).
3. M. Haba, A. Hatachi, JP 49048420, JP 1970ꢀ28018.
4. V. М. Talanov, G. М. Zhytnyi, Ionnye ravnovesiya v vodnykh
rastvorakh [Ionic Equilibria in Aqueous Solutions], Akademiya
Estestvoznaniya, Moscow, 2007, 95 pp. (in Russian).
5. M. J. Jorgenson, D. R. Hartter, J. Am. Chem. Soc., 1963, 85, 878.
6. Spravochnik po electrokhimii [Electrochemistry Handbook] Ed.
А. М. Sukhotin, Khimiya, Moscow, 1981, p. 488 (in Russian).
7. R. J. Gillespie, E. A. Robinson, Nonꢀaqueous Solvent System,
Ed. T. C. Waddington, Acad. Press, New York, 1965, p. 117.
8. R. J. Gillespie, T. E. Peel, E. A. Robinson, J. Am. Chem.
Soc., 1971, 93, 5083.
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mans, Green and Co., London—New York, 1936, 449 pp.
0. R. N. Helevin, Zhurn. Obshch. Khimii [Russ. J. Gen. Chem.],
986, 56, 1150 (in Russian).
1. R. N. Helevin, Zhurn. Obshch. Khimii [Russ. J. Gen. Chem.],
988, 58, 603 (in Russian).
2. H. G. Adolph, J. C. Dacons, M. J. Kamlet, Tetrahedron,
963, 19, 801.
a suspension of TNT (2 g) in H SO4 (11 mL) with a known
2
concentration of H BO or its solution in 100% H SO was added
3
3
2
4
if required by the experimental conditions. Then CrO (2.1 g) was
gradually added and the mixture was maintained for 2 h at
5—55 °С. The reaction mixture was then poured in the glass with ice
15 g). On mixing with ice, the main portion of the TNBA preꢀ
3
1
1
4
(
cipitated together with the unreacted TNT. The precipitate was
filtered, washed with ice water, dried, and weighted. To determine
the yield of TNBA and detect the possible byꢀproducts of the
reaction, the filtrate was diluted with water (125—150 mL) followed
by extraction with ethyl acetate, and the organic compounds
dissolved therein were separated from chromium compounds as
well as from boric acid. The extract and the precipitate were
1
1
1
1
1
2
2
2
2
1
analysed separately by the Н NMR spectroscopy and TLC and
the amount of the products obtained was calculated according to
1
the Н NMR spectral data. In some cases TNBA was separated
from TNT by dissolving the mixture in an aqueous alkaline soluꢀ
tion.19 Losses of TNBA during purification were 5—7% from its
analytically detected amout in the mixture. Melting point of the
purified TNBA is 220 °С.
1Н NMR spectra were registered on a «Bruker АМꢀ300»
spectrometer (working frequency 300 MHz) in (CD ) CO or
1
1
1
3
2
3. О. Yu. Okhlobystin, Perenos electrona v organicheskikh
reaktsiyakh [Electron Transfer in Organic Reactions], Izdꢀvo
Rostov University, RostovꢀonꢀDon, 1974, 118 pp. (in Russian).
4. D. W. Rogers, A. A. Zavitsas, N. Matsunaga, J. Phys.
Chem. A, 2009, 113, 12049.
CD OD. Amounts of TNT, TNBA, and PA were calculated
3
from the integral intensities of the corresponding signals of aroꢀ
1
matic protons in the Н NMR spectra.
2
2
2
1
2
2
,4,6ꢀTrinitrobenzoic acid. Н NMR (CD OD), δ: 9.18 (s, 2 H).
3
,4,6ꢀTrinitrophenol (picric acid). 1Н NMR (CD OD), δ:
3
5. Zh. Wen, Z. Li, Zh. Shang, J.ꢀP. Cheng, J. Org. Chem.,
8
3
.94 (s, 2 H).
,4,6ꢀTrinitrotoluene. Н NMR (CD OD), δ: 8.88 (s, 2 H);
2
001, 66, 1466.
1
2
3
6. A. V. Makarevich, M. B. Shcherbinin, A. G. Bazanov, I. V.
.26 (s, 3 H).
Tselinskii, Russ. J. Org. Chem. (Engl. Transl.), 1999, 35, 684
[
Zh. Org. Khim., 1999, 35, 710].
7. А. P. Rudenko, F. Pragst, Russ. J. Org. Chem. (Engl. Transl.),
998, 34, 1589 [Zh. Org. Chem., 1998, 34, 1660].
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