Oxidation of 4-Aminotoluene by ozone-air mixture in the presence of a stop-reagent

Article abstract of DOI:10.1134/S1070427212100187

A study of the liquid-phase oxidation of 4-aminotoluene by ozone to 4-amino benzaldehyde in the presence of a manganese/bromide catalyst demonstrated that introduction of potassium bromide into the oxidizing system makes it possible to obtain 4-aminobenzaldehyde in the form of 4-(acetylamino)benzylidene diacetate (84.5%) as the main product. A reaction scheme accounting for the results obtained was suggested. Pleiades Publishing, Ltd., 2012.

Full text of DOI:10.1134/S1070427212100187

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2012, Vol. 85, No. 10, pp. 1586−1589. © Pleiades Publishing, Ltd., 2012.
Original Russian Text © A.G. Galstyan, I.A. Zema, A.S. Bushuev, 2012, published in Zhurnal Prikladnoi Khimii, 2012, Vol. 85, No. 10, pp. 1653−1657.
ORGANIC SYNTHESIS
AND INDUSTRIAL ORGANIC CHEMISTRY
Oxidation of 4-Aminotoluene by Ozone–Air Mixture
in the Presence of a Stop-Reagent
A. G. Galstyan, I. A. Zema, and A. S. Bushuev
Institute of Chemical Technologies, Dal’East-Ukrainian National University, Rubezhnoe, Lugansk oblast, Ukraine
e-mail: ozon@megabit.com.ua
Received March 30, 2012
Abstract—A study of the liquid-phase oxidation of 4-aminotoluene by ozone to 4-amino benzaldehyde in the
presence of a manganese/bromide catalyst demonstrated that introduction of potassium bromide into the oxidiz-
ing system makes it possible to obtain 4-aminobenzaldehyde in the form of 4-(acetylamino)benzylidene diacetate
(84.5%) as the main product. A reaction scheme accounting for the results obtained was suggested.
DOI: 10.1134/S1070427212100187
4-Aminobenzaldehyde (4-AB) is widely used
in manufacture of pharmaceutical preparations and
organic dyes [1–4]. It is for the most part industrially
produced by reduction of 4-nitrotoluene with sulfides in
an alkaline medium [5]. Among disadvantages of this
method are the low yield of the product and formation
of large amounts of a toxic wastewater.
of a catalyst, manganese(II) acetate. The main oxidation
products are 4-(acetylamino)benzyl acetate (4-AABA,
65%) and 4-(acetylamino)benzylidene diacetate
(4-AABDA, 19.5%). It is impossible to oxidize 4-AT to
predominantly obtain 4-AB (in the acylated form) under
these conditions.
Proceeding with these studies, we examined the
possibility of making deeper the oxidation of 4-AAT
by its ozonation in the presence of a mixed manganese/
bromide catalyst, because it is known [8, 9] that the
catalytic activity of variable-valence metal ions grows
in a mixture with alkali metal bromides.
This study is devoted to analysis of the oxidation
process of 4-aminotoluene (4-AT) by an air–ozone
mixture in order to develop a low-waste method for
production of 4-AB in high yield.
It has been shown previously [6] thatAT-4 reacts with
ozone as 4-acetylaminotoluene (4-AAT) in a solution of
acetic anhydride in the presence of sulfuric acid. This
fact is accounted for by the high rate of acylation by
acetic anhydride, which ends under the experimental
conditions while a mixture for ozonation is prepared.
Ozone attacks 4-AAT predominantly at the aromatic
ring [reaction (2)], and the selectivity of oxidation at the
methyl group [reaction (1)] does not exceed 26%:
Our study demonstrated (Table 1) that addition of
potassium bromide to the oxidizing system at 30°C
does improve the selectivity and depth of oxidation at
the methyl group of 4-AAT with predominant formation
of 4-AABDA. The content of 4-AABDA in the
reaction products increases with the potassium bromide
concentration and reaches the maximum value at [KBr]
= 0.08 M (Table 1). At a [ArCH3] : [Mn(OAc)2] :
[KBr] ratio of 5 : 1 : 1, the reaction yields, at an overall
selectivity in the system of 94.8%, 84.5% 4-AABDA,
10.3% 4-AABA, and trace amounts of 4-(acetylamino)
benzyl bromide (4-AABB).
AcNHArCH3 + O3 → AcNHArC· H2 + OН· + O2,
AcNHArCH3 + O3 → Ozonolysis products.
(1)
(2)
It was shown in [7] that the selectivity of oxidation
at the ethyl group increases to 84.5% in the presence
Based on our experimental results and published data
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OXIDATION OF 4-AMINOTOLUENE BY OZONE–AIR MIXTURE
1587
Table 1. Effect of the reagent concentrations on the composition of the products formed in oxidation of 4-acetylaminotoluene
with ozone in acetic anhydride at 30°C
Overall selectivity
of oxidation at the
methyl group, %
Concentration of reagents, M
Concentration of reaction products, M (%)
Run no.
ozone
4-AAT
0.40
0.40
0.40
0.40
0.40
0.40
0.40
0.40
0.40
0.40
0.40
Mn(Oac)2
0.080
0.080
0.080
0.080
0.040
0.020
0.005
0.080
0.080
0.080
0.080
KBr
0.08
0.08
0.08
0.08
0.08
0.08
0.08
0.10
0.04
0.01
–
4-ААBА
Trace amounts
0.020 (5.5)
0.025 (6.4)
0.040 (10.3)
0.024 (6.1)
0.024 (6.1)
0.017 (4.5)
0.039 (10.0)
0.073 (18.2)
0.106 (26.1)
0.224 (56.0)
4-ААBDА
0.106 (27.0)
0.216 (54.0)
0.292 (73.5)
0.338 (84.5)
0.305 (76.5)
0.239 (59.0)
0.179 (44.3)
0.338 (84.5)
0.280 (70.0)
0.203 (52.5)
0.075 (18.7)
1
2
0.50 × 10–4
1.00 × 10–4
2.00 × 10–4
4.00 × 10–4
4.00 × 10–4
4.00 × 10–4
4.00 × 10–4
4.00 × 10–4
4.00 × 10–4
4.00 × 10–4
4.00 × 10–4
27.0
59.5
79.9
94.8
82.6
65.1
48.8
94.5
88.2
78.6
74.7
3
4
5
6
7
8
9
10
11
[9–13], we suggested the following scheme of catalytic
oxidation of 4-AAT:
system makes it possible to maintain a sufficiently high
concentration of the active form of the catalyst, Mn2+Br·,
and, consequently, also a high formation rate of benzyl
radicals.
Mn2+ + Br− → Mn2+Br−,
(3)
(4)
At [O2]0 >> [O3]0, the benzyl radical being formed
are converted to peroxide radicals [reaction (9)] whose
further transformation occurs due to their recombination
by reaction (10) to give oxidation products.
Mn2+Br− + О3 + H+ → Mn2+Br· + HО· + О2,
AcNHArCH3 + Mn3+ → AcNHArCH2· + Mn2+ + H+, (5)
АсNHArCH2OАс + Mn3+
→ АсNHArCH·OAc + Mn2+ + H+,
(6)
Without ozone, 4-AAT is slowly oxidized by mo-
lecular oxygen in the presence of the manganese bro-
mide catalyst. Only 10% of the substrate is consumed
in 11 h of the reaction. Trace amounts of 4-AABA and
5% 4-AABDA (relative to the consumed 4-AAT) were
AcNHArCH3 + Mn2+Br·
→ AcNHArCH2· + Mn2+Br– + H+,
(7)
AcNHArCH2ОАс + Mn2+Br
→ AcNHArCHОАс + Mn2+Br– + H+,
(8)
(9)
AcNHArCH2· + О2 → AcNHArCH2О2·,
2AcNHArCH2О2· → Reaction products.
Table 2. Rate constants of the elementary stages of the
catalytic cycle at 20°C
(10)
Reaction
4-ААТ + О3 (1)
k, M–1 s–1
The catalytic activity of manganese(II) acetate
increases in the presence of potassium bromide due to
the formation of a highly active manganese bromide
complex Mn2+Br· [reactions (3) and (4) [8, 9], which
can oxidize the 4-AAT and 4-AABA molecules faster
than Mn3+ does (Table 2). At the initial instant of
time, when hardly any oxidation products are present
in the system, active species are formed by reactions
(5) and (7). The continuous delivery of ozone into the
0.850
4-ААТ + Mn3+ (2)
0.200
0.270
0.060
0.090
0.010
0.016
4-AAT + Mn2+Br· (3)
4-ААBА + Mn3+ (4)
4-ААBА + Mn2+Br· (5)
4-ААBDА + Mn3+ (6)
4-ААBDА + Mn2+Br· (7)
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GALSTYAN et al.
c, M
(a)
(b)
c, M
(c)
(d)
τ, min
τ, min
Effect of temperature on the kinetics of 4-AAT oxidation in acetic anhydride. [ArCH3]0 = 0.4, [O3]0 = 4 × 10–4, [Mn(OAc)2]0 = 0.08,
[KBr]0 = 0.08, [H2SO4]0 = 0.8 M; gas flow rate 8.3 × 10–3 L s–1. (c) Concentration of 4-AAT and its conversion products and (τ) process
duration. T (°C): (a) 10, (b) 20, (c) 30, (d) 40. (1) 4-AAT, (2) 4-AABA, (3) 4-AABDA, and (4) 4-AABA.
found among the reaction products. Introduction of
ozone into the system leads to a substantial increase in
the oxidation rate of the methyl group, with predomi-
nant formation of 4-AABDA (Table 1). As indicated by
the experimental data (Table 1), the oxidation selectiv-
ity depends on the ozone concentration in the ozone–air
mixture: a decrease in this concentration from 4.0 × 10–4
to 0.5 × 10–4 M results in that the overall selectivity falls
by nearly 68%.
27 × 10–2 M–1 s–1, where k is the reaction rate constant;
r4 = 4.8 × 10–2, r7 = 6.1 × 10–3 M s–1, i.e., r4 : r7 = 23).
At [O3]0 = 1.25 M, r4 : r7 = 7 (r is the reaction rate).
In the presence of sulfuric acid, k4 decreases to
12.1 M–1 s–1 (T = 30°C), which leads to a strong decel-
eration of reaction (4). Under these conditions, r4 : r7 =
0.46, i.e., the rate of manganese reduction by reaction
(7) substantially exceeds the rate of its oxidation by re-
action (4). It is known that, in a system simultaneously
containing oxidized and reduced forms of manganese,
binuclear complexes with a strongly lowered redox
potential and lower catalytic activity than that of Mn3+
are formed [14]. Thus, as the ozone concentration in
the ozone–air mixture decreases, reaction (7) becomes
slower and the r2 : r7 ratio increases (in the presence of
sulfuric acid, k2 remains nearly unchanged and equal to
2.68 M–1 s–1, and just this circumstance leads to a de-
crease in the oxidation selectivity for the methyl group).
An analysis of the data obtained demonstrated that
this dependence, only observed in the presence of
sulfuric acid, is due to the change in the reaction rate
ratio r2 : r4 : r7. If ozonation is performed without sulfuric
acid, varying the ozone concentration within the range
under study hardly affects the oxidation selectivity.
The reason is that r4 always exceeds r7 (r4 : r7 = 21–
66) and manganese acetate is present in solution in the
oxidized form, which promotes ozonation of 4-AAT at
the methyl group (at [O3] = 4.0 × 10–4, [4-AAT] = 0.4,
[Mn(OAc)2] = 0.08, [KBr] = 0.08 M; k4 = 2.0 × 103, k7 =
The rate and selectivity of 4-AAT oxidation to
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OXIDATION OF 4-AMINOTOLUENE BY OZONE–AIR MIXTURE
1589
4-AABDA depend on the reaction temperature. The
reaction model.
optimal temperature is 30°C. At lower temperatures,
the overall selectivity remains nearly unchanged, but
the fraction of the acylated alcohol grows. Raising the
temperature leads to an increase in the rate and depth of
oxidation: 4-AABDA becomes an intermediate product
further converted to 4-(acetylamino)benzoic acid (4-
AABA) (see figure).
CONCLUSIONS
(1) It was shown that 4-aminotoluene can be oxidized
to the corresponding 4-(acetylamino)benzylidene
acetate with ozone in acetic anhydride in the presence of
a mixed manganese/bromide catalyst. The yield of the
acylated aldehyde is 84.5%.
Thus, it was found that, with the reaction of ozone
with 4–AAT catalyzed by a mixture of manganese(II)
acetate and potassium bromide in acetic anhydride in
the presence of sulfuric acid at a temperature of 30°C,
it is possible to preclude ozonolysis of the aromatic ring
and direct the process to oxidation of the methyl group
to give predominantly 4-AABDA (84.5%).
(2) A reaction scheme accounting for experimental
data was considered.
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