Study of 4-acetylaminotoluene ozonation in acetic acid

Article abstract of DOI:10.1134/S0965544109050119

The reaction of 4-aminotoluene with ozone in acetic acid was studied. It was found that the reaction proceeds at a high rate, giving predominantly tars. The product composition changes after acylation of the amino group: aliphatic peroxides prevailed among the oxidation products and 4-acetylaminobenzoic acid was identified (16%) as well. It was shown that the presence of a catalyst (cobalt(II) acetate) had no substantial effect on the oxidation selectivity for the methyl group (32%) and only the addition of potassium bromide increased the activity of the catalyst; the yield of 4-acetylaminobenzoic acid reached 70%. The mechanism of oxidation in the presence of a cobalt-bromide catalyst explaining the obtained results was discussed.

Full text of DOI:10.1134/S0965544109050119

ISSN 0965-5441, Petroleum Chemistry, 2009, Vol. 49, No. 5, pp. 397–400. © Pleiades Publishing, Ltd., 2009.
Original Russian Text © A.G. Galstyan, A.S. Bushuev, Yu.A. Shumilova, 2009, published in Neftekhimiya, 2009, Vol. 49, No. 5, pp. 417–420.
Study of 4-Acetylaminotoluene Ozonation in Acetic Acid
A. G. Galstyan, A. S. Bushuev, and Yu. A. Shumilova
Institute for Chemical Technologies, East-Ukrainian National University, Rubezhnoe, Ukraine
e-mail: ozon@megabit.com.ua
Received July 29, 2008
Abstract—The reaction of 4-aminotoluene with ozone in acetic acid was studied. It was found that the reaction
proceeds at a high rate, giving predominantly tars. The product composition changes after acylation of the
amino group: aliphatic peroxides prevailed among the oxidation products and 4-acetylaminobenzoic acid was
identified (16%) as well. It was shown that the presence of a catalyst (cobalt(II) acetate) had no substantial effect
on the oxidation selectivity for the methyl group (32%) and only the addition of potassium bromide increased
the activity of the catalyst; the yield of 4-acetylaminobenzoic acid reached 70%. The mechanism of oxidation
in the presence of a cobalt–bromide catalyst explaining the obtained results was discussed.
DOI: 10.1134/S0965544109050119
The reaction of ozone with aminobenzenes in the attacks mainly π electrons of the aromatic ring—with
liquid phase has been little studied. It was reported [1,
2] that ozone in carbon tetrachloride reacts predomi-
nantly at the lone electron pair of nitrogen with the for-
mation of the bipolar ion ëç3–ArNH2–O–3, which sub-
sequently gives tarry products. It was also noted that
azotoluenes and quinoimines are produced in small
amounts. Aromatic compounds with the oxidized
methyl group were absent from the products.
In this work aiming to synthesize aminobenzoic
acids, we studied the feasibility of the selective oxida-
tion of aminobenzenes at the methyl group, using as an
example the reaction of ozone with 4-aminotoluene in
glacial acetic acid. The experimental procedures and
the analytical techniques were described earlier [3].
the formation of aliphatic peroxides—and, to a lesser
extent, the methyl group (16%), rather than the lone
electron pair of nitrogen. The structure of hydroperox-
ides and their formation mechanism are beyond the
scope of this study, but we suppose that they are similar
to those for benzene hydroperoxides [1]. 4-Acetylami-
nobenzaldehyde and 4-acetylaminobenzoic acid were
detected among the methyl-group oxidation products
(Fig. 1).
+
A certain increase in the oxidation selectivity for the
methyl group is observed in the presence of transition
metal salts as catalysts. The maximum yield of 4-acety-
laminobenzoic acid is reached at 95°ë in the presence
of cobalt(II) acetate. However, the selectivity for
RESULTS AND DISCUSSION
Table 1. Kinetic parameters of the reactions involved in the
catalytic cycle of oxidation of 4-acetylaminotoluene with ozone
in acetic acid at 20°C, [ArCH3]0 = 0.4, [Co(OAc)2]0 = 0.1,
[KBr]0 = 0.1, and [O3]0 = 3.2 × 10–4 mol/l
Oxidation of 4-aminotoluene with an ozone–air
mixture in glacial acetic acid at 20°ë, as documented in
[1], primarily affects the amino group to gives tars of an
unidentified structure and insignificant amounts of
4-nitrotoluene (10%) and toluquinone (≈10–4 mol l–1).
The absence of aromatic compounds oxidized at the
methyl group from the oxidate is due to the low reactiv-
ity of 4-aminotoluene in this direction (kef is 0.8 [4] and
2.5 × 103 l mol–1 s–1 for toluene and 4-aminotoluene,
respectively) (Table 1).
The high reactivity of 4-aminotoluene is lost after its
N-acylation (acetic acid, í = 110°ë, τ = 2 h). The prod-
uct 4-acetylaminotoluene resembles methylbenzenes in
reactivity toward ozone (kef = 7.0 l mol–1 s–1, Table 1)
[4], thereby suggesting a change of the oxidation mech-
anism. It is obvious that under these conditions, the
ozone molecule in accordance with the classical con-
cept of the ozone reaction with alkylbenzenes [4]
Reaction
E,
kJ/mol
Reaction
k, l/(mol s)
rate,
mol/(l s)
O3 + 4-aminotolu- (2.5 × 103) 200
0.3
ene
O3 + 4-acetylami-
notoluene
7.0 0.7
29.9
0.9 × 10–3
Co2+ + O3
(9.3 × 102) 90
(1.3 × 103) 100
(1.7 0.2) × 10–2
5.2 × 10–2
5.4 × 10–2
0.7 × 10–3
Co2+Br• + O3
Co3+ + 4-acety-
laminotoluene
31.6
47.8
Co2+Br• + 4-acety- (2.5 0.2) × 10–2
1.0 × 10–3
laminotoluene
397

398
GALSTYAN et al.
c, mol/l
c, mol/l
0.4
0.4
0.3
0.2
0.1
2
1
1
2
0.3
0.2
0.1
O
3
4
4
3
3
20
40
60
80
τ, min
5
10
15
20
τ, min
Fig. 2. Change in the concentration of the reaction compo-
nents during the oxidation of 4-acetylaminotoluene with
ozone in acetic acid at 95°ë,[AcNHArCH ] = 0.4, [O ] =
Fig. 1. Change in the concentration of the reaction compo-
nents during the oxidation of 4-acetylaminotoluene with
ozone in acetic acid at 20°ë,[AcNHArCH ] = 0.4, [O ] =
3 0
3 0
–4
−1
4.2 × 10 ; [Co(OAc) ] = 0.1; [KBr] = 0.1 mol l , and
2 0
0
–3
–1
ν
= 8.3 × 10 l s (the arrow marks the cessation of
air
3 0
–3 –1
3 0
–4
–1
ozone supply to the system): (1) 4-acetylaminotoluene,
(2) 4-acetylaminobenzoic acid, (3) 4-acetylaminobenzyl
3.2 × 10 mol l , and ν = 8.3 × 10 l s : (1) 4-acety-
air
laminotoluene, (2) peroxides, (3) 4-acetylaminobenzalde-
hyde, and (4) 4-acetylaminobenzoic acid.
2+
•
bromide, and (4) Co Br .
is explained by the low rate of reaction (3) (k3 ꢀ k1,
Table 1).
methyl group does not exceed 32% even in this case;
the oxidation predominantly affects the aromatic ring.
A further increase in the oxidation selectivity for
methyl group (70%) is observed when potassium bro-
mide is introduced into the system (Fig. 2). It is likely
that a more active species than Co3+ is formed in the
presence of bromide. The nature of this species is
debated. It has been supposed [7, 8] that the high cata-
lytic activity of the mixed catalyst is due with the initi-
ation of the oxidation of methylbenzenes by the bro-
mine atom:
The selectivity in the presence of cobalt(II) acetate
can be enhanced as a result of two-stage oxidation [5]:
ozone primarily reacts with ëÓ2+ (k2 = 9.3 × 102; k1 =
7.0 l mol–1 s–1, Table 1) to give the oxidized form Co3+,
which reacts with 4-acetylaminotoluene at the methyl
group (reaction (3)):
AcNHArCH3 + O3
(1)
Aliphatic peroxides,
ëÓ3+ + Br–
ëÓ2+Br•
Co3+Br–
ëo2+Br•,
(6)
(7)
Co2+ + O3 + H+
Co3+ + HO• + é2,
(2)
(3)
Co2+ + Br•,
AcNHArCH3 + Co3+
AcNHArCH•2 + Co2+ + H+,
•
•
ArCH3 + Br
ArCH2 + HBr,
(8)
(9)
•
•
ArCH2 + O2
ArCH2O2.
•
•
AcNHArCH2 + O2
AcNHArCH2O2 ,
(4)
(5)
However, a Japanese group [9] believes that the
cobalt–bromide complex formed in the system via reac-
tion (6) is fairly stable and the oxidation is initiated
according to the reaction of methylbenzene with the
reactive cobalt–bromide radical ion:
•
2AcNHArCH2O2
Products.
In a dioxygen atmosphere, the acetylaminobenzyl
radical obviously transforms into a peroxide radical
(reaction (4)) and further to molecular products includ-
ing 4-acetylaminobenzoic acid.
•
ArCH3 + ëÓ2+Br•
ArCH2 + ëÓ2+Br– + H+. (10)
In this connection, the test AcNHArCH3–O3–KBr
The low methyl-group oxidation selectivity, even at system in which bromide anions are also actively oxi-
commensurable substrate and catalyst concentrations, dized into bromine atoms is of great interest. If the
PETROLEUM CHEMISTRY Vol. 49 No. 5 2009

STUDY OF 4-ACETYLAMINOTOLUENE OZONATION IN ACETIC ACID
399
catalysis indeed involves reactions (6)–(8), the ozona-
tion of 4-acetylaminotoluene in the presence of potas-
sium bromide must yield oxygenated aromatic prod-
ucts. However, the investigation has shown that reac-
tions (11)–(14) actually proceed in the AcNHArCH3–
O3–KBr system:
log[O ]
3
–4.0
–2.0
–3.7
–1.7
–3.4
–1.4
4
–3.1
log[KBr]
–1.1
logW
–1.1
•
Br– + O3 + H+
Br + O2 + HO•,
(11)
3
2
1
•
AcNHArCH3 + Br•
AcNHArCH2 + HBr, (12)
–1.4
–1.7
•
•
AcNHArCH2 + Br
AcNHArCH2Br,
(13)
(14)
•
•
Br + Br
Br2.
It is the addition of cobalt(II) acetate to the system
that results in the active involvement of 4-acetylamino-
toluene in the oxidation at the methyl group to give the
corresponding aromatic carboxylic acid (Fig. 2). The
obtained data argue in favor of initiation of the selective –2.0
oxidation through reaction (10).
Thus, the high catalytic activity of cobalt(II) acetate
in the presence of potassium bromide is due to the for-
mation of the highly reactive cobalt–bromide radical
ion complex via reaction (15):
–1.9
–1.0
–1.6
–0.7
–1.3
–1.0
log[Co(OAc) ]
2
Co2+Br– + O3 + H+
(Co3+Br–)Co2+Br• + O2 + HO•,
–0.4
–0.1
log[ArCH ]
3
(15)
Fig. 3. Dependence of the rate of 4-acetylaminotoluene oxi-
dation on the concentration of (1) cobalt(II) acetate,
(2) potassium bromide, (3) the substrate, and (4) ozone at
95°ë (for the conditions, see the legend to Fig. 2).
which involves the substrate in the oxidation at the
methyl group (reaction (10)).
The subsequent transformations of the 4-acetylami-
nobenzyl radical can be described by the following
reaction scheme:
icals (W19) is considerably higher than the propagation
rate growth (W17 and W18).
•
•
(2) The cessation of the ozone supply to the system
AcNHArCH2 + O2
AcNHArCH2O2,
(16)
(17)
•
leads to the full termination of the process, with Co2+Br
•
AcNHArCH2O2 + AcNHArCH3
transforming into Co2+Br– (Fig. 2). This fact is an addi-
tional piece of evidence for the secondary role of reac-
tion (18), according to which the active form of the cat-
•
AcNHArCH2O2H + AcNHArCH2,
•
alyst, Co2+ Br , could be regenerated in the absence of
•
AcNHArCH2O2 + Co2+Br– + H+
ozone. The calculations have shown that the rate of
reaction (18) (W18 ≈ 2.1 × 10–5 mol/(l s)) is indeed three
orders of magnitude below that of reaction (13) (W14 ≈
(18)
AcNHArCH2O2H + Co2+Br•,
•
2AcNHArCH2O2
Products.
(19) 5.4 × 10–2 mol/(l s)).
Under the reaction conditions when [O2]/[O3] ≈ 102,
the 4-acetylaminobenzyl radical predominantly trans-
forms into the peroxide radical (reaction (16)), which
subsequently recombines according to reaction (19).
The participation of the peroxide radical in propagation
reactions (17) and (18) can be ignored. This neglect
stems from the following considerations:
(3) According to the kinetic data, the oxidation reac-
tion of 4-acetylaminotoluene is zero order in oxygen
and first order in the substrate, the catalyst, or ozone
(Fig. 3). The consumption of ozone per mole of the sub-
strate is close to the theoretical amount of the oxidant
required for conversion of 4-acetylaminotoluene into
4-acetylaminobenzoic acid.
(1) According to published data [8] for toluene at
(4) The oxidation selectivity for methyl group sub-
70°ë W17 ≈ 0.6 × 10–6; W18 ≈ 2.1 × 10–5, and W19 ≈ 1 × stantially depends on the catalyst concentration
10−4 mol/(l s) ([ArCH3]0 = 0.4; [Co2+Br–]0 = 0.01 mol/l, (Table 2) and reaches high values at commensurable
k17 = 1.6, k18 = 2.1 × 103, k19 = 108 l/(mol s) [10]). Hence, concentrations of 4-acetylaminotoluene and the cata-
it follows that the recombination rate for peroxide rad- lyst when W15/W1 ≥ 2 (Table 1).
PETROLEUM CHEMISTRY Vol. 49 No. 5 2009

400
GALSTYAN et al.
Table 2. Influence of the cobalt(II) acetate concentration on
the selectivity of 4-acetylaminotoluene oxidation with ozone
in acetic acid at 95°C in the presence of potassium bromide
(see Table 1 for conditions)
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of 4-acetylaminotoluene at the methyl group with
ozone was shown. Preliminary acetylation of the amino
group in the substrate precludes the destructive attack
of ozone and makes it possible in the presence of a
mixed cobalt–bromide catalyst to run mild selective
oxidation of 4-acetylaminotoluene into 4-acetylami-
nobenzoic acid with a yield of 70% which, if need be,
can be readily hydrolyzed (EtOH, H+, 60°C) to 4-ami-
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