Catalysis of reaction between ozone and 4-hydroxytoluene in acetic anhydride

Article abstract of DOI:10.1134/S1070363214080118

Kinetics and products of 4-hydroxytoluene oxidation with ozone-air mixture in the presence of transition metal acetates as catalysts have been studied. Main steps of the catalytic series have been considered, and a mechanism of redox catalysis has been proposed which conforms to the experimental data and enables control over the direction, depth, and selectivity of the oxidation. Only manganese(II) acetate has been found to exhibit high catalytic activity in the presence of catalytic amounts of mineral acids. Manganese(II) acetate largely suppresses electrophilic reaction of ozone with the aromatic ring, so that the main reaction direction is oxidation of the methyl group with formation of 4-acetoxybenzyl acetate as the major product (62.6%) and 4-acetoxybenzylidene diacetate as a minor one (10.2%).

Full text of DOI:10.1134/S1070363214080118

ISSN 1070-3632, Russian Journal of General Chemistry, 2014, Vol. 84, No. 8, pp. 1510–1514. © Pleiades Publishing, Ltd., 2014.
Original Russian Text © A.G. Galstyan, A.A. Sedykh, G.A. Galstyan, 2014, published in Zhurnal Obshchei Khimii, 2014, Vol. 84, No. 8, pp. 1284–1288.
Catalysis of Reaction between Ozone and 4-Hydroxytoluene
in Acetic Anhydride
A. G. Galstyan, A. A. Sedykh, and G. A. Galstyan
Institute of Chemical Technology, Dal’East Ukrainian National University,
ul. Lenina 31, Rubezhnoe, Lugansk oblast, 93000 Ukraine
e-mail: ozon@megabit.com.ua
Received January 23, 2014
Abstract—Kinetics and products of 4-hydroxytoluene oxidation with ozone–air mixture in the presence of
transition metal acetates as catalysts have been studied. Main steps of the catalytic series have been considered,
and a mechanism of redox catalysis has been proposed which conforms to the experimental data and enables
control over the direction, depth, and selectivity of the oxidation. Only manganese(II) acetate has been found to
exhibit high catalytic activity in the presence of catalytic amounts of mineral acids. Manganese(II) acetate
largely suppresses electrophilic reaction of ozone with the aromatic ring, so that the main reaction direction is
oxidation of the methyl group with formation of 4-acetoxybenzyl acetate as the major product (62.6%) and 4-
acetoxybenzylidene diacetate as a minor one (10.2%).
Keywords: 4-hydroxytoluene, 4-acetoxytoluene, ozone, acetic anhydride, catalyst, oxidation
DOI: 10.1134/S1070363214080118
We previously showed that 4-hydroxytoluene
reacted with ozone in the form of acetoxytoluene in the
sulfuric acid–acetic anhydride medium, and that the
process involved competing reactions of ozone with
the methyl group and aromatic ring (ozonolysis) [1]
(Scheme 1).
present work we examined the reaction of 4-hyd-
roxytoluene with ozone in the presence of transition
metal salts in order to enhance the selectivity of the
methyl group oxidation [2].
The ozonation of 4-acetoxytoluene in acetic
anhydride at 278 K in the presence of transition metal
salts revealed improved selectivity for the methyl
group oxidation, and the major products were 4-acet-
oxybenzyl acetate and 4-acetoxybenzylidene diacetate.
The transition metal salts acted as oxidation catalysts,
and ozone reacted preferentially with metal cation Mn+
In all the cases, oxidation of methyl group stopped
at the stage of formation of acetylated benzyl alcohol
and benzaldehyde, the final product being 4-
acetoxybenzoic acid. The selectivity with respect to the
methyl group oxidation did not exceed 20.5%. In the
Scheme 1.
Ac2O
H+
H3C
OAc
H3C
OH
Products of ozonolysis
CH2OAc
O3, Ac2O, H+
CH(OAc)2
COOH
+
OAc
OAc
OAc
1510

CATALYSIS OF REACTION BETWEEN OZONE AND 4-HYDROXYTOLUENE
1511
Table 1. Selectivity of oxidation of 4-acetoxytoluene with ozone in acetic anhydride in the presence of sulfuric acid and
transition metal salts; 278 K; [4-AT]0 = 0.4, [M(OAc)2]0 = 0.1, [H2SO4]0 = 1.2, [O3]0 = 4.0×10–4 mol/L
Yield of oxidation products, %
Metal salt
Er298 M/M+
4-acetoxybenzyl acetate
4-acetoxybenzylidene diacetate
Co(OAc)2
Mn(OAc)2
Cr(OAc)2
Ni(OAc)2
1.810
1.510
0.740
0.407
13.7
63.3
36.1
30.2
7.3
14.0
6.3
3.1
to give its oxidized form M(n+1)+ (1) which was in turn
reduced according to reaction (2) with formation of
benzyl radicals, and the latter were oxidized into final
products [3].
sulfuric acid the oxidation was described by Eq. (3),
and in the presence of sulfuric acid Eq. (4) was held.
Mn2+ + O3 + 2H+ → Mn4+ + O2 + H2O,
Mn2+ + O3 + H+ → Mn3+ + O2 + HO.
(3)
· (4)
Mn+ + O3 + H+ → M(n+1)+ + O2 + HO·,
(1)
(2)
In the absence of sulfuric acid, manganese(II)
rapidly reacted with ozone, and the k2 value attained
4.45 × 103 L mol–1 s–1 at 303 K. Sulfuric acid slowed
down the oxidation of Mn(II): the rate constant k2 at
303 K was lowered by two orders of magnitude in the
presence of 1.2 mol/L of sulfuric acid (Table 2).
Likewise, the reaction of cobalt(II) acetate with ozone
was of the first order with respect to the both reactants,
and Co(II) was oxidized to Co(III).
·
AcOC6H4CH3 + Mn+1 → AcArCH2 + Men+ + H+.
The selectivity of the oxidation of 4-acetoxytoluene
depended on the redox potential of the Mn+/M(n+1)+
couple and was found to increase in the Ni(II) < Cr(II) <
Mn(II) series, being the highest in the presence of
manganese(II) acetate (Table 1).
Despite the highest redox potential and high
catalytic activity in the oxidation of arenes [3, 4],
cobalt(II) acetate showed no catalytic activity in the
presence of H2SO4.
Co2+ + O3 + H+ → Co3+ + O2 + HO·.
(5)
[O3]0 × 104, mol/L
In order to get a deeper insight into the mechanism
of redox catalysis in reactions of arenes with ozone, we
examined the reaction of ozone with manganese(II)
and cobalt(II) acetates and the reactions of Mn(III) and
Co(III) acetates with 4-acetoxytoluene in acetic
anhydride in the presence of sulfuric acid. The reaction
of O3 with Mn(OAc)2 at 278–303 K was of the first
order with respect to the both reactants (Fig. 1). The
kinetic parameters of that reaction at different
temperatures are given in Table 2. In the absence of
sulfuric acid, the electronic absorption spectra of
ozonized solutions of manganese acetate were similar
to those of Mn(IV) solutions (Fig. 2) [5, 6]. The
solution was brown, also typical of Mn(IV) ions. In the
presence of sulfuric acid exhaustive oxidation of
Mn(II) yielded Mn(III), as confirmed by the absorption
spectrum of the resulting solution (Fig. 2).
[Mn(OAc)2]0× 10, mol/L
In agreement with the published data [7–9],
ozonation of transition metal complexes with organic
ligands followed mainly a two-electron intramolecular
mechanism. According to our data, in the absence of
Fig. 1. Plots of the rate of oxidation of 4-acetoxytoluene at
278 K versus concentrations of (1) manganese(ІІ) acetate at
[O3]0 = 4.0× 10–4 mol/L and (2) ozone at [Mn(OAc)2]0 =
0.1 mol/L.
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GALSTYAN et al.
Table 2. Rate constants of the reaction of ozone with manganese(ІІ) acetate in acetic anhydride
Initial concentration, mol/L
Temperature,
k, L mol–1 s–1
E, kJ mol–1
A, L mol–1 s–1
[Mn(OAc)2]0 ×
103
[O3]0 ×105
[H2SO4]0
K
278
283
288
303
278
283
288
303
2.3–18.1
2.3–18.1
2.3–18.1
2.3–18.3
2.3–18.1
2.3–18.1
2.3–18.1
2.3–18.3
1.6–4.5
1.6–4.5
1.6–4.5
1.6–4.5
1.6–4.5
1.6–4.5
1.6–4.5
1.6–4.5
1.0–1.2
19.2±2.0
24.5±2.4
31.6±3.0
59.4±6.0
1000±100
1380±130
1915±190
4450±400
1.0–1.2
26.6±3.0
48.3±4.0
(2.1±0.2)×106
1.0–1.2
1.0–1.2
–
–
–
–
(1.1±0.1)×1012
The rate constant k5 was of 9.1 × 102 L mol–1 s–1 at
303 K in the absence of sulfuric acid. Addition of the
latter reduced k5 to 1.0 L mol–1 s–1. The findings may
be rationalized taking into account the structure of the
transition complex [M+ · · · O3] whose stability should
be determined by the ligand environment. In the acetic
anhydride medium, acetate anions and acetic anhydride
molecules could act as ligands [M(OAc)x(Ac2O)y].
Presumably, in the presence of sulfuric acid ligand
exchange with participation of hydrogen sulfate anion
led to the [Me(OAc)x–a(Ac2O)y–b(HSO4–)a+b] complex,
which hindered electron transfer from the metal to
ozone in the intermediate complex [Mn+ · · · O3] [7].
Almost no oxidation of 4-acetoxytoluene was observed
under those conditions (Fig. 3). Within the observation
time the color of the solution remained pink which was
typical of the reduced form of the catalyst. That is
difficult to explain in terms of the existing views:
cobalt generally exists mainly in the oxidized form
under the conditions of methylbenzenes ozonation [2].
Cobalt should also be present in the oxidized form in
the course of 4-acetoxytoluene oxidation, for the rate
of accumulation of Co(III) (r5 = 7.2 × 10–5 mol L–1 s–1;
k5 = 1.0 L mol–1 s–1, [O3]0 = 4.0 × 10–4 mol L–1,
[Co(OAc)2]0 = 0.18 mol L–1) was an order of
magnitude higher than the rate of consumption of 4-
acetoxytoluene in the reaction with Co(III) in the
presence of sulfuric acid, as calculated from the data in
Fig. 3 (curve 1). The pattern is very untypical of
ozonolysis catalysis by cobalt acetate; we believe that
it is possible only if there are other preferential paths
of Co(III) consumption.
It was surprising that cobalt(II) acetate showed no
catalytic activity in the presence of sulfuric acid.
D
As follows from the data in Fig. 3, exhaustive
ozonation of cobalt(II) acetate in acetic anhydride
yielded Co(III) which fairly slowly reacted with acetic
anhydride; according to our data, the effective rate
constant of the reaction of Co(III) with acetic
anhydride at 293 K was of 3.4 × 10–4 L mol–1 s–1. As a
result, the concentration of ozone in the reaction
solution was almost constant after termination of its
feeding to the system (Fig. 3, curve 3). Addition of
sulfuric acid promoted fast reduction of Co(III) to
Co(II) (Fig. 3, curve 3). Characteristically, 4-acetoxy-
toluene added to the system simultaneously with
sulfuric acid almost was not oxidized within the ob-
servation period.
λ, nm
Fig. 2. Electronic absorption spectra of (1) Mn(OAc)2,
(2) Mn(OAc)3, and ozonized Mn(OAc)2 (3) in the presence
and (4) in the absence of sulfuric acid.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 84 No. 8 2014

CATALYSIS OF REACTION BETWEEN OZONE AND 4-HYDROXYTOLUENE
Our experimental data indicated fast reduction of
1513
τ, min
Co(III) in the reaction with acetic anhydride in the
presence of sulfuric acid. That fast reaction was likely
allowed due to exchange of coordinated acetate ion
and acetic anhydride molecule for hydrogen sulfate ion
at the metal cation, which facilitated electron transfer
from acetic anhydride molecule to Co(III). The kinetic
data were consistent with the above conclusion: in the
presence of sulfuric acid at 293 K the effective rate
constant of the Co(III) reaction with acetic anhydride
increased from 3.4 × 10–4 to 0.31 L mol–1 s–1, i.e., by
three orders of magnitude, so that the ratio r[Co(III) +
Ac2O]/r[Co(III) + 4-AT] attained ~103.
Similar studies carried out with manganese acetate
showed that Mn(III) reacted with acetic anhydride in
the presence of sulfuric acid at a considerably lower
rate than did Co(III) (kef = 1.7 × 10–4 L mol–1 s–1 at
293 K), which was responsible for the high catalytic
activity in the ozonation of 4-acetoxytoluene (Table 3).
At 293 K, [4-AT] = 0.4 mol L–1, [Ac2O]0 = 10 mol L–1,
[Mn(OAc)3]0 = 0.1 mol L–1, [O3] = 4 × 10–4 mol L–1,
we found that k[Mn(III) + Ac2O] = 1.7 × 10–4 L mol–1 s–1,
k1 = 31.6 L mol–1 s–1, and k[Mn(III) + 4-AT] = 2.1 ×
10–2 L mol–1 s–1; r[Mn(III) + 4-AT] = 0.8 × 10–3 mol L–1 s–1
and r3[Mn(III) + Ac2O] = 1.7 × 10–4 mol L–1 s–1.
τ, min
Fig. 3. Kinetics of (1) oxidation of 4-acetoxytoluene with
Co(III) in the presence of sulfuric acid, (2) accumulation of
Co(III) in the reaction of Co(II) with ozone in the absence of
sulfuric acid, and (3) consumption of Co(III) in the presence
of sulfuric acid and 4-acetoxytoluene; temperature 278 K;
solvent acetic anhydride; [H2SO4]0 = 1.2, [4-AT]0 = 0.4 mol/L.
with GLC using the LKhM-8MD chromatograph
equipped with a flame-ionization detector and a
3 m × 4 mm column packed with 5 wt % of SE-30 on
Inerton AW-DMCS; other conditions were as follows:
injector temperature 523 K, oven temperature 463 K;
carrier gas (nitrogen) and hydrogen flow rate 1.8 L/h;
air flow rate 18 L/h.
To conclude, manganese(II) acetate is an efficient
catalyst for the oxidation of 4-acetoxyoluene at the
methyl group in acetic anhydride in the presence of
sulfuric acid. At 273 K the oxidation results in forma-
tion of 4-acetoxybenzyl acetate (62.6%) and 4-acet-
oxybenzylidene diacetate (10.2%). Cobalt(II) acetate
loses its catalytic activity in the presence of sulfuric acid.
The effective rate constants of ozone reactions with
Mn(II) and 4-acetoxytoluene were determined by
spectrophotometry as described in [10]. The rate
constants of Mn(III) reaction with 4-acetoxytoluene
EXPERIMENTAL
Experiments were carried out in a glass column
equipped with a porous baffle to disperse the ozone–air
mixture and maintained at a constant temperature in
the range from 273–303 K; the reactions occurred in
the kinetic mode. The column was charged with 10 mL
of acetic anhydride, 0.4 mol L–1 of 4-hydroxytoluene,
0.8 mol L–1 of sulfuric acid, and the corresponding
metal salt. 4-Hydroxytoluene underwent acylation
during the reactants mixing; therefore, the oxidation of
4-acetoxytoluene was discussed above. Ozone–air
mixture containing (1.0–4.0) × 10–4 mol L–1 of ozone
was then fed at a flow rate of 30 L/h. Ozone
concentration in the gas phase was determined by
spectrophotometry at λ 254–290 nm. Aromatic oxida-
tion products were identified and quantified in solution
Table 3. Rate constants
Reaction
Co(II) + O3 + H+ →
k, L mol–1 s–1 r, mol L–1 s–1
1.0
4.0×10–5
3.4×10–3
0.31
Co(III) + Ac2O →
3.4×10–3
Co(III) + Ac2O + H+ → 0.31
Co(III) + ArCH3 →
Mn(II) + O3 + H+ →
8.2×10–3
3.3×10–4
1.3×10–3
1.7×10–4
8.4×10–4
31.6
Mn(III) + Ac2O + H+ → 1.7 ×10–4
Mn(III) + ArCH3 →
2.1×10–2
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GALSTYAN et al.
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