Oxidation of benzyl alcohol and benzaldehyde with ozone in acetic acid

Article abstract of DOI:10.1134/S1070427210070141

Ozonation of benzyl alcohol and benzaldehyde in acetic acid was studied. A method for ozonation of these compounds is suggested.

Full text of DOI:10.1134/S1070427210070141

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2010, Vol. 83, No. 7, pp. 1243−1247. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © E.V. Potapenko, P.Yu. Andreev, 2010, published in Zhurnal Prikladnoi Khimii, 2010, Vol. 83, No. 7, pp. 1137−1141.
ORGANIC SYNTHESIS
AND INDUSTRIAL ORGANIC CHEMISTRY
Oxidation of Benzyl Alcohol and Benzaldehyde
with Ozone in Acetic Acid
E. V. Potapenko and P. Yu. Andreev
Institute for Chemical Technology, Dal’Eastern-Ukrainian National University, Rubezhnoe,
Lugansk oblast, Ukraine
Received September 3, 2009
Abstract—Ozonation of benzyl alcohol and benzaldehyde in acetic acid was studied. A method for ozonation
of these compounds is suggested.
DOI: 10.1134/S1070427210070141
Liquid-phase oxidation of methylbenzenes with ozone
has been rather long considered a promising way to obtain
aromatic carboxylic acids [1]. In the course of ozonation,
the primary comparatively stable products of oxidation
of the methyl group are aromatic alcohols and aldehydes.
Despite that their concentration in the system is rather
low, these substances undoubtedly play an important role
in deep conversion stages of methylbenzenes.
amount of benzaldehyde accumulates in the reaction
mass. This is indicative of the successive nature of their
formation. Already in the first minutes of ozonation,
cobalt becomes trivalent and after that its concentration
remains unchanged.
In contrast to benzyl alcohol, the main reaction
pathway in ozonation of benzaldehyde is the destruction
of the aromatic ring (52.3%), with the yield of benzoic
Apparently, to extend the concepts of processes
for oxidation of methylbenzenes, it is also necessary
to understand the fundamental kinetic aspects and
mechanism of the reaction of ozone with aromatic
alcohols and aldehydes.
In this context, it is of interest to study the
fundamental aspects of ozonation of benzyl alcohol
and benzaldehyde in acetic acid under the conditions
characteristic of oxidation of methylbenzenes.
The interaction of benzyl alcohol with ozone in
acetic acid is accompanied by preferential oxidation of
the oxymethyl group (Fig. 1). Benzoic acid accumulates
as the main product (83.7%), with the amount of
ozonides not exceeding 15.2%. Benzaldehyde is formed
in parallel with benzoic acid in minor amounts.
τ, min
Fig. 1. Kinetic curves of (1, 1') consumption of benzyl alcohol
and accumulation of (2, 2') benzoic acid, (3, 3') benzaldehyde,
and (4) ozonides in ozonation (1–4) in the absence of a catalyst
and (1'–3') in the presence of cobalt(II) acetate in acetic acid at
60°C. [ArCH2OH] = 0.069 M, [O3] = 4 × 10–4 M, [Co(OAc)2] =
0.08 M. (c) Concentration and (τ) reaction duration; the same
for Fig. 2.
Introduction of cobalt(II) acetate into the oxidizing
system makes it possible to raise both the yield of
benzoic acid (95.4%) and the consumption rate of the
alcohol (Fig. 1). It is noteworthy that benzoic acid is
formed in the presence of the catalyst after a sufficient
1243

1244
POTAPENKO ANDREEV
(1)
A study of the effect of the concentrations of ozone,
catalyst, and substrate in catalytic oxidation of benzyl
alcohol demonstrated that the consumption rate of the
alcohol is described by a third-order kinetic equation
(Fig. 4):
(2)
The dependence of the oxidation rate of benzaldehyde
on the concentrations of ozone, catalyst, and substrate is
more complicated, which points to a chain consumption
mechanism of benzaldehyde (Fig. 5):
τ, min
Fig. 2. Kinetic curves of (1, 1') consumption of benzaldehyde
and accumulation of (2, 2') benzoic acid and (3) ozonides
in ozonation (1–3) in the absence of a catalyst and (1', 2')
in the presence of cobalt(II) acetate in acetic acid at 60°C.
[ArCHO] = 0.08 M, [O3] = 4 × 10–4 M, [Co(OAc)2] = 0.08 M.
(3)
acid equal to 43.2%. By using the catalyst, it is possible
to change the reaction direction toward formation of
aromatic acid (91.2%) (Fig. 2). As also in the case of
benzyl alcohol, cobalt is present in the reaction mixture
in the oxidized form.
On the basis of the experimental results of this study
and published data, the following oxidation scheme of
benzyl alcohol can be considered, and primarily just
those reactions which enable understanding of the role
The consumption kinetics of the alcohol and
aldehyde in the reaction with ozone is described by the
bimolecular law and has the first order with respect to
each of the reactants (Fig. 3):
–ln c(ArCH2OH) [M]
c (ArH) × 102, M
–ln c(O3), [M]
–ln c(Co(OAc) 2, [M]
c (O3) × 102, M
Fig. 3. Dependence of the initial rate W of consumption
of (1) benzyl alcohol and (2) benzyl alcohol on the ozone
concentration c(O3) at 60°C.
Fig. 4. Dependence of the initial rate W of oxidation of benzyl
alcohol on the concentrations c of (1) benzyl alcohol, (2)
ozone, and (3) cobalt(II) acetate at 60°C.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 83 No. 7 2010

OXIDATION OF BENZYL ALCOHOL AND BENZALDEHYDE
1245
–ln c(ArCHO) [M]
Kinetic parameters of elementary reactions in catalytic
oxidation of benzyl alcohol and benzaldehyde with ozone in
acetic acid at 25°C
k
А
Е,
J mol–1
Reaction
M–1 s–1
(4)
3.7
2.0 × 105
2.6 × 108
2.3 × 108
1.1 × 106
2.3 × 109
27000
31000
55000
32000
62000
(10)
(11)
(16)
(21)
9.6 × 102
5.3 × 10–2
2.7
3.1 × 10–2
[reaction (5)] or destruction products of the aromatic
ring [reaction (6)]. Hydrotrioxide can be consumed by
reactions (7)–(9) [2, 3]. If the fact that the yield of the
acid does not exceed 43% in oxidation of the aldehyde
(Fig. 2) and is 83.7% in oxidation of the alcohol is
taken into account, then the most probable pathway for
formation of benzoic acid is reaction (7).
–ln c(O3), [M]
–ln c(Co(OAc) 2, [M]
Fig. 5. Dependence of the initial rate W of oxidation of
benzaldehyde on the concentrations c of (1) benzaldehyde,
(2) ozone, and (3) cobalt(II) acetate at 60°C.
In the presence of cobalt(II) acetate, ozone primarily
reacts with the reduced form of the metal [reaction (10)];
W10 > W4 (see table).
played by each of the reactants:
Direct oxidation of benzyl alcohol occurs by reaction
(11) with cobalt(III) acetate. The increase in the alcohol
consumption rate and oxidation selectivity in the
presence of the catalyst is accounted for by the fact that
W11 > W4 (see table). Further, oxyalkyl radicals interact
with oxygen by reaction (12) and oxyperoxide radicals
formed in this case are consumed in accordance with
reactions (13)–(15) [4].
(ArCH2OH + O3 → [ArCH2OH·O3],
[ArCH2OH·O3] → ArCH(OH)OOOH,
[ArCH2OH·O3]
(4)
(5)
→ products of disintegration of the aromatic ring, (6)
ArCH(OH)OOOH → ArCOOH + H2O2,
ArCH(OH)OOOH → ArCHO + H2O + O2,
ArCH(OH)OOOH → ArC·HOH + НО· + O2,
Co2+ + O3 + H+ → Co3+ + НО· + О2,
ArCH2OH + Co3+ → ArC·HOH + Co2+ + H+,
ArC·HOH + O2 → ArCH(OH)OO·,
(7)
(8)
The hydroxy radicals formed in the system by
reactions (9) and (10) are mostly consumed in the
reaction with acetic acid [5].
(9)
Benzaldehyde is consumed in accordance with the
scheme [4, 6], which is in rather good agreement with
the experimental data obtained:
(10)
(11)
(12)
(13)
ArCНO + O3 → [ArCHO·O3],
[ArCHO·O3] → ArC(O)OOOH,
(16)
(17)
ArCH(OH)OO· → ArCHO + HOO·,
[ArCHO·O3] → disintegration products
ArCH(OH)OO· + HOO· → molecular products, (14)
2ArCH(OH)OO· → molecular products.
(15)
of the aromatic ring,
(18)
(19)
(20)
(21)
ArC(O)OOOH → ArCOOH + O2,
ArC(O)OOOH → ArC·O + НО· + O2,
ArCHO + Co3+ → ArC·O + Co2+ + H+,
In the case of a noncatalytic oxidation, ozone react
with the alcohol to give a π-complex [reaction (4)],
whose further transformation yields either hydrotrioxide
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 83 No. 7 2010

1246
POTAPENKO ANDREEV
(22)
ArC·O + O2 → ArC(O)OO·,
EXPERIMENTAL
ArC(O)OO· + ArCHO → ArC(O)OOН + ArC·O, (23)
ArC(O)OOН + Co2+ → ArC(O)O· + Co3+ + OH–, (24)
ArC(O)OOН + Co3+ → ArC(O)ОO· + Co2+ + H+, (25)
The oxidation was performed in a thermostated
glass column with a porous partition for gas dispersion.
The reactor was charged with 0.05 l of acetic acid and
calculated amounts of the substrate and catalyst, and
the ozone–air mixture was delivered at a rate of 1 ×
10–2 l s–1. The ozone concentration in the gas phase
was found spectrophotometrically from the absorption
at 254–259 nm. The starting substances and oxidation
products were determined by gas-liquid chromatography
[7].
2ArC(O)OO· → molecular products.
(26)
In the absence of a catalyst, benzaldehyde is mostly
oxidized with disintegration of the aromatic ring by
reaction (18). Benzoic acid can be formed directly from
hydrotrioxide [reaction (19)] or via reactions (20), (22),
and (26). Even if the benzoyl peroxide radical interacts
with the aldehyde in this process [reaction (23)], it
has no effect on the kinetics because benzaldehyde is
consumed in noncatalytic ozonation by the nonchain
pathway [Eq. (1)].
The rate constants of the interaction between ozone
and cobalt(II) acetate were determined by the procedure
described in [8]. To find the rate constants of interaction
of Co3+ with benzyl alcohol and benzaldehyde, the Co3+
consumption rate was measured. The Co3+ concentration
in the reaction mixture was found using the photometric
method by measuring the optical density with a KFK-
2 instrument (λ = 315 nm, 30-mm cuvette). The rate
constants were calculated by the equation
In the presence of the catalyst, the chain oxidation
is initiated by the interaction of benzaldehyde with
trivalent cobalt (W21 > W16) (see table). Further
transformation of the benzoyl radical occurs in
accordance with the commonly accepted mechanism
[reactions (22)–(26)]. With the fact that [Co3+] >> [Co2+]
under the experimental conditions taken into account,
reaction (25) may play a noticeable role in the catalytic
decomposition of perbenzoic acid.
It was assumed in the calculations that the reaction
hindrance by divalent cobalt can be neglected at the
initial instant of time.
The rate of chain initiation is, naturally, determined
by the limiting stage, i.e., the reaction of trivalent cobalt
with the aldehyde [reaction (21)]. In the steady reaction
mode, the rates of chain initiation and termination are
equal:
CONCLUSIONS
(1) The yield of benzoic acid is 83.7% in ozonation of
benzyl alcohol and 43.2% in ozonation of benzaldehyde.
2k21[Co3+][ArCHO] = k26[ArC(O)OO·]2,
[ArC(O)OO·] = (2k21[Co3+][ArCHO]/k26)0.5.
(27)
(28)
(2) Comparison of the oxidation products of benzyl
alcohol and benzaldehyde shows that benzoic acid
accumulates directly in the interaction of the alcohol
with ozone, rather than via intermediate formation of
the aldehyde.
Consequently, the expression for the benzaldehyde
consumption rate has the form
(3) Use of Co(II) acetate as a catalyst makes it
possible to raise the consumption rates of benzyl alcohol
and benzaldehyde. The yield of benzoic acid increases
to 95.4% in oxidation of the alcohol and to 91.2% in
oxidation of the aldehyde.
(29)
Because the concentration of trivalent cobalt is
proportional to the rate of interaction between ozone
and divalent cobalt at a current instant of time ([Co3+] ~
k[Co2+][O3]) [6], it can be concluded that Eq. (29) is in
good agreement with the experimental data [Eq. (1)].
(4) Results of a study of the oxidation kinetics
demonstrate that benzyl alcohol is only consumed by
the nonchain mechanism. Benzaldehyde is consumed by
the nonchain mechanism in the reaction with ozone, and
by the chain mechanism in catalytic ozonation.
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OXIDATION OF BENZYL ALCOHOL AND BENZALDEHYDE
1247
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