Kinetics and products of acetophenone oxidation with ozone-air mixture in acetic acid medium

Article abstract of DOI:10.1134/S1070363215080046

Various factors affecting kinetics of acetophenone oxidation with ozone in acetic acid medium at 15-40°C have been examined. Scheme of the acetophenone oxidation with ozone via the radical mechanism has been proposed.

Full text of DOI:10.1134/S1070363215080046

ISSN 1070-3632, Russian Journal of General Chemistry, 2015, Vol. 85, No. 8, pp. 1816–1820. © Pleiades Publishing, Ltd., 2015.
Original Russian Text © A.S. Bushuev, A.O. Kolbasyuk, M.A. Lagutenko, G.A. Galstyan, 2015, published in Zhurnal Obshchei Khimii, 2015, Vol. 85, No. 8,
pp. 1257–1261.
Kinetics and Products of Acetophenone Oxidation
with Ozone–Air Mixture in Acetic Acid Medium
A. S. Bushuev, A. O. Kolbasyuk, M. A. Lagutenko, and G. A. Galstyan
Institute of Chemical Technology, Dal’ East Ukrainian National University,
ul. Lenina 31, Rubezhnoe, Lugansk oblast, 93009 Ukraine
e-mail: bas131982@mail.ru
Received March 2, 2015
Abstract—Various factors affecting kinetics of acetophenone oxidation with ozone in acetic acid medium at
15–40°C have been examined. Scheme of the acetophenone oxidation with ozone via the radical mechanism
has been proposed.
Keywords: acetophenone, ozone, oxidation, acetic acid, mechanism, kinetics
DOI: 10.1134/S1070363215080046
Acetophenone is a stable intermediate of ozone
reaction with ethylbenzene; further oxidation converts
acetophenone into benzoic acid. However, details of
kinetics and mechanism of these transformations have
remained unclear so far. In this study we examined
various factors affecting kinetics of ozone reaction
with acetophenone in acetic acid medium at 15–40°C.
chain not changing noticeably (Table 1). Replacement
of the ozone–air mixture with the ozone-oxygen one
practically did not affect the oxidation process.
Similarly, increasing the reaction temperature from
15 to 40°C accelerated the process by more than three
times, whereas the oxidation selectivity remained
constant (Table 2).
The preliminary studies revealed that oxidation of
acetophenone with air oxygen at 15–40°C was very
slow, only traces of benzoic acid were detected after
passing air through acetophenone solution in acetic
acid during 15 h at 15°C.
The acetophenone oxidation developed without any
induction period (Fig. 1). The initial rate of aceto-
phenone consumption was of 1.6 × 10–5 mol L–1 s–1,
that of ozone being of 1.7 × 10–5 mol L–1 s–1 (the latter
value was calculated from the difference of ozone
concentrations at the reactor inlet and outlet). Hence,
rO2/rArH = 1.06, and kinetics of ozone reaction with
acetophenone at temperature up to 15°C was described
by the second-order equation: rO2 = kapp[O3]0[ArC(O)CH3]0
with rO2 being the initial ozone consumption rate and
kapp being the apparent reaction rate constant, L mol–1 s–1.
Similar conclusion on the reaction rate order with
respect to the reactants was made from analysis of the
initial rate of acetophenone oxidation as function of the
reactants concentration (Fig. 2). The stoichiometric
coefficient for ozone at 15°C was of 2.04 over a wide
range of the [O3]0/[ArC(O)CH3]0 ratios. The apparent
bimolecular rate constant for ozone consumption was
of 0.040±0.003 L mol–1 s–1.
Using air–ozone mixture instead of air accelerated
the reaction; however, its rate of 1.6 × 10–5 mol L–1 s–1
remained significantly lower than those of oxidation of
ethylbenzene (8.3 × 10–5 mol L–1 s–1) and α-hyd-
roxyethylbenzene (4.4 × 10–4 mol L–1 s–1) under similar
conditions [1]. The relatively low oxidation rate was
primarily responsible for the accumulation of sig-
nificant amounts (34% [1]) of acetophenone in the
ethylbenzene oxidation products. The major product of
ozone reaction with acetophenone at 15°C was benzoic
acid (96.2%); the aliphatic peroxides (3.2%) were
formed as well via destructive ozonolysis of the
benzene ring along with carbon dioxide (Fig. 1).
Variation of ozone concentration in the mixture led
to the proportional change in the reaction rate, the
selectivity of the acetophenone oxidation at the side
Heating above 15°C led to deviation of the kinetics
from the first-order with respect to the reactants. The
1816

KINETICS AND PRODUCTS OF ACETOPHENONE OXIDATION
log [ArH]0
1817
log r
τ, h
Fig. 1. Kinetics of acetophenone oxidation with ozone in
log [O3]0
acetic acid at 15°C. [ArC(О)CH3]0=0.4 mol/L, [O3]0
=
5.2 × 10–4 mol/L, mixture volume 0.01 L, and ozone–air
mixture feeding rate 30 L h–1. Curves 1–3 describe the
concentrations of acetophenone, benzoic acid, and aliphatic
peroxides, respectively.
Fig. 2. Rate of acetophenone oxidation with ozone as
function of the substrate (1) and ozone (2) concentrations at
15°C. [ArC(О)CH3]0 = 0.4 mol/L and [O3]0 = 5.2 ×
10–4 mol/L.
kinetic data shown in Fig. 3 confirmed that the linear
dependence (1) was held at a constant temperature
within the 15–40°C range, and the experimental
expression for the ozone consumption rate
corresponded to Eq. (2).
The first term of Eq. (2) is a typical expression for
the second-order reaction rate (3) describing the
primary reaction of ozone with acetophenone via a
non-chain mechanism both at the α-carbon atom of the
side chain and at the benzene ring.
kapp = k' + k''·√[O3]0/[ArC(O)CH3]0,
(1)
r' = k'[O3]0[ArC(O)CH3]0.
O2
(3)
r
O2 = k'[O3]0[ArC(O)CH3]0 + k''[O3]01.5[ArC(O)CH3]00.5. (2)
Interpretation of the above-described kinetic data
according to the existing views on ketones oxidation
[2, 3] and ozone reactions with aromatic compounds
[4, 5] suggested the two possible schemes of non-chain
oxidation of acetophenone: at the benzene ring via the
Criegee mechanism (Scheme 1) [5] and at the side
chain α-carbon atom via the radical mechanism
(Scheme 2) [2–4].
Here, k 'and k'' are temperature-dependent empirical
parameters: k' = 1.4 × 102exp(–19500/RT) and k'' =
6.2 × 1012exp(–74000/RT), L mol–1 s–1.
Table 1. Effect of the ozone concentration on the selectivity
of acetophenone oxidation at the side chain S with the
ozone-air mixture at 15°Cа
[O3]0 × 10–4,
Scheme 1 accounted for the experimentally
determined number of the hydroperoxide groups in the
products of the benzene ring ozonolysis (one group per
r0 × 105,
mol L–1 s–1
0.45
τ, h
S, %
mol/L
1.2
10.4
6.2
4.6
2.1
1.6
96.1
95.9
96.2
96.0
95.8
Table 2. Effect of temperature on acetophenone oxidation
with ozone–air mixture in acetic acid
3.5
0.91
5.2
1.60
T, °С
r0 × 105, mol L–1 s–1
S, %
6.7
2.96
15
20
30
40
1.6
2.2
3.3
5.5
96.2
95.9
96.1
96.0
7.7
5.30
[ArC(O)CH3]0 = 0.4 mol/L; ozone-air mixture feed rate 30 L h–1,
and mixture volume 0.01 L; (τ) is time of acetophenone
concentration half-decrease and r0 is the initial oxidation rate.
a
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1818
BUSHUEV et al.
Scheme 1.
O
O
O
O
O3
O3
O
+
−O
O
O
O
O
O
O
AcOH
O
OAc
O
O +
OOH
O−
Scheme 2.
O
O
Ar
+ O3
O
Ar
C
+ O2 + HO
(4)
(5)
CH3
CH2
O
Ar
C
+ O2
Ar
C
CH2
CH2
OO
C
O
O
O
O
(6)
+
H
Ar
Ar
C
Ar
C
C
2
CH2OH
CH2
OO
O
C
O
O
(7)
(8)
+ O2
Ar
C
+ CO2
Ar
C
H
OH
O
O
+ OH + O2
+ O3
Ar
C
Ar
C
CHOH
CH2
OH
OH
CH
O
C
O
(9)
+ O2
Ar
C
Ar
OO
CH
OH
O
OH
CH
O
O
C
O
C
O
C
+
C
Ar
C
Ar
OO
H
Ar
2
OH
(10)
molecule), and Scheme 2 was developed in view of the
ozone consumption [two mol of ozone per a mol of
acetophenone via reactions (4) and (8)].
Scheme 2 assumed that acetophenone oxidation
proceeded via a radical non-chain mechanism under
the experimental conditions. The chain oxidation of the
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 85 No. 8 2015

KINETICS AND PRODUCTS OF ACETOPHENONE OXIDATION
1819
ketone via reactions (11), (12) did not occur according
to the experimental data: acetophenone hydroperoxide
was not formed, and the oxidation completely stopped
when the ozone supply was terminated.
O
O
O
O
(11)
+
Ar
C
Ar
O
C
Ar
C
+ Ar
C
CH2
CH3
CH2OOH
CH2
OO
O
(12)
OH
+
Ar
C
Ar
C
CH2OOH
CH2O
The non-chain reaction mechanism was further
confirmed by the calculations suggesting that the
reaction between the peroxide radicals formed in the
system was faster than that of the radical with aceto-
phenone. For example, assuming that r4 = r6 according
to the Bodenshtein–Semenov stationary concentration
principle [2], at 15°C with [ArC(O)CH3]0 = 0.4 mol/L,
EXPERIMENTAL
Acetophenone (“chemical pure” grade) was used as
received; glacial acetic acid (“chemical pure” grade)
was purified via repeated fractional freezing and
distillation at 118.5°C.
Acetophenone concentration in solution was
determined by gas-liquid chromatography using a
chromatograph equipped with a flame ionization
detector and a 1 m × 3.5 mm column packed with
Inerton Super carrier coated with FFAP stationary
phase (5 wt % of the carrier). Other conditions were as
follows: thermostat heating from 115 to 175°C within
10 min; 1.8 L/h of nitrogen, 1.8 L/h of hydrogen, and
18 L/h of air. 4-Nitrochlorobenzene was used as
internal reference. Concentrations of the peroxide
products and the carboxylic acids were determined by
titration (iodometric and neutralization, respectively).
[O3]0 = 5.2 × 10–4 mol/L, k4 = 0.04, k6 ≈ 1.7 × 106, and
–1
k11 ≈ 0.02 L mol
s–1 (the average values for the
known ketones were taken to estimate k6 and k11 [5]),
the calculation gave r6 ≈ 8.3 × 10–6 mol L–1 s–1and r11 ≈
1.8 × 10–8 mol L–1 s–1, i.e., r11:r6<< 1.
The second term in Eq. (2) describes the rate of
radical chain process of ozone consumption (13).
r'' = k''[O3]01.5[ArC(O)CH3]00.5.
(13)
Ozone consumption via the chain mechanism was
also indicated by the dependence of the stoichiometric
coefficient for ozone (n) on the reaction temperature:
n = 2.04 (15°С), n = 2.6 (40°С), and n = 3 (60°С).
Oxidation of acetophenone was carried out in a
temperature-controlled (15–40°C) glass column with a
A possible route of ozone consumption via a chain
mechanism can be described by Eqs. (14), (15) [4].
ROO• + O3 → RO• + 2O2,
RO• + O3 → RO2• + O2,
(14)
(15)
R stands for any peroxide radical in the equations above.
To conclude, it was found that acetophenone
oxidation with ozone–air mixture in glacial acetic acid
at 15–40°C proceeded predominantly at the side chain
to form benzoic acid (96.2%); 3.2% of acetophenone
underwent destructive ozonolysis of the aromatic ring.
A possible scheme of non-chain radical ozonation of
acetophenone has been proposed, with initiation of the
oxidation at the side chain proceeding via a reaction
with ozone. At higher temperature, ozone consumption
via a chain mechanism accompanied the radical non-
chain oxidation of acetophenone.
√[O3]0/[ArH]0 × 102
Fig. 3. Apparent rate constant of ozone reaction with
acetophenone as function of the reactants concentration at
(1) 15, (2) 20, (3) 30, and (4) 40°C.
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1820
BUSHUEV et al.
2013, vol. 66, no. 6/6, p. 8.
porous glass separator for dispersing the ozone–air
mixture, in the kinetic regime. The column was
charged with 10 mL of glacial acetic acid and 0.4 mol/L
of acetophenone, and then the ozone–air mixture was
fed at 30 L h–1. The ozone-air mixtures with ozone
concentration of 10–4–10–3 mol/L were prepared using
a laboratory ozone generator described in [6]. Ozone
concentrations in the gas phase at the column inlet and
outlet were determined by spectrophotometry at 254–
290 nm. The ozonation reaction rate constant was
determined via the procedure described in [6] using an
ideal-mixing reactor equipped with a constant-
temperature jacket. The gas and the liquid phases were
mixed in the reactor by shaking. Under those
conditions, the oxidation proceeded in the kinetic
regime.
2. Emanuel’, N.M., Denisov, E.T., and Maizus, Z.K.,
Tsepnye reaktsii okisleniya uglevodorodov v zhidkoi faze
(Liquid-Phase Chain Oxidation of Hydrocarbons),
Moscow: Vysshaya Shkola, 1965.
3. Denisov, E.T., Mitskevich, N.I., and Agabekov, V.E.,
Mekhanizm
soderzhashchikh soedinenii (The Mechanism of the
Liquid-Phase Oxidation of Oxygen-Containing
zhidkofaznogo
okisleniya
kislorod-
Compounds), Minsk: Nauka i Tekhnika, 1975.
4. Galstyan, G.A., Tyupalo, N.F., and Razumovskii, S.D.,
Ozon i ego reaktsii s aromaticheskimi soedineniyami v
zhidkoi faze (Ozone and Its Reactions with Aromatic
Compounds in the Liquid Phase), Lugansk: Vost.-Ukr.
Nats. Univ., 2004.
5. Bailey, P.S., Ozonation in Organic Chemistry: Olefinic
Compounds, New York: Academic, 1978, vol. 1.
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