The liquid-phase oxidation of isomeric methoxy-( 1 -methylethyl) benzenes with oxygen to the hydroperoxides

Article abstract of DOI:10.1002/kin.20043

The kinetics of liquid-phase oxidation of 1-methoxy-2-(1-methylethyl) benzene (1a), 1-methoxy-3-(1-methylethyl)benzene (1b), and 1-methoxy-4-(1- methylethyl)benzene (1c) with oxygen as oxidant to yield the corresponding 1-methyl- 1-(2-methoxyphenyl)ethyl

Full text of DOI:10.1002/kin.20043

The Liquid-Phase Oxidation
of Isomeric Methoxy-(1-
methylethyl)benzenes with
Oxygen to the
Hydroperoxides
JAN ZAWADIAK, BARTLꢀ OMIEJ JAKUBOWSKI, ZBIGNIEW STEC
Department of Chemical Organic Technology and Petrochemistry, Silesian University of Technology, ul. Krzywoustego 4,
44-100 Gliwice, Poland
Received 16 February 2004; accepted 22 July 2004
DOI 10.1002/kin.20043
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: The kinetics of liquid-phase oxidation of 1-methoxy-2-(1-methylethyl)benzene (1a),
1-methoxy-3-(1-methylethyl)benzene (1b), and 1-methoxy-4-(1-methylethyl)benzene (1c) with
oxygen as oxidant to yield the corresponding 1-methyl-1-(2-methoxyphenyl)ethyl (2a), 1-
methyl-1-(3-methoxyphenyl)ethyl (2b), and 1-methyl-1-(4-methoxyphenyl)ethyl (2c) hydroper-
oxides was studied. The oxidizabilities of 1a, 1b, and 1c were established over the temperature
range 50–120◦C. The overall activation energies of oxidation were determined for 1b and 1c over
the temperature range 50–120◦C. Thermal stability of 2a and 2b and the initiating properties
of hydroperoxides 2a, 2b, and 2c were studied. Long-term noncatalytic oxidations of 1b and 1c
to respective hydroperoxides were carried out. ꢀ 2004 Wiley Periodicals, Inc. Int J Chem Kinet
C
37: 10–16, 2005
compounds in the liquid phase with an oxygen. 4-
Methoxyphenylethanone is used as a component of
suntan creams providing UV protection [1], a poly-
merization photoinitiator [2], and intermediate in the
syntheses of pharmaceuticals [3].
The primary products of oxidation of 1-methoxy-
4-(1-methylethyl)benzene (1c) or 1-methoxy-4-ethyl-
benzene in the liquid phase with oxygen include the
corresponding hydroperoxides that in the presence of
transition metal salts are further converted to yield ke-
tones and other products. The factor controlling a po-
tential susceptibility of a compound for oxidation is
its oxidizability. Our earlier studies [4] showed that
the oxidizability of 1c over the temperature range
INTRODUCTION
Our studies on the kinetics of oxidation of isomeric
methoxy-(1-methylethyl)benzenes are intended to es-
tablish whether it is possible to prepare 4-methoxy-
phenylethanone by catalytic oxidation of alkylaromatic
Correspondence to: Jan Zawadiak; e-mail: janzaw@polsl.
gliwice.pl.
Contract grant sponsor: Ru¨tgers Chemicals AG (Germany).
Contract grant sponsor: The State Committee for Scientific Re-
search in Poland.
Contract grant number: 4T09B10725.
2004 Wiley Periodicals, Inc.
c
ꢀ

THE LIQUID-PHASE OXIDATION OF ISOMERIC METHOXY-(1-METHYLETHYL)BENZENES
11
50–110◦C was about 4.6–1.8 times higher than that
for 1-methoxy-4-ethylbenzene.
The compound 2c has been found to be unstable. Our
earlier studies [4] showed the thermal stability of 2c to
be lower than that of cumyl hydroperoxide (CHP). So
far, hydroperoxides 2a, 2b have not been described and
the initiating properties of hydroperoxides 2a, 2b, and
2c have not been studied.
In this work, the kinetics of noncatalytic oxidation
of 1a and1b in the presence of azo-initiators over the
temperature range 50–120◦C and of 1c at 120◦C was
studied. The determined oxidizability data were used to
evaluate the overall energies of activation for the oxida-
tion of 1b and 1c over the temperature range 50–120◦C.
Thermal stabilities of 2a and 2b were determined and
the initiating properties of 2a, 2b, and 2c were estab-
lished. Studies on long-term noncatalytic oxidation of
1b and 1c were also carried out.
A mixture of isomeric methoxy-(1-methylethyl)-
benzenes, containing 1-methoxy-2-(1-methylethyl)-
benzene (1a) and 1c as the major components, can
be used as a raw material for the catalytic oxidation.
Such a mixture is a product of alkylation of methoxy-
benzene [5,6]. It is necessary to know the kinetics of
noncatalytic oxidation of each isomer before using the
mixture to catalytic oxidation process. In the case of
differences in activation energies, it would be possi-
ble to select temperature allowing to obtain the highest
selectivity of 4-methoxyphenylethanone.
The literature on noncatalytic oxidation of isomeric
methoxy-(1-methylethyl)benzenes is rather scarce.
Russel and Williamson [7] and Howard and Ingold
[8] studied the kinetics of oxidation of 1c at 60◦C
and 30◦C. At these temperatures, the oxidizabilities
RESULTS AND DISCUSSION
The kinetics of noncatalytic oxidation of isomeric
methoxy-(1-methylethyl)benzenes with molecular
oxygen was studied in the presence of azo-initiators.
Table I lists the oxidizability data (kp/kt1/2) for
compounds 1a, 1b, and 1c [4] and for cumene [10,11]
over the temperature range 50–120◦C.
In the case of oxidation of 1a over the tempera-
ture range 50–90◦C, no chemisorption of oxygen was
observed. Over the temperature range 100–120◦C, the
oxidizability of 1a was as low as the experimental er-
ror of the method. At 120◦C, the ratios of the oxidiz-
abilities of the studied compounds were 1a:1b:1c =
1.0:29.5:81.5.
of 1c are 2 × 10−3 and 6.75 × 10−3 (dm3/(mol·s))1/2
,
respectively, and they are 1.3 and 1.5 times higher than
those of cumene. Howard’s value of the oxidizability
of 1-methoxy-3-(1-methylethyl)benzene (1b) at 30◦C
is 0.54–0.6 × 10−3 (dm3/(mol·s))1/2 [8]. The kinetics
of oxidation of 1a has not been studied so far.
Figure 1 shows the logarithm of oxidizability for
compounds 1b and 1c versus reciprocal absolute
temperature.
Hydroperoxide 2c has been prepared by oxidizing
2-(4-methoxyphenyl)propan-2-ol with 30% H2O2 in a
solution of diethyl ether in the presence of H2SO4 [9].
The obtained data were used to evaluate, using the
Arrhenius equation, the overall energies of activation
for compounds 1b and 1c over the temperature range
Table I The Oxidizabilities of 1a, 1b, 1c, and Cumene in Liquid-Phase Oxidation with Oxygen in the Presence
of Azo-Initiators
kp/k1/2 × 103 ((dm3/(mol·s))1/2
)
t
No.
t (◦C)
1a
1b
1c [4]
Cumene [10,11]
1
2
3
4
5
6
7
8
50
60
70
80
90
100
110
120
–
–
–
–
1.0 0.1
1.4 0.1
2.0 0.2
2.7 0.2
3.2 0.1
3.7 0.2
4.8 0.3
5.9 0.2
3.7 0.4
4.7 0.2
6.0 0.2
7.6 0.3
9.5 0.3
11.4 0.3
13.7 0.3
16.3 0.3
4.22
5.70
7.65
–
0.0 0.1
0.1 0.1
0.2 0.1
13.6
17.2
20.8

12
ZAWADIAK ET AL.
Figure 1 The negative log(oxidizability) of compounds 1b (I) and 1c (II) versus reciprocal absolute temperature. (I) y =
(1.3525 0.0570)x − (1.2206 0.1602), R2 = 0.9895. (II) y = (1.1652 0.0134)x − (1.1738 0.0376), R2 = 0.9992.
50–120◦C (Table II) (the activation energy for cumene
over 60–120◦C is also included [11]).
Since the overall energies of activation are rela-
tively close in value, the effect of temperature on the
oxidation rates of compounds 1b, 1c, and cumene, is
similar.
The hydroperoxides that are primarily formed in the
oxidation reactions of alkylaromatic hydrocarbons are
relatively unstable and undergo partial decomposition,
initiating further chains of the free-radical oxidation.
Thermal stability of the hydroperoxides was studied
using differential scanning calorimetry (DSC). Ther-
mograms of compounds 2a, 2b, and 2c are presented
in Fig. 2.
The temperatures at which the decomposition rates
of 2a, 2b, 2c, and literature data of CHP attained
maximum are 161, 198, 151, and 176◦C [4], respec-
tively. The initiating properties of the hydroperoxides
were investigated in the model reaction of oxidation of
cumene at 120◦C. Determined initiation rates are listed
in Table III.
its higher thermal stability. The initiating properties of
2c were found to be only slightly weaker than that of
CHP.
Long-term oxidation of 1b and 1c carried out over
the temperature range 60–110◦C, in the presence of
azo-initiators, showed the reaction rate to slow down
as the reaction was continued. It is probably due to
the formation of inhibitors as well as the decrease of
azo-initiator concentration during the oxidation. In or-
der to accelerate the reaction and to achieve higher
conversion of the substrates, it was necessary to add
the azo-initiator several times during the reaction. The
courses of oxidation of 1b and 1c at 70◦C are presented
in Fig. 3. The reaction courses recorded at other exper-
imental temperatures were found to be similar.
The final concentrations of the hydroperoxides
formed in long-term oxidation of 1b and 1c are listed
in Tables IV and V.
Table III Initiation Rates of Oxidation of Cumene at
120◦C in the Presence of 2a, 2b, 2c, and CHP
The presented data showed that 2a and 2b were
initiators 2–10 times weaker than CHP. Weaker ini-
tiating properties of 2b were probably associated with
CHP
(mol/dm3)
ri × 108
No.
1
Hydroperoxide
(mol/(dm3·s))
2a
0.1
0.3
0.1
0.3
0.1
0.3
0.1
0.3
3.3
11.5
16.4
30.5
26.5
79.3
28.8
97.2
Table II The Overall Energies of Activation for 1b, 1c,
and Cumene in the Oxidation Reaction
2
3
4
2b
2c
No. RH
Ea = Ep − 1 Et (kJ/mol)
2
1
2
3
1b
1c
Cumene
26
22
28
2
2
1
(50–120◦C)
(50–120◦C) [4]
(60–120◦C) [11]
CHP

THE LIQUID-PHASE OXIDATION OF ISOMERIC METHOXY-(1-METHYLETHYL)BENZENES
13
Figure 2 DSC thermograms for compounds 2a, 2b, and 2c.
At temperatures of 60 and 70◦C, the conversions of
1b and 1c are higher than those at other temperatures.
The obtained products contained about 35% of the hy-
droperoxide. At 110◦C, the concentration of 2b was
higher than that of 2c formed under identical condi-
tions. The observed differences are likely to be due to
different thermal stabilities of the hydroperoxides.
Sincetheoxidizabilityof1awasverylow, studieson
long-termoxidationofthiscompoundwereabandoned.
However, its influence on the long-term oxidation of
1c was examined. Long-term oxidations of 1a/1c mix-
tures as well as of 1c in inert solvent at 100◦C were
performed. The courses of reactions are presented in
Fig. 4.
It was found that 1a had a negative influence on
the oxidation of 1c. The similar effect was observed
by Bondarenko in the process of oxidation of cymenes
mixtures [12].
EXPERIMENTAL
Materials
Lancaster Synthesis’ 98+% pure 1-methoxy-4-(1-
methylethyl)benzene (1c) was used. Compounds 1a
and 1b were prepared in 63% and 57% yields us-
ing the Williamson method [13] of etherification of
Figure 3 Long-term oxidation of 1b (II) and 1c (I) at 70◦C. Arrows indicate the moments when subsequent initiator portions
were added. The measured amount of chemisorbed oxygen was reduced in the graph by the quantity of oxygen that reacted with
the azo-initiator.

14
ZAWADIAK ET AL.
Table IV Final Concentrations of 2b Formed in
Long-Term Oxidation of 1b with Oxygen, in the Presence
of Azo-initiators
nitrogen evaluated in the decomposition of the azo-
initiator), CRH is concentration of hydrocarbon oxi-
dized, ri is initiation rate calculated as ri = 2ekdCi,
where Ci is initiator concentration, e is initiation ef-
fectiveness coefficient, which for the initiators used
in the experiments was assumed to be e = 0.6 [16]),
kd is initiator decomposition rate constant for AIBN
kd = 1.6 × 1015 exp(−30800/RT) [17] and for ACHN
kd = 5.24 × 1016 exp(−34500/RT) [18]. At each tem-
perature, the initial oxidation rate was measured five
times.
No.
t (◦C)
Time (h)
C2b (%)
1
2
3
4
70
80
100
110
26.0
18.7
17.7
20.0
35
29
30
24
1-hydroxy-2-(1-methylethyl)benzene and 1-hydroxy-
3-(1-methylethyl)benzene, respectively. The isomeric
methoxy-(1-methylethyl)benzenes were purified by
distilling over sodium in a nitrogen atmosphere (1a,
bp 34–35/3 mmHg; 1b, bp 74–78◦C/11–13 mmHg; 1c,
bp 78–79◦/9 mmHg). Purity of the compounds was de-
termined by GC and 1H-NMR.
Long-term oxidation of 1a (2 cm3, 1.908 g), 1b
(2 cm3, 1.926 g), and 1c (2 cm3, 1.890 g) was performed
until the chemisorption of oxygen was 0.3 cm3/10 min.
The initiator was added three times (at the start of the
reaction, and then twice after the chemisorption of oxy-
gen had dropped to 0.3 cm3/10 min). The content of
the hydroperoxide was determined by iodometry [19].
Long-term oxidations of (1:1 v/v) mixtures of tert-
butylbenzene + 1c as well as 1a + 1c were carried out
in 14 cm3 bubbling reactor at 100◦C in the presence of
0.0300 g ACHN. The content of the hydroperoxide in
the product was determined by iodometry [19].
Oxidation
Compounds 1a, 1b, and 1c were oxidized at atmo-
spheric pressure with an oxygen in the presence of azo-
initiators, viz., 2,2ꢁ-azo-bis(isobutyronitrile) (AIBN)
over the temperature range 50–90◦C and 1,1ꢁ-azo-
bis(cyanocyclohexane) (ACHN) over the temperature
range 100–120◦C. The oxidation reactions were car-
ried out in gasometric apparatus using the procedure
described in the literature [14]. The initial oxidation
rates were measured by determining the amount of
Studies on Initiating Properties
of Hydroperoxides
The initiating properties of hydroperoxides 2a, 2b,
and 2c were studied in the model reaction of oxida-
tion of cumene. Cumene was purified by rectification
with concentrated sulfuric acid and by distillation over
sodium in a nitrogen atmosphere. Oxidation of cumene
with oxygen in the presence of 2a, 2b, or 2c was per-
formed at 120◦C at atmospheric pressure. The rate of
oxidation observed within the first 15 min of the oxi-
dation was determined via the amount of chemisorbed
oxygen. The reported cumene oxidizability data at
120◦C [10] and the oxidation rates measured for
the isomeric methoxy-(1-methylethyl)benzenes were
used to evaluate the initiation rates for the studied
hydroperoxides.
chemisorbed oxygen (rO ). In the applied conditions
2
only the isopropyl groups of 1a, 1b, and 1c undergo
oxidation. Under these conditions, the amounts of the
resulting hydroperoxide are rather small and therefore,
the effect of the hydroperoxide and of the products
of its decomposition on the kinetics of the oxidation
may be neglected. The oxidation rate data were used to
calculate the oxidizabilities of compounds 1a, 1b, and
kp
1
√
√
1c. The applied equation:
= rox
×
, where
ri∗CRH
kt
√
kp/ kt is oxidizability, rox is oxidation rate (when the
kinetic chains were short (rox/ri = 6–25 in this work),
the rate of oxidation was calculated from the equation
rox = rO − 1.5ri [15]; additionally with corrected for
2
Synthesis of Hydroperoxides
2c was synthesized by oxidation of 2-(4-methoxyph-
enyl)-2-propanol (10 g, 0.06 mol) in benzene (100 cm3)
with 50% hydrogen peroxide (24.2 cm3, 0.6 mol) at
40◦C in the presence of H2SO4 (0.023 cm3, 2.6 ×
10−4 mol). The hydroperoxide 2c was isolated from
the reaction mixture as the sodium salt that was then
saturated with carbon dioxide. The hydroperoxide was
purified twice by precipitation of the sodium salt and
then saturation. 1.9 g (0.01 mol) of 2c in 17% yield was
obtained.
Table V Final Concentrations of 2c Formed in
Long-Term Oxidation of 1c with Oxygen, in the Presence
of Azo-initiators
No.
t (◦C)
Time (h)
C2c (%)
1
2
3
4
5
60
70
80
100
110
17.8
11.5
7.0
5.2
3.7
40
37
29
17
13

THE LIQUID-PHASE OXIDATION OF ISOMERIC METHOXY-(1-METHYLETHYL)BENZENES
15
Figure 4 The concentrations of hydroperoxides formed in long-term oxidation versus reaction time. Substrates: (I) 1c +
t-butylbenzene (1:1 v:v); (II) 1c + 1a (1:1 v:v), temperature 100◦C, initiator - ACHN.
13C NMR2b (CDCl3) δ = 159.8, 146.4, 129.6,
The hydroperoxides 2a and 2b were prepared by a
similar oxidation method using 2-(2-methoxyphenyl)-
2-propanol and 2-(3-methoxyphenyl)-2-propanol as
substrates, respectively. Hydroperoxides were isolated
from the reaction mixture by using the column chro-
matography (column, φ = 3 cm and L = 60 cm;
packing, Silicagel-60; eluent, (1:2 v/v) hexane-diethyl
ether). The yields of 2a and 2b were 7% and 9%,
respectively.
Obtained hydroperoxides were purified by addi-
tional column chromatography (column, φ = 1.5 cm
and L = 30 cm; packing, Silicagel-60; eluent, (1:2 v/v)
hexane-diethyl ether).
117.7, 112.4, 111.5 (C-ar.), 83.9 ( C(CH3)2 ), 55.2
( OCH3), 26.1 ( CH3).
2c. Anal. Calcd: C, 65.91; H, 7.75; O, 26.34. Found:
C, 65.57; H, 7.34; Ocalc., 27.09.
1H-NMR2c (CDCl3/TMS) δ = 1.60 (s, 6H,
Ar C(CH3)2C O O , 3.81 (s, 3H, CH3OPh), 6.91
(d, J = 9 Hz, 2H, H-2, H-6), 7.40 (d, J = 9 Hz, 2H,
H-3, H-5), 7.43 (s, 1H, C O OH).
13C NMR2c (CDCl3) δ = 158.8, 136.4, 126.8,
113.8 (C-ar.), 83.6 ( C(CH3)2 ), 55.3 ( OCH3), 26.0
( CH3).
Analytical Instruments
Analysis of Products
GC analyses of the reactants were run on a Carlo Erba
Strumentazione FV 4061 gas chromatograph (capillary
column SGE BP5, 25 m long, internal diameter 0.22
mm, film thickness 1 ꢀm, FID).
2a. Anal: Calcd: C, 65.91; H, 7.75; O, 26.34. Found:
C, 65.52; H, 7.71; Ocalc., 26.77.
1H-NMR2a (CDCl3/TMS) δ = 1.65 (s, 6H,
Ar C(CH3)2C O O ), 3.85 (s, 3H, CH3OPh), 6.93
(d, J = 4 Hz, 1H, H-6), 6.98 (t, J = 7 Hz, 1H, H-4),
7.27 (t, J = 7 Hz, 1H, H-5), 7.48 (d, J = 4 Hz, 1H,
H-3), 7.73 (s, 1H, C O OH).
Elementary analyses were carried out on a Perkin-
Elmer 2400 Series II instrument.
1
The H-NMR and 13C NMR analyses of the hy-
droperoxides and reactants were run on a Varian Unity
Inowa 300 spectrometer.
13C NMR2a (CDCl3) δ = 156.6, 132.6, 128.4,
127.2, 120.7, 111.9 (C-ar.), 84.6 ( C(CH3)2 ), 55.5
( OCH3), 25.4 ( CH3).
2b. Anal. Calcd: C, 65.91; H, 7.75; O, 26.34. Found:
C, 66.05; H, 7.70; Ocalc., 26.25.
CONCLUSIONS
1H-NMR2b (CDCl3/TMS) δ = 1.59 (s, 6H,
Ar C(CH3)2C O O ), 3.82 (s, 3H, CH3OPh), 6.84
(d, J = 4 Hz, 1H, H-6), 7.03 (s, 1H, H-2), 7.04 (d,
J = 6 Hz, 1H, H-4), 7.30 (t, J = 8 Hz, 1H, H-5), 7.45
(s, 1H, C O OH).
Studies on the kinetics of noncatalytic oxidation of iso-
meric methoxy-(1-methylethyl)benzenes 1a, 1b, and
1c carried out over the temperature range 50–120◦C
showed that isomer 1c exhibited the highest oxidiz-
ability. Over the experimental temperature range, the

16
ZAWADIAK ET AL.
oxidizability of isomer 1c was 3.7–2.7 times higher
than that of 1b. The oxidizability of 1a was close to
zero over the temperature range 50–120◦C. Based on
the oxidizability data, overall energies of activation
(Ea) for the reactions of oxidation were calculated and
compared with the data reported for cumene. Ea val-
ues for 1b and 1c were, respectively, 26 2 kJ/mol
and 22 2 kJ/mol and for cumene 28 1 kJ/mol. The
influence of temperature on the rate of oxidation of
isomers 1b and 1c were close and similar to that for
cumene.
The primary products of the oxidation reaction were
relatively unstable hydroperoxides 2a, 2b, and 2c. Hy-
droperoxides 2a and 2b exhibited weak initiating prop-
erties. The ratesof initiation of thecumeneoxidationby
these hydroperoxides were 2–10 times lower than those
of CHP. The initiating properties of hydroperoxide 2c
were comparable to those of CHP. In the long-term ox-
idations of 1b and 1c in the presence of azo-initiators,
the reaction rate was decreasing. Some inhibitors were
probably formed in that process.
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