Mechanism of Methylcyclohexane Ozonolysis

Article abstract of DOI:10.1023/A:1026029824365

A mathematical model of selective oxidation of methylcyclohexane with ozone-oxygen mixtures was substantiated.

Full text of DOI:10.1023/A:1026029824365

Russian Journal of Applied Chemistry, Vol. 76, No. 5, 2003, pp. 785 790. Translated from Zhurnal Prikladnoi Khimii, Vol. 76, No. 5,
2003, pp. 814 819.
Original Russian Text Copyright
2003 by Syroezhko, Begak, Proskuryakov.
ORGANIC SYNTHESIS
AND INDUSTRIAL ORGANIC CHEMISTRY
Mechanism of Methylcyclohexane Ozonolysis
A. M. Syroezhko, O. Yu. Begak, and V. A. Proskuryakov
St. Petersburg State Technological Institute, St. Petersburg, Russia
Mendeleev Russian Research Institute of Metrology, Federal State Unitary Enterprise, St. Petersburg, Russia
Received January 22, 2003
Abstract A mathematical model of selective oxidation of methylcyclohexane with ozone oxygen mixtures
was substantiated.
The methylcyclohexane molecule has three non-
equivalent C H bonds (at primary, tertiary, and five
secondary carbon atoms at the -, -, and -positions
relative to the methyl substituent). Therefore, selective
oxidation of this molecule at a definite C H bond can
hardly be expected, the more so as thermal oxidation
of methylcyclohexane with oxygen occurs at a notice-
able rate only at elevated temperatures (above 120 C)
and is low-selective [1].
secondary and tertiary methylcyclohexanols and iso-
meric methylcyclohexanones.
Among carboxylic acids, ozonolysis of methylcy-
clohexane yields monocarboxylic, dicarboxylic, and
keto acids. The ratio of the yield of monocarboxylic
acids to the total yield of keto and dicarboxylic acids
in the examined temperature range (20 80 C, [O ] =
3
4 vol %) is 0.6 0.7 at the methylcyclohexane conver-
sion of 12 82%. This ratio varies within a narrow
range (0.4 0.7) [1] as the ozone concentration is
varied within 2 4 vol %. The composition of the
acids is given in Tables 2 and 3. Among monocarbox-
The procedures for oxidation and analysis of reac-
tion products were described in [2]. Ozonolysis of
methylcyclohexane is active at 20 C (Fig. 1) and
yields hydroperoxides, alcohols, ketones, acids, and
esters.
[H2O2], M
[ROOH], M
(a)
(b)
Among methylcyclohexyl hydroperoxides formed
by oxidation of methylcyclohexane, the major isomer
(95 96%) is 1-methylcyclohexyl hydroperoxide.
Among methylcyclohexanols, all possible isomers are
detected (Table 1), with 1-methylcyclohexanol pre-
vailing. The yield of the tertiary alcohol decreases
with increasing conversion, temperature, and ozone
concentration (Table 1, Fig. 1). Among isomeric
methylcyclohexanones, 2-methylcyclohexanone pre-
vails (Table 1). As the methylcyclohexane conversion
increases, the yield of the alcohols and ketones de-
creases, with a simultaneous slight increase in the
relative yield of hydroperoxides, acids, and esters
(Fig. 2). The tertiary hydroperoxide at low tempera-
tures (20 C) is consumed in the reaction with ozone
slowly (rate constant 19 10 l mol s ); therefore,
its yield increases with conversion of methylcyclo-
hexane. Ozonolysis of methylcyclohexane is highly
selective with respect to the yield of 1-methylcyclo-
hexanol at conversions of up to 12%. An increase in
the yield of acids in the developed ozonolysis of
methylcyclohexane is due to further oxidation of
(c)
[R=O], M
[ROH], M
(d)
(e)
(f)
[RCO2R ], M
[RCO2H], M
3
1
1
, h
, h
Fig. 1. Kinetic curves of accumulation of (a) hydroperox-
ides ROOH, (b) hydrogen peroxide H O , (c) ketones R=O,
(d) alcohols ROH, (e) acids RCO H, and (f) esters RCO R
in oxidation of methylcyclohexane ([O ] = 4 vol %).
( ) Time; the same for Fig. 3. T, C: (1) 20, (2) 40, and
(3) 80; the same for Figs. 2 and 3.
2
2
2
2
3
1070-4272/03/7605-0785$25.00 2003 MAIK Nauka/Interperiodica

786
Table 1. Composition of neutral products of methylcyclohexane ozonolysis ([O3] = 4 vol %)*
Alcohols, mol % Ketone, mol %
SYROEZHKO et al.
™,
C
**
,** h
1-MCHL
2-MCHL
3- + 4-MCHL
2-MCHN
3- + 4-MCHN
20
12
2
4
6
2
4
6
2
4
6
93
93
90
85
81
75
80
75
95
5.5
5.0
8.0
12
15
16
15
17
4
1.5
2.0
2.0
3
4
9
5
8
95
93
90
90
86
80
85
80
96
5
7
35
57
20
55
77
35
82
26
10
10
14
20
15
20
4
40
80
40***
Traces
* 1-, 2-, 3-, and 4-MCHL denote the respective isomers of methylcyclohexanol, and 2-, 3-, and 4-MCHN, the respective isomers of
methylcyclohexanone.
** ( ) Degree of conversion and ( ) ozonolysis time.
*** [O ] = 2 vol %.
3
.
ylic acids, acetic acid prevails (70 87%). We also
identified propionic, butyric, valeric, and capric acids.
Among dicarboxylic and keto acids, we identified
-ketoenanthic (16.2 35.1%), - and -methyladipic
(40.7 53.2% in total), oxalic, succinic, methylsuccinic,
- and -methylglutaric, and glutaric acids (Table 3).
As the ozone concentration and temperature are in-
creased, the yield of acetic and -ketoenanthic acids
decreases. Keto acids are formed by ozonolysis of the
tertiary hydroperoxide, 1- and 2-methylcyclohexanols,
and 2-methylcyclohexanone [3 6].
species are alkoxy radicals rather than less active RO
2
radicals. Alkoxy radicals are apparently generated by
recombination of tertiary peroxy radicals and by reac-
.
tion of RO radicals with ozone. The tertiary hydro-
2
.
peroxide is formed by cross recombination of RO
2
.
and HO radicals and is mainly consumed with forma-
2
tion of 1-methylcyclohexanol.
At low (4 7%) methylcyclohexane conversions,
ozonolysis occurs to 92 93% at the tertiary C H
bond. For example, at 20 C ([O ] = 0.5 2.5 vol %),
the total yield of 1-methylcyclohexanol and 1-methyl-
cyclohexyl hydroperoxide is 92 93%, that of other
methylcyclohexanols and of methylcyclohexanones,
6 6.5%, and that of acids, 0.5 2%. The whole set of
the data obtained allow us to propose the following
mechanism of methylcyclohexane ozonolysis at low
conversions:
3
Low-temperature ozonolysis of methylcyclohexane
is a radical-chain reaction. Calculations show that, as
in oxidation of cyclohexane, the chain-propagating
i, %
(b)
(a)
i, %
CH3
= 0.07
+ O2
OH
(c)
(e)
i, %
i, %
CH3
OH
(d)
i, %
k = 8.6
CH3
1
3
1 1
= 0.59
= 0.24
10 l mol
s
+ O2
+ O3
CH3
.
.
ci, M
of (a) hydroperoxides ROOH, (b) alcohols
ci, M
+ O
+ OH
2
Fig. 2. Yield
i
= 0.1
ROH, (c) ketones R=O, (d) acids RCO H, and (e) esters
2
RCO R as a fuinction of methylcyclohexanol conversion
2
(1)
HOOO CH3
at [O ] = 2 vol %. ( c ) Sum of ozonolysis products.
3
i
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 76 No. 5 2003

MECHANISM OF METHYLCYCLOHEXANE OZONOLYSIS
787
(2)
CH3
OOOH
CH3
.
O
3
1
k
= 6.8 10
s
.
2
+ HO2
,
CH3
CH3
OOOH
.
O
1
1
k
l mol
s
= 0.25
.
3
3
,
+ HO
+ O2
+ O3
(3)
Slowly
.
.
,
HO3
HO + 2.5O2
+ O3
(4)
(5)
6
1
1
**= 6.4 10
k
l mol
s
5
.
.
HO2
+ HO3
,
H2 O2 + 1.5O2
H2 C
CH2
.
.
OO
7
1
1
**= 2.5 10
6
k
l mol
s
,
+ O2
(6)
(7)
CH3
.
O
5
1
1
**= 3 10
k
l mol
s
7
,
+ O2
2
H3 C
H3 C
.
.
OO
OO
CH3
CH3
O O
+
5
1
1
k
= 10 l mol
s
7
,
+ O2
CH3
OOH
CH3
.
.
OO
6
1
1
k
= 10 l mol
8
s
.
(8)
(9)
+ O2
HO2
,
+
CH3
OO
3
1
1
k
= 1.1 10
l mol
s
9
+
,
2 O2
O3
+
.
H3 C O
.
.
CH3
O
CH3
HOO
H3 C
CH3
OH
OO
4
1
1
**
k
= 5 10
l mol
s
,
10
+
+
(10)
(11)
.
OO
H2 C OOH
H3 C
3
1
1
1
k
11 = 19 10 l mol
s
.
+ O3
+
,
+ O2
HO
CH3
CH3
.
OH
H3 C
** = 5 10
4
1
k
l mol
s
12
,
+
+
(12)
(13)
.
O
CH3
**
10
1
1
k
= 10 l mol
s
13
.
.
,
+
HO
HO
H2 O2
l mol
CH3
CH3
6
1
1
CH3
k
CH3
= 1.6 10
s
13
(14)
,
+ O2
+
+
.
.
OO
OO
OH
O
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 76 No. 5 2003

788
SYROEZHKO et al.
Table 2. Content of monocarboxylic acids formed by ozonolysis of methylcyclohexane ([O3] = 4 vol %)
Acid, mol %
T,
C
, %
, h
C2
C3
C4
C5
C6
20
12
35
57
20
55
77
35
82
26
2
4
6
2
4
6
2
4
6
69.8
71.0
74.0
74.1
75.0
76.0
76.0
80.0
87.0
13.4
12.5
10.0
10.0
10.0
9.5
9.5
8.3
1.0
5.0
4.7
5.0
5.2
5.1
4.8
2.9
3.5
Traces
5.0
5.0
5.3
8.0
7.0
6.0
7.0
5.0
Traces
6.8
6.5
6.1
2.3
2.9
3.6
4.5
3.0
12.0
40
80
40
Table 3. Content of keto and dicarboxylic acids formed by ozonolysis of methylcyclohexane ([O3] = 4 vol %)
Acid, mol %
T,
C
, %
, h
methylsuccinic +
succinic
glutaric and
methylglutaric
oxalic
-ketoenanthic ( + )-methyladipic
20
12
35
57
20
55
77
35
82
26
2
4
6
2
4
6
2
4
6
15.0
13.0
12.5
10.5
8.5
6.1
4.0
2.7
10.0
7.5
7.0
6.9
5.1
4.3
3.1
2.7
2.1
5.9
11.6
12.0
12.4
8.0
10.3
12.5
10.8
11.5
8.3
20.6
18.8
16.2
28.1
27.5
26.6
32.3
30.5
35.1
45.4
49.2
52.0
48.3
49.4
51.7
50.1
53.2
40.7
40
80
40
where k** are the estimated values of the constants,
refined during further optimization.
The initial reactant concentrations are [RH] = 7.8,
3
3
1
1
[O ] = 1.3 10 , and [O ] = 5.6 10 mol l s .
3
2
The concentrations of the radicals are found from the
following relationships:
The steady-state concentrations of the ith radical
are determined from the condition d[X ]/d = 0.
i
Taking into account the yields of products formed by
the attack of ozone and radicals at the tertiary and
secondary C H bonds of the methylcyclohexane mol-
ecule, we can write
[RO2] = [(2R2 + R4 + R11
+
R1)/(2k7)]1/2
,
R1 + R12
[O2]k6
[R ] =
,
.
OO
H3C
CH3
[ OH] = [( R1 + R4 + R11)/(2k13)]1/2
[HO3] = R3/([O3]k4 + k5[HO2]),
,
.
=
0.007
OO
.
The rates of elementary reactions (1) (14) of meth-
ylcyclohexane ozonolysis are expressed as follows:
[RO ] = (R2 + R3 + 2R7 + R9)/(k10[ROOH] + k12[RH]),
[HO2] = (R2 R3 + R4)/([RO ]k8).
The constants k k , k , k , k , and k were
R
= k [RH][O ], R = k [ROOOH], R = k
1
1 3 2 2 3 3
[ROOOH][O ], R = k [O ][HO ], R = k [HO ]
[HO ], R = k [O ][R ], R = k [RO ] , R = k
3
4
4
3
3
5
5
2
2
3
6
6
2
7
7
2
8
8
[RO ][HO ], R = k [RO ][O ], R = k [ROOH]
2
2
9
9
2
3
10
10
5
7
10 12 13
13
[RO ], R = k [O ][ROOH], R = k [RH][RO ],
11
11
3
12
12
taken from [7 12], and the constants k , k , and k
1 9 11
2
2
R
= k [HO ] , and R = k [RO ] .
were measured by us in special experiments.
13
13
14
13
2
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 76 No. 5 2003

MECHANISM OF METHYLCYCLOHEXANE OZONOLYSIS
The rates of consumption of methylcyclohexane [RH], M
789
(RH) and accumulation of 1-methylcyclohexanol
(ROH ), 1-methylcyclohexyl hydroperoxide (ROOH),
t
hydrogen peroxide, sum of secondary methylcyclo-
hexanols (ROH ), methylcyclohexanones (R=O),
s
and hydrotrioxide (ROOOH) are described by the
differential equations
d[RH]/d = R1 R12, d[ROH]t/d = R10 + R12
d[ROOH]/d = R8 R10 R11,
+ R1,
+
, h
d[H2O2]/d = R5 + R13, d[ROH]s/d = R1 + k13[R O2]2,
d[R=O]/d = k13[R O2]2, d[ROOOH]/d = R1 R2 R3.
Fig. 3. Kinetic curves of consumption of methylcyclohex-
ane RH ([O ] = 1.3 10 M): (I) experiment and (II) cal-
3
3
culation.
4
1
1
A
10 , counts s 1 mol
104, mol l 1 min
Integration of this sytem of differential equations
allows refinement of the constants k k** and coeffi-
1
13
cients
, , , and . The values of these parameters
are given in the scheme of the mechanism of methyl-
cyclohexane ozonolysis. The calculated data reasona-
bly agree with the experiment (Figs. 1 3). The steady-
state concentrations of the radicals are as follows:
5
[RO ] = (1.32 1.36) 10 , [RO ] = (3.62 3.68)
2
10
8
10 , [ OH] = (3.13 3.21) 10 , [HO ] = (5.48
5.72) 10 , and [HO] = (1.04 1.07) 10 M ( =
10 60 min).
2
7
7
, h
Fig. 4. Variation of the (1) specific activity A and of the
rates of (2) formation and (3) consumption of 1-methyl-
cyclohexyl hydroperoxide in the course of methylcyclohex-
According to the suggested mechanism, methyl-
cyclohexane is consumed by the nonchain and chain
pathways. Linear consumption (Fig. 3) is observed
only at small conversions. In the developed process,
the kinetic curves of methylcyclohexane (RH) con-
sumption are nonlinear and are described by the fol-
lowing equation in the entire range of reactant con-
centrations up to deep conversions:
ane ozonolysis (20 C, [O ] = 4 vol %).
3
NMR [13]) reacts with ozone at the same rate as does
nonassociated 1-methylcyclohexyl hydroperoxide. To
reveal the role of this reaction in the overall ozonol-
ysis of methylcyclohexane, we added at the stage of
the developed reaction (20 C, [O ] = 4 vol %, 30 min
3
14
/
CH3
after the start of the experiment) C-labeled 1-meth-
/
ylcyclohexyl hydroperoxide. The specific activity of
the added hydroperoxide decreases (Fig. 4) owing to
both its consumption and dilution with the forming
nonradioactive 1-methylcyclohexyl hydroperoxide.
The labeled compound transforms into the following
/
/
kapp[RH] = k1app[RH][O3]
d
=
d
/
/
k[([RH]0 [RH]) [RH] .
+
/
14
C-labeled products: 1-methylcyclohexanol, -keto-
Computer processing of the experimental data gave
enanthic acid, and acetic acid. From variation of the
specific activity of the hydroperoxide in the course of
the experiment, we calculated the rates of its forma-
tion. Taking into account the experimental rates of
1-methylcyclohexyl hydroperoxide accumulation, we
calculated the rates of its consumption (Fig. 4).
2
the following expressions: k
= 1.25 10
1app
(25500 3500)RT
1
1
1
e
e
l mol
l mol s .
s
and k = 3.5 10
(25500 3500)RT
1
1
The bimolecular rate constant of the reaction of
1-methylcyclohexyl hydroperoxide with ozone is low.
However, in the developed reaction of methylcyclo-
hexane ozonolysis, it is possible that the associate of
Among the products of 1-methylcyclohexyl hydro-
peroxide transformations, 1-methylcyclohexanol com-
prises only 6 7%. An increase in the rate of formation
of the tertiary hydroperoxide with the conversion of
1-methylcyclohexyl hydroperoxide and 1-methylcy-
1
clohexanol (their association was detected by
H
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 76 No. 5 2003

790
SYROEZHKO et al.
the hydrocarbon is apparently due to increase in the
kov, V.A., Zh. Prikl. Khim., 1976, vol. 49, no. 3,
pp. 588 592.
5. Vikhorev, A.A., Syroezhko, A.M., and Proskurya-
kov, V.A., Zh. Prikl. Khim., 1975, vol. 48, no. 9,
pp. 2059 2062.
.
rate of HO generation. Unstable methylcyclohexyl
2
.
hydrotrioxide may be a source of HO . A decrease in
2
the yield of hydrogen peroxide (Fig. 1), seemingly
contradicting this assumption, may be due to catalysis
of its decomposition with acids.
6. USSR Inventor’s Certificate 745891.
7. Emanuel’, N.M., Denisov, E.T., and Maizus, Z.K.,
Tsepnye reaktsii okisleniya uglevodorodov v zhidkoi
faze (Chain Reactions of Liquid-Phase Oxidation of
Hydrocarbons), Moscow: Nauka, 1965.
8. Denisov, E.T., Mitskevich, N.I., and Agabekov, V.E.,
Mekhanizm zhidkofaznogo okisleniya kislorodsoder-
zhashchikh soedinenii (Mechanism of Liquid-Phase
Oxidation of Oxygen-Containing Compounds), Minsk:
Nauka i Tekhnika, 1975.
9. Denisov, E.T., Mechanism of Liquid-Phase Homolysis
of Molecules, Itogi Nauki Tekh., Ser.: Kinet. Katal.,
1981, vol. 9.
10. Denisov, E.T., Konstanty skorosti gomoliticheskikh
zhidkofaznykh reaktsii (Rate Constants of Homolytic
Liquid-Phase Reactions), Moscow: Nauka, 1971.
CONCLUSION
In contrast to nonselective high-temperature non-
catalytic oxidation of saturated hydrocarbons contain-
ing primary, secondary, and tertiary C H bonds, their
low-temperature ozonolysis at small conversions, as
demonstrated by the example of methylcyclohexane,
occurs selectively at the tertiary C H bond and yields
the corresponding alcohols. The tertiary alcohols are
.
.
formed by recombination of the R and OH radicals
generated by primary initiation, by radical-chain trans-
formations of tertiary hydroperoxides, and by attack
.
of the starting substrate by RO radicals.
11. Emanuel’, N.M., Zaikov, G.E., and Maizus, Z.K.,
Rol’ sredy v radikal’no-tsepnykh reaktsiyakh okisle-
niya organicheskikh soedinenii (Role of the Medium
in Radical-Chain Oxidation of Organic Compounds),
Moscow: Nauka, 1973.
12. Antonovskii, V.L., Organicheskie perekisnye initsi-
atory (Organic Peroxide Initiators), Moscow: Khi-
miya, 1972.
13. Vikhorev, A.A. and Syroezhko, A.M., in Issledova-
niya v oblasti khimii i tekhnologii produktov perera-
botki goryuchikh iskopaemykh (Studies in the Field of
Chemistry and Technology of Products of Fossil Fuel
Processing), Leningrad: Leningr. Tekhnol. Inst. im.
Lensoveta, 1977, pp. 33 37.
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RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 76 No. 5 2003