Inhibitory effect of 2-aminothiazole derivatives in oxidation reactions

Article abstract of DOI:10.1023/A:1013978608299

The kinetics and mechanism of inhibitory effect of 2-aminothiazole derivatives in the radical-initiated oxidation of cumene were studied.

Full text of DOI:10.1023/A:1013978608299

Russian Journal of General Chemistry, Vol. 71, No. 9, 2001, pp. 1484 1486. Translated from Zhurnal Obshchei Khimii, Vol. 71, No. 9, 2001,
pp. 1567 1570.
Original Russian Text Copyright
2001 by Karpov, Pekarevskii, Potekhin.
Inhibitory Effect of 2-Aminothiazole Derivatives
in Oxidation Reactions
K. A. Karpov, B. V. Pekarevskii, and V. M. Potekhin
St. Petersburg State Institute of Technology, St. Petersburg, Russia
Received July 7, 2000
Abstract The kinetics and mechanism of inhibitory effect of 2-aminothiazole derivatives in the radical-
initiated oxidation of cumene were studied.
Compounds of the thiazole series are widely used
as pharmaceuticals due to their antiphlogistic, anti-
(Fig. 2). As follows from the shape of the kinetic
curves, compounds I IV show antiradical activity,
tumor, psychotropic, antibacterial, fungicide, anti- participating in chain termination via reaction with
tuberculous, and antihelminthic activity [1, 2]. Studies
on the biochemical properties of thiazole derivatives
have been performed; among these, the most interest-
ing (in terms of our work) are those concerned with
antioxidant properties of thiazoles [3 5]. Thiazoles
and benzothiazoles are also used as vulcanization
peroxide radicals. The kinetic curves resemble those
observed in autooxidation of cumene. However, the
inhibitory effect of aminothiazoles I IV depends on
the reaction conditions. In the initiated oxidation of
cumene compound II does not exert the maximum
inhibitory effect as in the autooxidation. By contrast,
accelerators in rubber-processing industry [6] and as aminothiazole IV shows the reverse relation. Cumene
efficient antioxidant and antiwear lubricant additives
6, 7].
oxidation in the presence of radical initiator is
characterized by similar initial rates and induction
periods, regardless of the aminothiazole added.
[
There are no reliable published data on the
mechanism of antioxidant action of such compounds.
The present work was aimed at the studying of 2-
aminothiazole antioxidant properties in the cumene
liquidphase oxidation. The study was performed by
the test procedure including determination of rate
constants of key reactions involving the antioxidant
The miechanism of oxidation of hydrocarbons in
the presence of inhibitors is determined by contribu-
tions of elementary steps, i.e., by the rates of key
reactions involving antioxidants [8]. These contribu-
c
102, 4
O2
1
4
2 5
3
[
8]. The data thus obtained will allow us to establish
M
the mechanism of inhibitory effect and determine the
kinetic parameters for the oxidation of hydrocarbons.
3
2
2-Aminothiazole derivatives I IV were examined
as model antioxidants.
R
N
S
NH2
1
I IV
I, R = H; II, R = Me; III, R = Ph; IV, R = 1-naphthyl.
0
5
10
15
,
Addition of aminothiazoles I IV to the substrate
cumene) inhibits the oxidation process. The initial
min
(
rate of the process decreases, and the induction period
increases, depending on the substituent in position 4
of the inhibitor (Fig. 1). The ability of aminothiazoles
to terminate oxidation chain was studied by carrying
out the reaction in the presence of radical initiator
Fig. 1. Absorption of oxygen ( c ) in the oxidation of
O2
0
0
3
cumene at 130 C; c
7.17, c
10 M: (1) in
cumene
inhibitor
the absence of inhibitor and in the presence of amino-
thiazole (2) I, (3) II, (4) III, and (5) IV.
1
070-3632/01/7109-1484$25.00 2001 MAIK Nauka/Interperiodica

INHIBITORY EFFECT OF 2-AMINOTHIAZOLE DERIVATIVES
1485
(3)
[HIn] [RH]0.7vi 0.5[O ]
1.5
F
102,
0 0 2 0
c
M
1
O2
3 2 5 4
4
3
Such dependences of reaction orders on v and F
indicate a complicated mechanism of inhibition,
which is typical of linear chain termination through
reactions of inhibitor with both RO and R :
2
2
1
k1
RO + HIn
ROOH + In;
Products.
(4)
(5)
2
k2
R + HIn
The stoichiometric inhibition coefficient was cal-
culated using Eq. (6):
0
5
10
15
,
min
f = indv/[HIn]0.
(6)
Fig. 2. Absorption of oxygen ( c ) in the radical-
O2
0
initiated oxidation of cumene at 80 C; c
7.17,
8.86
Here,
is the induction period, s.
ind
cumene
0
3
c
5 10
M, initiation rate
v
inhibitor
6
i
The value of f ( 1, Table 1) and the non-zero order
of the reaction with respect to oxygen suggest that
inhibitor can react with oxygen [8] according to
scheme (7):
1
1
10
mol l
s : (1) in the absence of inhibitor and in
the presence of aminothiazole (2) I, (3) II, (4) III, and
5) IV.
(
tions were estimated by the kinetic method on the
basis of the orders of reaction with respect to each
reactant.
k3
HIn + O2
In + HO2.
(7)
In addition, the rate of initiation v increases. On
i
The initial rate v of cumene oxidation in the pres-
ence of inhibitors was determined from the slope of
the tangent at the initial part of the kinetic curve [9].
The respective order of reaction was calculated from
the change in the initial rate upon variation of the
initial concentration of inihibitor, substrate, initiator,
and oxygen. We thus obtained Eq. (1) which relates
the initial oxidation rate to the component concentra-
tions and vi:
the other hand, the proximity of f to unity indicates
that radicals derived from the inhibitor could react
with each other [8]:
k4
In + In
Products.
(8)
In keeping with published data [8], the kinetic
scheme for compounds I IV is described by Eq. (9).
1
0.6
0.2
k3[RH]vi
k1[HIn]
2k5 k [O ][RH] k [O ]v
3 2
6 2 i
v
[HIn] [RH] v [O ]
.
(1)
0
0
i
2 0
v =
+
+
.
(9)
k1
k2[HIn]
Hereinafter, v is the initial rate of cumene oxida-
1
1
Here, k is the rate constant for the reaction of
5
peroxide radicals with the substrate, l mol s ; is
the probability for active radicals to appear in the bulk
tion, mol l s ; [HIn] and [RH] are the initial con-
0
0
1
1
centrations of the inhibitor and substrate, respectively,
1
1
M; and v is the rate of initiation, mol l s . The
i
solution; k is the rate constant for the reaction of
6
inhibitory effect can be characterized by the parameter
F which is calculated as the ratio of the oxidation
chain lengths in the absence and in the presence of
inhibitor [8] [Eq. (2)].
1
1
alkyl radical with oxygen, l mol s ; k , k , and k
1
3
2
are the rate constants of reactions (4), (7), and (5),
1
1
respectively, l mol s ; and [O ] is the concentration
2
of oxygen, M. The second and the third terms in
Eq. (9) are negligible under our conditions; therefore,
oxidation chains are terminated mainly through the
reaction with peroxide radicals.
v2
v0
v
F =
1
.
(2)
2
v
0
Here, v is the rate of radical chain oxidation of
0
1
1
The energies of activation for the reactions of
inhibitors with peroxide radicals were determined
cumene with no inhibitor, mol l s . Kinetic analysis
of the oxidation scheme allows us to express the
inhibitory effect by Eq. (3), provided that chain
termination occurs in one rate-determining stage:
from the temperature dependences of k in the range
1
from 70 to 90 C. The ratio k /k can be determined
5
1
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 9 2001

1486
KARPOV et al.
Table 1. Parameters of cumene oxidation in the presence
Table 2. Yields, melting points, and Rf values of
2-aminothiazole derivatives I IV
of 2-aminothiazoles I IV; 80 C, c
7.2 M, v 8.86
cumene
i
6
mol l 1 s )
1
1
0
Comp.
no.
Rfa
Yield, %
mp,
C
4
Comp.
no.
k1 10 ,
l mol
E ,
kJ/mol
104
a
f
k5/k
1
1
1
s
I
II
III
IV
90
60
80
80
88 1, 90 [9]
40 41, 42 [10]
152 153, 151 152 [10]
165 1
0.44
0.49
0.61
0.68
I
II
III
IV
0.79
1.03
1.17
0.86
4.07
5.09
4.31
3.42
0.84
20.1
22.5
18.1
23.7
0.67
0.79
1.00
a
Eluent 2-propanol octane, 1:1.
as the slope of the plot of oxidation rate versus com-
REFERENCES
ponent concentration or v values calculated from the
i
first term of Eq. (9). Table 1 summarizes the kinetic
and thermodynamic parameters of cumene oxidation
in the presence of aminothiazoles I IV.
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no. 3, p. 102.
The rate constants k for chain termination in-
1
volving the inhibitor and the energies of activation Ea
for the compounds under study are fairly similar. This
fact leads us to presume that the main reaction center
in molecules I IV is the amino group.
3
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pp. 2070 2077.
4
5
6
. US Patent no. 5643932, 1990; Ref. Zh., Khim., 1999,
no. 8O112.
Thus we have shown that 2-aminothiazoles are
effective acceptors of peroxide radicals; therefore,
they act as inhibitors in the oxidation of alkylaromatic
hydrocarbons. Substituted aminothiazoles can react
with alkyl radicals and also can be oxidized with
oxygen, but the contribution of these processes is
negligible. The main reaction center in the molecules
of compounds I IV is the amino group, whereas
the substituent in position 4 has no appreciable effect
on the inhibitory activity.
. US Patent no. 5677319, 1990; Ref. Zh., Khim., 1999,
no. 12O101.
. Kurzer, F., Organic Compounds of Sulphur, Sele-
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8
. Sosulina, L.N., Barabanova, G.V., and Kosso-
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pp. 12 13.
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EXPERIMENTAL
1
15.
2
-Aminothiazole derivatives I IV were synthesized
according to Hantzsch by reactions of thiourea with
-dichlorodiethyl ether [10], iodine and acetone
11, 12], iodine and acetophenone [10, 11], and iodine
9. Charykov, A.K., Matematicheskaya obrabotka re-
zul’tatov khimicheskogo analiza (Mathematical
Processing of the Results of Chemical Analysis),
Leningrad: Khimiya, 1984.
,
[
and 1 -acetonaphthone, respectively. The structure
and purity of compounds I IV were proved by TLC
and IR and NMR spectroscopy. Their yields, melting
10. Organic Reactions, Adams, R., Ed., New York:
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Chem. Soc., 1924, vol. 46, no. 12, p. 2864.
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f
11. Dodson, R.M. and King, L.C., J. Am. Chem. Soc.,
1
945, vol. 67, no. 12, pp. 2242 2243.
Oxidation of cumene. The oxidation was carried
out using a manometric setup whose action is based
on the measurement of the volume of absorbed
oxygen while the system is maintained at a constant
pressure [13]. Azobis(isobutyronitrile) was used as
1
2. Byers, J.R. and Dickey, J.B., Organic Syntheses,
New York: Wiley, 1943, coll. vol. 2, pp. 31 32.
13. Golubeva, M.V., Ivanov, V.A., Potekhin, V.M., and
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zhidkofaznykh reaktsii (Rate Constants of Homolytic
Liquid-Phase Reactions), Moscow: Nauka, 1971.
radical initiator, the initiation rate constants k being
i
5
5
4
equal to 4.02 10 , 7.63 10 , 1.42 10 ,
4
4
1
2
7
.61 10 , and 4.70 10
5, 80, 85, and 90 C, respectively [14].
s
for cumene at 70,
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 9 2001