Ethylbenzene oxidation in the presence of acetic anhydride

Article abstract of DOI:10.1007/s11167-005-0314-z

The kinetics of formation and subsequent transformation of the products of ethylbenzene oxidation in the presence of acetic anhydride at 100°C was studied by solution of the inverse kinetic problem.

Full text of DOI:10.1007/s11167-005-0314-z

Russian Journal of Applied Chemistry, Vol. 78, No. 3, 2005, pp. 443 447. Translated from Zhurnal Prikladnoi Khimii, Vol. 78, No. 3,
2005, pp. 447 451.
Original Russian Text Copyright
2005 by Nosacheva, Revkov, Perkel’.
ORGANIC SYNTHESIS
AND INDUSTRIAL ORGANIC CHEMISTRY
Ethylbenzene Oxidation in the Presence of Acetic Anhydride
I. M. Nosacheva, O. A. Revkov, and A. L. Perkel’
Kuzbass State Technical University, Kemerovo, Russia
Received May 28, 2004; in final form, December 2004
Abstract The kinetics of formation and subsequent transformation of the products of ethylbenzene oxidation
in the presence of acetic anhydride at 100 C was studied by solution of the inverse kinetic problem.
Secondary hydroperoxides are the main primary
molecular intermediates of liquid-phase oxidation of
saturated aliphatic, alicyclic, and some alkylaromatic
hydrocarbons [1 4]. These intermediates can further
react with carboxylic acid anhydrides (important prod-
ucts of oxidative degradation of saturated hydrocar-
bons and their oxygen derivatives) [4, 5].
The total peroxide content in the EB oxidation
products was determined colorimetrically after treat-
ment of the sample with the reagent containing Fe
2+
and N,N-dimethyl-p-phenylenediamine [9]. The con-
tent of 1-phenylethyl peroxyacetate (PEPA) was cal-
culated as the difference between the content of
1-phenylethyl acetate (PEA) in the sample reduced
with triphenylphosphine and in the oxidized sample.
Acetic anhydride was decomposed with a methanol
pyridine mixture (1 : 1 v/v) immediately after the
sampling. The content of 1-phenylethyl hydroperoxide
(PEHP) was determined as the difference between
the total peroxide content and the PEPA content. To
determine the content of benzoic acid by GLC, it was
converted into methyl benzoate by treatment with di-
azomethane. The GLC analysis of nonperoxide prod-
ucts of EB oxidation was performed at 140 C on a
2000 3-mm column; stationary phase 15% Car-
bowax-6000 on Chromaton N-AW-DMCS, internal
reference nitrobenzene.
Carboxylic acid anhydrides react with hydroper-
oxides to form peroxy esters [4 7]. Synthesis of per-
oxy esters of secondary hydroperoxides and their
properties are poorly studied. This is probably due to
the fact that peroxy esters of secondary hydroperox-
ides are substantially less stable than those of primary
and especially tertiary hydroperoxides and decompose
even at low temperatures [6, 7].
In this work we studied the kinetics of ethylben-
zene (EB) oxidation in the presence of acetic anhy-
dride (AA). 1-Phenylethyl hydroperoxide formed in
EB oxidation reacts with AA to form 1-phenylethyl
peroxyacetate. In this study we examined the main
relationships in formation and subsequent reaction of
1-phenylethyl peroxyacetate.
Ethylbenzene was oxidized in a manometric unit
under the conditions of the kinetic control of the
oxygen uptake.
The parameters of the kinetic equations were calcu-
lated by the least-squares procedure using the Del-
phi 5.5 program. A set of differential equations was
solved in each step by the fourth-order Runge Kutta
method.
EXPERIMENTAL
Ethylbenzene (pure grade) was treated with concen-
trated sulfuric acid to remove oxygen-containing com-
pounds [8] and then was distilled in an argon flow. As
determined by gas liquid chromatography, the con-
centration of methylphenylcarbinol and acetophenone
in the resulting product was lower than 0.003 and
0.005 wt %, respectively.
Peroxy esters of secondary hydroperoxides are ther-
mally unstable and can be studied only under special
conditions [6, 7] which substantially differ from the
conditions of hydrocarbon oxidation. The general
features of formation and subsequent transformations
of peroxy esters of secondary hydroperoxides under
the autooxidation conditions were studied with an
example of PEPA. This compound was prepared in
situ by reaction of AA introduced into the reaction
Acetic anhydride (analytically pure grade) was
purified by distillation after conversion of the acetic
acid impurity into methyl acetate by the reaction with
diazomethane.
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444
NOSACHEVA et al.
described by the equation [1, 2]
d[RH]/d = kp(2kt) 0.5[RH](wi0)0.5
cO2, M
,
(2)
where k and 2k are the rate constants of chain propa-
p
t
0
gation and termination, respectively; w is the rate of
i
initiation by chain generation.
0.5
4
0.5 0.5
cO2 103, M
Substitution of k (2k )
=22.2 10 lmol
s
p
t
[2], the rate constant of radical-chain oxidation of EB
in the absence of acetic anhydride (w = 3.9
rc
6
1
1
10 mol l s ), and [RH] = 8.2 M into Eq. (2) gives
0
8
1
1
w
= 4.5 10 mol l
s
at 100 C.
i
The kinetic curve of oxygen uptake in the presence
cO2, M
of AA is more complex. In the first steps, the curve is
typical of an autocatalytic process and then it becomes
linear (see figure). Along with PEHP, PEPA is ac-
cumulated by acylation of PEHP with AA:
cO2 102, M
(Ac) O
2
PhCH(OOH)Me
PhCH(OOAc)Me (PEPA). (3)
AcOH
Acceleration of EB oxidation in the presence of
AA is due to increasing contribution of degenerate
branching caused by homolytic decomposition of
PEPA. Probably, radical decomposition of PEPA in
the course of BE oxidation is faster than that of the
initial PEHP [5]. The main oxidation product of EB in
the presence of AA is AP. Its yield is substantially
higher than that in thermal oxidation of EB at 120 C
in the absence of AA [2]. We suggest that AP is
formed by pericyclic transformation of PEPA [scheme
(4), pathway a]. The relative yield of MPC in the
presence of AA (see figure) is substantially lower than
that in the absence of AA [2]. Probably, methyl-
phenylcarbinol is formed by the reaction of 1-phenyl-
ethoxy radical (PER) with EB and partially reacts
with AA to form PEA [scheme (4), pathway b]. Clear-
ly, homolytic decomposition of PEPA yields BAl
which is further oxidized to BAc [scheme (4), path-
way c].
, min
(1 ) Kinetic curves of oxygen uptake in EB oxidation at
100 C in the absence of AA and kinetic curves of (1) total
accumulation of the products of EB oxidation at 100 C
in the presence of AA and accumulation of (2) PEHP,
(3) PEPA, (4) AP, (5) MPC, (6) BAl + BAc, (7) PEA, and
(8) BAc. (C ) concentration of absorbed oxygen and the
O
2
oxidation products and ( ) time. (1 7) Calculated curves
and (1 , 8) experimental curves.
mixture with PEHP formed in radical-chain oxidation
of EB.
Ethylbenzene was oxidized with oxygen at 100 C
in the absence of AA and in the presence of 1.1 M
AA. The experimental kinetic curves of oxygen up-
take (in the absence of AA), total accumulation of the
EB oxidation products, and accumulation of PEHP,
PEPA, methyphenylcarbinol (MPC), acetophenone
(AP), PEA, benzaldehyde (BAl) + benzoic acid
(BAc), and BAc are shown in the figure.
The rate of ethylbenzene oxidation to form PEHP
at 100 C in the absence of AA is almost constant (see
figure)
b
*
RO , O , RH
2
2
PhCH2Me
PhCH(OOH)Me (PEHP). (1)
PER
(4)
c
Clearly, PEHP decomposition under these condi-
tions is slow and is initiated mainly by chain genera-
tion. The rate of radical-chain oxidation of EB is
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ETHYLBENZENE OXIDATION IN THE PRESENCE OF ACETIC ANHYDRIDE
445
Calculated rate constants of formation and transformations of PEPA in EB oxidation in the presence of AA
ki 104
ki 102
i
(ki /ki) 102
1
1
1
1
s
l mol 1 s
9.57
l mol 1 s
0.19
l2 mol 2 s
16.68
0
1
2
3
4
5
10.00
1.90
174
32.5
6.13
2.52
0.76
0.22
0.0016
2.34
3.59
0.063
0.00
2.57
0.000
app app
Thus, EB oxidation in the presence of AA can be
described by the following kinetic scheme:
amount of oxygen taken up, M; k
k
are the ap-
1
5
parent rate constants of the steps of reaction (5); and
app
k
is the apparent rate constant of initiation by
0
*
,
O
2
,
RH
RO
PEPA decomposition.
2
PEHP + AA
AP
EB
app
Since formation and subsequent transformations of
1-hydroxycyclohexyl peroxydodecanoate are acid-
catalyzed [10, 11], we suggest that the corresponding
apparent rate constants can depend on the acetic acid
(AAc) concentration
k
k
2
app
3
app
app
2
k
k
5
(5)
PEPA
MPC
PEA
app
4
k
BAl + BAc.
kai pp = ki + k i[AAc],
(16)
Kinetics of reactions (1), (3), and (4) can be de-
scribed by the following set of equations:
where k is the autocatalytic constant.
i
d[EB]/d = kp(2kt) 0.5[EB](wi0) + ka0pp[PEPA]
,
(6)
0.5
The rate constants calculated by solution of the
inverse kinetic problem are presented in the table, and
the calculated kinetic curves are shown in the figure.
ka0pp[PEPA]
0.5
d[PEHP]/d
=
kp(2kt) 0.5[EB](w0)
+
i
ka1pp[PEPA][AA],
(7)
As seen from the figure, the calculated kinetic
curves fit well the experimental results (the sum of
d[PEHP]/d
=
ka1pp[PEPA][AA]
+ ka4pp)[PEPA],
(ka2pp + ka3pp
4
squares of the deviations is 4.9 10 ). Comparison
of the rate constants shows that the conversion of
(8)
(9)
app
PEPA into AP is about 75%. Although all k , except
i
d[AP]/d = ka2pp[PEPA],
d[MPC]/d = ka3pp[PEPA] ka5pp[MPC][AA],
app
for k , depend on the AAc concentration, the auto-
5
catalytic contribution to reactions steps, as indicated
(10)
(11)
(12)
by the k /k ratio (see table), noticeably differ. The in-
i
i
fluence of AAc on acylation of PEHP with AA is
the most pronounced. Clearly, the catalytic effect in
this case is caused by formation of AAc AA (CCA)
complex which is more reactive than the free an-
hydride with respect to the nucleophilic attack of the
d([BAl] + [BAc])/d = ka4pp[PEPA],
d[PEA]/d = ka5pp[MPC][AA],
d[AA]/d = k1app[PEHP][AA] + ka5pp[MPC][AA], (13)
d[AA]/d = k1app[PEHP][AA] + (ka2pp + k3app + k4app)[PEPA]
+
+
hydroperoxide (
>
):
2
1
+
+
C1
O
Me
Ac
C2
O
HO
(CCA)
+ ka5pp[MPC][AA],
(14)
Me
K
Ac,
+ AcOH
[O2] = [EB]0
[EB] = [PEHP] + [PEPA] + [AP]
(15)
O
O
(17)
Ac
+ [MPC] + [BAl] + [BAc] + [PEA],
k"
2
where [X] is concentration of the reactants, M; [O ],
(CCA) + PEHP
PEPA + AAc,
(18)
2
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 78 No. 3 2005

446
NOSACHEVA et al.
where k" is the rate constant of reaction of PEHP
2
with CCA and K is the equilibrium constant of reac-
tion (17).
Taking into account reactions (17) and (18), the
PEHP consumption can be described by the kinetic
equation of an autocatalytic reaction
(20)
d[PEHP]cat/d = k"2 K[AAc][AA][PEHP]
(CCAAc)
= k2 [AAc][AA][PEHP].
Hence, k = k"K.
(19)
This constant should be related, by definition, to
the apparent rate constants of PEPA decomposition
into MPC and BAl by the equation
2
2
Acylation of MPC with AA should be seemingly
also acid-catalyzed. However, our previous kinetic
study of this reaction in chlorobenzene at 80 110 C
[12] shows that AAc has no effect on this reaction. The
k0 + k0[AAc] = 2e(k3 + k3 [AAc]) + (k4 + k4 [AAc]), (21)
app
4
1
1
k
determined in this study (2.57 10 l mol s )
5
where e is the coefficient of radical escape from the
app
4
1
1
is close to k
from the dependence lnk = (10.9 2.9) (59300
8900)/RT [12].
= 2.66 10 l mol
s
calculated
5
solvent cage.
0
At [AAc] = 0.2 M, substitution of the constants in
Eq. (21) gives e = 0.57 which is close to the known
value (e = 0.6) for aromatic solvents [13].
The reason of this distinction is not clear. We as-
sume that it is due to the fact that the steric hindrance
in the attack of the carbon atom of the carbonyl group
in CCA by the hydroxy group of MPC is stronger
than that in the reaction of CCA with the hydroperoxy
group of PEHP.
Thus, carboxylic acid anhydrides substantially
catalyze decomposition of secondary hydroperoxides
and change the decomposition mechanism. Contrary
to the previous data [7], about 25% of the peroxy ester
of the secondary hydroperoxide decomposes homolyt-
ically, i.e., this compound initiates the radical-chain
oxidation but to a lesser extent as compared to peroxy
esters of tertiary hydroperoxides [7, 14]. Since car-
boxylic acid anhydrides are consumed mainly by reac-
tions with hydroperoxides [4], formation and decom-
position of peroxy esters of secondary hydroperoxides
can contribute significantly to both initiation of oxida-
tion of organic compounds and formation of carbonyl-
containing products.
The influence of AAc in the step of PEPA decom-
position is weaker than that in the step of its forma-
tion. The ratio k /k (table) shows that the contribution
i
i
of the autocatalytic reaction to the overall process is
substantially lower [particularly, in the case of reac-
tion (4, pathway c)] than in the case of decomposition
of 1-hydroxycyclohexyl peroxydodecanoate (inter-
mediate of Baeyer Villiger oxidation of cyclohexa-
none with peroxylauric acid) [10, 11]. The ratio of the
constants for heterolytic rearrangement of 1-hydroxy-
cyclohexyl peroxydodecanoate ( -hydroxy peroxy
ester) into lactone (18 C) is 17.3, whereas for the
homolytic decomposition it is 41.8 [11]. Probably,
decomposition of the peroxy ester is less acid-sensi-
tive than that of -hydroxy peroxy ester owing to
lower probability of the decomposition by the mech-
anism of relay proton transfer [10, 11].
CONCLUSIONS
(1) Acetic anhydride substantially catalyzes radi-
cal-chain oxidation of ethylbenzene at 100 C. The
primary molecular product of ethylbenzene oxidation,
1-phenylethyl hydroperoxide, decomposes via 1-phen-
ylethyl peroxyacetate formation to give mainly aceto-
phenone by the pericyclic mechanism.
We suggest that the decomposition mechanism of
PEPA in the cyclic complex with AAc (CCAAc) to
form AP (20) is similar.
(2) Acceleration of ethylbenzene oxidation in the
presence of acetic anhydride is due to homolytic de-
composition of the peroxy ester into methylphenyl-
carbinol and benzaldehyde.
The apparent rate constant of initiation by homolyt-
app
ic decomposition of PEPA (k ) (see table) is almost
(3) Acetic acid formed in acylation of the hydro-
peroxide and peroxy ester decomposition catalyzes
0
independent of the presence of acetic acid.
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ETHYLBENZENE OXIDATION IN THE PRESENCE OF ACETIC ANHYDRIDE
447
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