Biphasic Autoxidation of Tetralin
J. Phys. Chem. B, Vol. 101, No. 4, 1997 601
k2k25[ML]2T
formed in the propagation chain. The catalyst’s selectivity
toward the hydroperoxide decomposition reactions is presumed
to determine relative distribution of R-tetralone (RdO) and
R-tetralol (ROH).
dNO
1
2
-
≈
[ML]2T at low [ML]T
(k2 + k7 + k8[RH])k9
V dt
(I-1)
In accord with other biphasic reactions that also show a weak
dependence on dissolved gas reactants2,3 and consistent with
the known existence of oxygenated manganese and chromium
complexes,33,34 the involvement of an oxygenated metal complex
is proposed in the propagation chain (step 5) to explain the weak
dependence of the biphasic autoxidation reaction on oxygen
pressure. As will be shown shortly, the same postulated
involvement of the oxygenated metal complex could also explain
the rate dependence on the catalyst.
dNO
k2k5k6[ML]T[RH]
1
2
-
≈
V dt
(k2 + k7 + k8[RH])k10
[ML]1T at intermediate [ML]T (I-2)
k2k26[RH]2
dNO
1
2
-
≈
[ML]0T at high [ML]T
(I-3)
V dt
(k2 + k7 + k8[RH])k11
To explain the shift to zeroth-order dependence on the catalyst
at a high metal concentration and to account for the lengthening
of the induction period in the manganese-catalyzed reaction,
metal species are presumed to participate in chain termination
reactions (steps 10 and 11). In conjunction with step 5 and the
proposed initiation sequence, steps 10 and 11 may be shown to
give rise to first- and zeroth-order rate dependence, respectively,
on the metal concentration. Step 9, which may be shown to
give a second-order rate dependence on catalyst, is supported
by the detection of 1,2-dihydronaphthalene (Rd) as a side
product in the reaction mixture.22,28
Thus, above an oxygen pressure of 0.15 atm, the model predicts
second-, first-, and zeroth-order rate dependence on the catalyst
at low-, intermediate, and high-catalyst concentration ranges,
respectively. The model results are in accord with the experi-
mental finding on the manganese-catalyzed reaction. They are
also in accordance with the experimental findings on the
chromium-catalyzed reaction if it is assumed that in the low-
catalyst concentration range in which rate data are available,
step 10 makes a comparable contribution to step 9 toward chain
termination. With this assumption, the model would predict,
for the low-catalyst concentration range, a limiting catalyst order
of about 1.5, which is in good agreement with the experimental
value of 1.4. For the intermediate and high-chromium concen-
trations, the model and experimental results are in agreement
on the first- and zeroth-order rate dependence on the catalyst.
The initiation sequence is presumed to be established during
the induction period during which hydroperoxide and other
reaction intermediates are built up to some “steady-state”
concentrations. The chain carrier, tetralyl radical (R•), is
presumed to derive from the decomposition of tetralyl hydro-
peroxide. The latter reaction would produce geminate radicals
in a solvent cage, and most of the radicals would react with
each other or with tetralin to form R-tetralone and R-tetralol
(steps 7 and 8), but some might break out of the solvent cage
to produce the tetralyl radical.
At a low-oxygen pressure (<0.15 atm), the metal complex
may be assumed to exist predominantly in a nonoxygenated
form, i.e., [ML(O2)] ) K[ML]PO ≈ K[ML]TPO , where K is
2
2
the stability constant of the oxygenated complex. Equation I
then reduces to the following limiting form for the intermediate
catalyst concentration range for which rate data are available:
The above reaction steps presumably occur at or near the
O-W interface. In order to rigorously model the biphasic
reaction, it would be necessary to have quantitative data on the
interfacial area and interfacial concentrations of the reacting
species. Such data, unfortunately, are not available presently,
and it is necessary to simplify the model. On account of the
vigorous and rapid mechanical stirring used in this study, the
reaction mixture could be treated as a uniform emulsion for
simplicity. With the dispersion and mass transfer effects being
averaged out by the rapid mixing process, the reaction ef-
fectively becomes a pseudohomogeneous reaction.
dNO
k2k5k6K[ML]TPO [RH]
1
2
2
-
≈
[ML]TPO (I-4)
2
V dt
(k2 + k7 + k8[RH])k10
Thus, at a low-oxygen pressure and an intermediate catalyst
concentration, the model predicts a first-order rate dependence
each on oxygen and on catalyst, which agrees with the
experimental results.
Conclusion
Assuming the reaction is pseudohomogeneous and applying
the usual steady-state approximations on the reactive intermedi-
ates, the proposed mechanism in Scheme 1 may be shown to
lead to the following model rate expression:
Biphasic autoxidation of tetralin has been carried out using
surface-active tetramethylethylenediamine complexes of man-
ganese, chromium, and nickel as catalysts, tetralin as the
substrate and organic phase, and dodecyl sodium sulfate as
emulsifier. The biphasic reaction was found to offer significant
advantages over the homogeneous and heterogeneous counter-
parts, including avoidance of the use of a troublesome solvent,
ease of catalyst recovery and substrate recycle, and attainment
of high reactivity, selectivity, and reproducibility under mild
reaction conditions. The main reaction products were R-tetra-
lone and R-tetralol. The selectivity for the former decreased
from 95% with the chromium complex to 90% with the nickel
complex and 60% with the manganese complex, and the activity
varied in a reverse order. The biphasic reaction was inhibited
by higher oxidation products that are radical scavengers.
dNO
1
2
-
)
V dt
k2k25[ML(O2)]2
k5 [ML(O2)]
2
k5[ML(O2)]
k6[RH]
(k2 + k7 + k8[RH]) k9 + k10
+ k11
{
(
) }
k6
[RH]
(I)
At an oxygen pressure above 0.15 atm, the majority metal
complex may be assumed to exist mainly in the oxygenated
form, i.e., [ML(O2)] ∼ [ML]T, where [ML]T represents the total
concentration of the transition metal-ligand complex, and eq I
reduces to the following limiting forms for different catalyst
concentration ranges:
The reaction order with respect to oxygen decreased from
1.0 to 0 above an oxygen partial pressure of 0.15 atm. The
reaction order with respect to catalyst decreased from 2.0 or
1.4 to 1.0 and then 0 with increasing metal concentration. A