T. Vidoczy, E. Danoczy, and D. Gal
020
Sequence Studies in Liquid Phase Hydrocarbon Oxidation. II. On the Mechanism of the Alcohol-Ketone Transition in the Oxidation of Ethylbenzene T. Vidoczy, E. Danoczy, and D. Gal* Central Research Institute tor Chemistry of the Hungarian Academy of Sciences, Budapest, Hungary
(Received October 10, 7973)
The initiated oxidation of phenylmethylcarbinol has been investigated between 60 and 110" using 14C alcohol and 1 8 0 in ~ order to establish the mechanism of oxidation. It can be assumed that the initiated oxidation is responsible for the alcohol-ketone transition observed earlier during the study of ethylbenzene oxidation. Our kinetic and isotope distribution data support the view that the role of main chain carriers can be assigned to HO2. radicals, and it is suggested that according to indirect evidence the activation energy of the chain propagation reaction is smaller than that for the ethylbenzene oxidation, in complete agreement with our present results.
Introduction In a previous paper1 we showed that methylphenylcarbinol (ROH) produced during the liquid phase oxidation of ethylbenzene formed acetophenone (R'COR'') and the rates of this transformation have been calculated for increasing degrees of ethylbenzene conversion. However, the kinetic study of the alcohol-ketone transformation provides only limited possibilities for establishing the mechanism of the transition. The oxidation of the ROH formed during the overall reaction could be assumed as a probable reaction route. We now have extended the previous investigation to the oxidation of methylphenylcarbinol in order to determine the kinetic parameters and the mechanism of the reaction. The oxidation of secondary alcohols in nonpolar solvents has been thoroughly studied in the case of cyclohexanol as parent substrate.2-8 In the course of cyclohexanol autoxidation at 110" a compound containing active oxygen and formed after a rather long induction period undergoes decomposition and accelerates the ~ x i d a t i o n . ~(At - ~ .lower ~ temperatures, in order to obtain measurable conversion, initiators have been applied.)2,3,5 Ketone (cyclohexanone) and peroxide-type compounds were identified among the products. Data of Parlant, et ~ l . with , ~ respect to the nature of the latter products are somewhat contradictory because the quantity of the alcohol consumed and that of the ketone formed (including some acidic by-products) are identical within the limits of the experimental error and consequently the peroxidic product is likely to be hydrogen peroxide (peroxides were usually measured iodometrically). The hydroxy hydroperoxide expected to be formed in a situation analogous to hydrocarbon oxidation processes is already unstable above 50" and decomposes into ketone and hydrogen peroxide.6 An examination of the available literature on the oxidation of secondary alcohols above 60" reveals the inconclusive nature of the evidence for answering a basic question: according to certain authors the chain carrier radicals are hydroxyperoxy radicals,2-5 while others attribute this role to the HO2. radicals.6 This latter view seems to be supported by the fact that during the oxidation at higher temperatures no hydroxy hydroperoxides were found. According to the data of Parlant, et ~ l . the , ~ termination rate constants of the chain carrier radicals in the oxidation of alcohols (differing in their chemical structure) The Journaiof Physical Chemistry, Vol. 78, No. 8, 1974
are closely identical, while the same values for hydrocarbon oxidation vary essentially depending on the structure of the parent hydrocarbon.1° Data of Parlant, et al., can be regarded as consistent with the view that the nature of the chain carrier radical does not depend on the structure of the parent alcohol and that they can be considered as common species in the oxidation of alcohols. Howard and Ingold have shown convincinglyll that the chain carriers in the oxidation of 1,4-cyclohexadiene are HOp radicals. EmanuellO suggested the possibility of the existence of an intramolecular hydrogen bridge in the case of hydroxy peroxy radicals
O",
H,."
I
which could yield HO2. radicals and stable molecules (ketone in the present study). The importance of the hydrogen bridge in the oxidation of alcohols was emphasized also by Rust and Youngman.12 Andre and Lemaire6 have found that in the photochemical oxidation of cyclohexanol at 25" both hydroxy peroxy and HOz. radicals take part in the chain-propagating step. In the thermal oxidation (80-90") they did not find spectroscopic evidence for hydroxy hydroperoxides. It was concluded that with increasing temperature the role of the HOz. radicals increased and above 85" it became the exclusive chain carrier. On the other hand, Howard and K ~ r c e k using ,~ an inhibitor method, have shown that in the oxidation of benzyl alcohol a t 30" the role of HOz. radicals was negligible (less than 10%of that expected). The oxidation of methylphenylcarbinol was first studied by Stephens,l3 who separated acetophenone, benzoic acid, and other (presumably dehydrated) products. Parlant, et al.,4 made a thorough investigation of the oxidation and calculated the "oxidizability" at different temperatures and estimated the termination rate constant for cooxidation studies kp(2kt)-l/2 = 1.15 x 104e-10,50a/RT dm3/2 mol-1/2 sec-1/2 (1) where k , is the rate constant for the chain-propagating step; kt, for the termination reaction. They have found that
829
Alcohol-Ketone Transition in the Oxidation of Ethylbenzene TABLE I: Oxidation of Phenylmethylcarbinol in the Presence of W-ROHa [R’COR” ] counts t,
hr
#I-1
[ROHI
mCi dm-3
counts
0.7 1.5 2.5 3.3 4.1 4.4
Total activity exptl counts
rl-1
mCi dm-3
rl-1
mCi dm-3
4815 4385 4250 3888 3700 3597
15.47 14.0 13.6 12.4 11.8 11.5
5040 4862 4030 4912 4995 4973
16.1 15.5 16.1 15.7 15.9 15.9
~~
!l
IEI
.-I
A
M Illli
Figure 1. Crosed system reaction vessel: (A) reaction volume; (B) gas introduction; (C) liquid supply; (D and E) cooling liquid circulation; (F) thermometer; ( G ) magnetic stirrer.
k,
>2x
din3 mol-’ sec-l
(2) According to the data of Howard and Korcekg in the initiated oxidation of methylphenylcarbinol at 30” h,(Zh,)-1/2 = $1 X 10-4 dm3/2 (3) lo7
Using the rotating sector method the termination rate constant they obtained was h , = 2.5 x 106 dm3 mol-’ sec-’ (4) Experimental Section Materials. Commercial chlorobenzene was washed with NaHS03 in order to remove traces of aldehydes and ketones, dried over CaC12, and followed by distillation. The drying-distillation procedure was repeated using a packed glass distillation column (bp 132”). Azobis(izobutyronitrile), AIBN, was recrystallized from chloroform four times (mp 101-101.5”). Based both on literature data5J4 and our measurements, the rate constant of the AIBN decomposition in chlorobenzene was h,AIBS = 2.41 x l@5e-31d00/RT sec-l (5) Azobis(1-phenylethane), APhE, was synthetized according to a modified literature description.16-18 The rate constant of its decomposition was hd*PhE = 1/56 X sec-” (6) at 110”, in good agreement with literature data.15-17Jg,20 The activation energy of the decomposition is approximately independent of the solvent; E = 31.6 kcal mol-l. For the radical yield, literature values were used:14q20 eAIBN 0.60; eAPhE = 0.71. l 8 0 z was obtained by the electrolysis of water containing 19.6 atom 70 l8O. Metallic sodium was added to the water in order to avoid incorporation of oxygen. Analysis. Ketone and alcohol were analyzed by gas chromatography and their activities measured by radio gas chromatography as described ear1ier.l Simultaneous iodometric measurements of “active oxygen” were carried out according to published procedures.11921 The mass spectrometric analyses were made on an AEI-type instrument (Ms-902) with double focusing. Resolution was
0 .oo 1.63 2.23 4.00 5.30 7.00
225 477 770 1024 1295 1376
T = 6 1 O ; rate of oxygen flow, 3.1 dm3 hr-1; [ROH] = 55 X 10-3 mol dm-8; solvent, chlorobenzene; [AIBN] = 10-1 mol dm-3; [1‘C-ROHlt-a = 0.480 mCi/30 cm3 = 16.0 mCi d m - 8 .
about 15,000 and thus the peaks of ROH and R’COR’’ (containing 1 8 0 ) were satisfactorily separable. The I80 content of the R’COR’’ was calculated from the ratio of the molecular peak intensities of the l80ketone to l e 0 ketone. Reaction Conditions. The experimental technique was similar to that described in part 1.l Some experiments, however, were also carried out in a closed system (Figure 1) and the oxygen uptake was followed by gas buret. The volume of the reaction vessel was 100 cm3. Mixing of the liquid and gas has been ensured by magnetic stirrer. Experimental Results It was shown earlier that under certain experimental conditions in the oxidation of ethylbenzene, acetophenone is an end product; and that methylphenylcarbinol as an intermediate yields no products other than acetophenone. The experimental results obtained studying the oxidation of ethylbenzene have been proved valid also for the oxidation of ROH. Using isotopically labeled 14C-ROH as substrate the sums of the radioactivities detected in R’COR’ and in ROH remained constant with increasing conversion and were identical with the radioactivity introduced to the system in the form of 14C-ROH. Experimental conditions and results are given in Table
I. Special runs were carried out to determine the extent of the autoxidation of ROH to obtain information on the role played by H202 in the process. As can be seen from Table II (No. 1)the autoxidation is extremely slow under our experimental conditions. Experiments in an oxygen atmosphere and in the presence of H202 (Table II, No. 2) provided evidence that H202 can be regarded as an initiator while results obtained in an argon atmosphere (Table II, No. 3) support the earlier opinion that products of the decomposition of H202 do not affect the formation of R’COR’’ significantly. That is, OH radicals formed on decomposition of H202 do initiate the oxidation, but their contribution in abstracting the second hydrogen from ROH is negligible. This indicates that the reaction sequence
H C ‘/
‘OH
’
-% ‘C-OH
+
+
>M 4- H202 (7)
and by Bolland and suggested earlier by Brodsky, et Cooper22 plays no significant role under the conditions used in our system. (HOz. radicals are even less active than OH radicals.) Further proof for the initiating role of H202 can be concluded from Figure 2. Though the AIBN initiator is conThe Journal of Physical Chemistry, Vol. 78, No. 8, 1974
T. Vidoczy, E. Danoczy, and D. Gal
830
TABLE 11: Preliminary Experimental Data of the Oxidation of ROH Autoxidation
Oxidation in the presence of HzOz
No. 1
No. 2
[ROH] = 0.51 mol dm-8
T
In the presence of oxygen [ROHl = 7.90 mol dm-3 T = 70‘ V = 30 cma 02 = 3.2 dma hr-1 [HzOz] = 0.129 mol dm-3
= 120”
V = 30cm8 OZ = 1.6 dm8 hr-1 Solvent chlorobenzene
[R’COR”], mol dm-3 X 108
t, hr
0 .o 3 .O 6.0 7.9 10 .o 13.1 14.6
t , hr
2 . llQ 2.42 2.63 2.79 2.79 2.96 3.13
In the presence of argon [ROH] = 7.90 mol dm-* T = 70’ V = 30 cma Ar = 2 dms hr-1 [HzOz] = 0.115 mol dm -8
[R’COR”], moldm-3 X 103
[HzOzl, moldm-a X 103
29.40 28.5 29.4 32.6 31 $ 8 32.6 36.4 40.4 42 . O 45.6
129 129 129 129 134 129 131 120 121 118
0 1 2 3 4 5 6 7 8
9 a
No. 3
t, hr
[R’COR”], moldm-3 X l o 3
[HzOzl, moldm-3 X 108
2 8 . 6a
115 106 101 91 81 72 60
0 1 2 3 4 5 7 9 10
28.6 28.6 28.6 30.2 30.2 30.2
50
The alcohol contained acetophenone initially.
Concenfration mole dm-3x IO
lancer?frofion mole drne3x10 X
GO x
55 -
O
Figure 2. Accumulation of [ROH] = 0.825 mol d m - 3 ;
= 70”.
X
R‘COR” HZ02
acetophenone and [AIBN] = 49 X
vs. time: mol d m - 3 ; T
H202
sumed continuously, the amount of R’COR” formed increases linearly, that is, a certain product (likely to be H202) formed during the reaction is capable of initiating the reaction. (We did not detect other products yielding radical species.) Since the initiating effect of the H202 is remarkable in the case of initiated reactions, the ratio of the rate constants characterizing the oxidizability of ROH can reasonThe Journal of Physical Chemistry, Vol. 78, No. 8, 1974
Figure 3. Accumulation of acetophenone and H202 vs. time (initial stage of the reaction): [ROH] = 0.232 mol d m - 3 ; [APhE] =
9.21 X
mol d m - 3 ; T = 110’.
ably be determined from measurements carried out at the initial stages of the process where the (constant) initiation rate is well established and secondary initiation stemming from the decomposition of H202 can be neglected and thus the Bodenstein principle can be applied.24 In this stage five samples were normally required to
Alcohol-Ketone Transition in the Oxidation.of Ethylbenzene
831
TABLE IV: Rates of Oxygen Absorption
TABLE 111: Experimental Data of t h e Oxidation of ROH [ROH], mol dm-3 (at the exptl temperature)
[Initiator] X 103, mol dm-a (at the exptl temperature)
0.23 0.23 0.24 0.26 0.30 0,.92 3.68 7.6q 7.60 7.60 7.60 0.23 7.68 7.68 7.68 7.68 7.84 7.84 7.84 7.84 7.84 0.11 0.22 0.95 3.82 7.88
9.21 9.85 4.60 4.60 4.60 4.59 4.59 4.57 4.57 4.57 4.57 4.65 4.62 9.24 18.48 27.72 4.72 9.43 18.86 28.29 37.72 9.54 9.54 9.54 962 9.50
T, ‘C
wi X 108, mol dm-3 sec-1
dm-3sec-1
110 110 110 110 110 110 110 110 110 110 110 100 100 100 100 100 80 80 80 80 80 73 73 73 73 73
2.48 2.63 1.24 1.24 1.24 1.24 1.03 0.92 0.92 0.92 0.92 0.30 0.30 0.58 1.16 1.74 0.73 1.46 2.93 4.21 5.85 0.60 0.60 0.60 0.60 0.60
3.5 4.2 2.6 2.7 2 .o 8.8 25.6 29.2 33.8 42.5 43.7 0.9 20.5 14.2 30.8 34.1 14.6 35.9 54 . O 59.6 69.7 0.67 1.18 2.72 7.90 12.65
-
rO,,
1-0+ ,. ROH -----t X),H ROH
+ O2
+ ROH
HOROO
4.85 4.34 4.67 4.51
Series II* 1.65 1.67 1.65 1.51
-
[ROH] = 7.84 mol dm-3; [AIBN] 23.5 X 10-8 mol dm-a; T = 79.3OC. [ROH] = 7.97 mol dm-3; [AIBN] = 0.161 X mol dm-3; T = 59.6‘.
Reactions 8-11 are identical in both mechanisms and r. is an alkyl radical formed upon decomposition of the initiator. For mechanism A then HOROO. ROH HOROOH ROH (12‘4) HOROOH ---t R’iCOIR” H202 (13‘4) (14‘4)’‘ 2HOR00. + 2R’COR” + HzOz 02
-
+
+
+
+
but for mechanism B HOROO. R’COR” + HO,* (13B) HOz* + R@H H202 + ROH (12W 2H0g --t H202 0 (14~) Applying the steady-state treatment the two mechanisms give identical differential equations
-+
Discussion Based on literature data two possible mechanisms can be assumed which correspond to the initial stages of the oxidation (above 300 TOR oxygen pressure, alkyl radicals react very fast with oxygen to yield peroxy radical@). r*+ 0,
1 2 3 4 1 2 3 4
provide sufficient accuracy and a least-squares analysis was carried out on these experimental points. As it can be seen from Figure 3 the “active oxygen” content of the mixture increased linearly during the reaction time studied, that is, the decomposition of H202 is negligible as expected. The rates of formation of R’COR’’ are given in Table III. (Concentrations are given at temperatures of the experiment .) The rate of oxygen consumption was also measured under the conditions given in Table IV. Since experiments carried out in the presence of oxygen-18 provide an alternative means to obtain evidence with respect to the mechanism of the oxidation we have oxidized methylphenylcarbinol (0.1 mol dm- 3, initiated by AIBN (0.1 mol dm-3) in chlorobenzene at 80” in an oxygen atmosphere containing 19.6% Is0. After 1.5 hr half of the ROH was converted into R’COR’’. (Since during this time about Y3 of the AIBN is decomposed the kinetic chain length becomes very small; its average value was 1.2.) The mixture was repeatedly distilled followed by mass spectrometric analysis. As a result, the R‘COR’’ contained only 1.5% l 8 0 .
+ N* +
