March 5 , 1956
THERMAL AND CATALYZED [CONTRIBUTION FROM
THE
OXIDATION OF I N D E N E :
1041
KINETICS
GENERAL ELECTRIC RESEARCH LABORATORY]
Oxidation of Unsaturated Compounds. IV. Kinetics of the Reaction of Indene with Oxygen; Evidence of the Identity of Active Intermediates in Thermal and Catalyzed Oxidations BY GLENA. RUSSELL RECEIVED AUGUST9, 1955 The reaction of indene and oxygen, which yields a low molecular weight polymeric peroxide, has been investigated a t 50" in the presence and absence of a,a'-azodiisobutyronitrile,a t atmospheric and reduced pressures, and in the presence and absence of inert diluents. The observed kinetics demonstrate that the reaction in the absence of free radical initiators involves the same active intermediates, alkyl and peroxy radicals, present in the initiated reaction. These kinetics are consistent with a thermal initiation reaction between indene and oxygen molecules t o produce two free radicals or a diradical. The thermal initiation reaction that occurs during the oxidation of other olefins is discussed.
were thus investigated in the oxidation of indene a t 50'. Results and Discussion -d[S]/dt = k[S]'.6[02]o.5 Rate of Oxidation of Indene.-Indene, when while in the presence of 0.001-0.1M a,d-azodiiso- shaken in an oxygen atmosphere in the presence or butyronitrile (AIBN) the kinetic equation fol- absence of AIBN a t SO", absorbs oxygen a t a constant rate t h a t is independent of the amount of lowed is' oxygen absorbed up to a t least one-fourth mole per -d[S]/dt = k'lS] [AIBX]',' These observations suggest an initiation reaction cent. oxidation for the thermal reaction and two mole per cent. oxidation for the reaction performed between styrene and oxygen molecules producing in the presence of 0.01MAIBN. The product of the two free radicals and that the complete rate exreactions is essentially a 1: 1 copolymer of indene pression is and ~ x y g e n . I~n Table I are summarized the rates -d[S]/dt = k"[S]Rio,5= of oxidation observed a t various indene and AIBN k"[S](k,[AIBN] ki'[On] [S])o.6 (1) concentrations and a t various oxygen pressures. where Ri is the rate of initiation, and ki and ki' are Other experiments demonstrated that an increase the rate constants for the initiation reactions. The or a decrease in the amount of agitation did not fact that a linear relationship between the rate of affect the rate of oxidation observed. Thus, the oxidation and [AIBNIoJ was observed' may mean rates of oxidation summarized in Table I are not complicated by diffusion controlled processes. that (1) is correct and that ki[AIBN] >>ki'[02] [SI, or alternatively, that (1) is incorrect and the TABLE I complete kinetics are OXIDATION OF INDENE AT 50" -d[S]/dt = k'[S][AIBN]".6 k [ S ] ' J [ 0 2 ] 0 . 6 (2) Oxygen Rate/ Ratel [Inpress.. [RH]l.j[RH]Calcd. Equation 1 is easily derived if both the thermal dene]' [.4IBNIa m m . b Ratec [ 0 ~ ] 0 . 6 d [AIBN]O.ja ratect/ and catalyzed oxidation reactions involve the same 8.35 0 750 0.0342 5.17 . . . . 0.035 active intermediates (see Discussion). On the 750 ,0339 5 . 1 3 .., . ,035 8.35 0 other hand, i t has been suggested that for oxidations 618 ,0301 5 . 0 5 .... ,032 8.35 0 where rate expressions similar to ( 2 ) are observed 8.35 0 587 ,0296 5 . 0 5 .,., ,031 thermal and catalyzed oxidations do not involve 8.35 0 486 ,0277 5 . 1 8 ., .. ,028 common intermediates.2a 426 ,0261 5 . 2 2 ... ,027 8.35 0 Mulcahy and Watt have concluded recently that 750 '0229 4.84 , . , , ,025 6.6ga 0 the oxidation of benzaldehyde in the presence of 4.18' 0 750 ,0115 4.90 . , . ,012 benzoyl peroxide, a t constant benzaldehyde and 8 . 3 5 0.0005 750 ,0398 .. 0,0203 ,039 oxygen concentrations, obeys the rate expression .. ,0126 ,052 8.35 ,0025 750 ,0500 8.35 .0050 750 ,0608 ~. .0103 ,062 -d[BzH]/dt = Ra k[Bzr02I0.' 8.35 ,0100 750 ,0809 , , ,0970 .080 where ROis the thermal rate of oxidation, and that 8.35 ,0150 750 ,0950 , , ,0928 ,095 benzoyl and perbenzoate radicals are not involved , , ,0915 ,058 8.35 .0050 443 .0540 in the thermal oxidation.2 Since it was suggested 4.18' ,0100 750 ,0341 .. ,0790 ,038 that other thermal oxidations might proceed by a Moles l i t e r 1 . Corrected for the vapor pressure of mechanism not involving, alkyl or peroxy radicals,2a indene. Moles liter-' hr.-l. Litefl.5 moles-0.6 mm.-0.5 the copolymerization of oxygen with an olefin was hr.-l. * LiteW m ~ l e s - ~hr.-l. J Calculated from equainvestigated under conditions where contributions tion 3 . 0 Bromobenzene diluent . to the total rate of oxidation by the catalyzed and From Table I, column 5 , it is seen that in the thermal reactions were about equal. The effect of absence of a catalyst the rate law obeyed is similar oxygen pressure, AIBN and indene concentrations to the rate law observed in the thermal oxidation of (1) A . A. Miller and F. R. Mayo, THISJOURNAL, 78, 1017 (1956). styrene.l The data indicate that the rate is pro(2) (a) M. F. R Mulcahr and I. C. Watt, PYOC. Roy. SOC.( L o n d o n ) , portional to [RH]1.6[O~]o-46. However, in the pres216A. 10, 30 (1951); J . Chcm. Soc., 2971 (1954): (b) see however, ence of 0.0005-0.015M AIBN the rate of oxidation T. A. Ingles and H. W. Melville, Proc. Roy. SOC. ( L o n d o n ) , 218A, 175 The copolymerization of styrene (S) and oxygen in the absence of a catalyst follows closely the rate expression
+
+
,
+
f
(1953).
(3) G. A. Russell, THISJOURNAL, 7 8 , 1035 (1956).
GLENA. RUSSELL
1042
of indene is not proportional to [RH][AIBNIOJ (column 6). That (1) correctly summarizes the kinetics of the oxidation of indene and that (2) is incorrect was demonstrated by plotting rate and (rate)2as functions of [AIBN]0.5and [AIBN] for the oxidation of indene (8.35 M ) a t 750 mm. oxygen pressure. A definite curvature occurs in the plot of rate versus [AIBiY]OJas would be expected from (1) but not from ( 2 ) . Moreover, a plot of (rate)2 versus [AIB N ] gave a straight line in agreement with (1) and not with (2). The kinetics demanded by (1) are consistent with the following chain reaction wherein e represents the efficiency of the radicals from the catalyst (R.') in initiating oxidation chains containing alkyl or alkenyl radicals (R.) and peroxy radicals (ROO.). 4 AIDS
k,
+ 2K.'
R H or
+ RH
___)
0 2
2eR
k,' ---+
0:f -CH2CHzCH2--
-CH2CHCH200 (or HOO R.
+ 0%
very fast
+ R.)
Vol. 78
ured rates of oxidation and the rate of oxidation calculated from (3) with the above constants. At lower oxygen pressures the rate expression is probably more complex than (3) because termination can involve interactions of alkyl radicals and peroxy r a d i ~ a l s . ~ Since the ratio of k3:k3' is a t least 4 : 1,3 k3ks-O.j is not less than 5.1 and k3'k6-0'5 is not more than 1.3 1.0.5 m ~ l e - ~hr.-0.5 .~ a t 50". The values of k3k6-O" determined in the oxidation of styrene, 1,l-diphenylethylene and methyl methacrylate a t SOo, are 5.4, 2.3, and 0.5 m01e-O.~ hr.-0.5, respectively.' Comparable values of k3'kg-0.j have been determined a t 45" in the presence of benzoyl peroxide for numerous olefins that are oxidized to hydroperoxides.8 These results indicate that the value of K 3 ' K 6 - 0 . 5 for allylbenzene is only 0.06 1.O.jmole-0.5 hr. -O.j. Thus, cyclization of allylbenzene into indene, a molecule which can give an aralkenyl radical having two equivalent resonance structures, L o a 5
eq
\2
ROO.
+-+
OA~ \ -
may increase the reactivity of the a-methylene group considerably. IL___, k,' It is interesting to compare the rate of polyROOH -CHCH=CH.merization of indene with the rate of copolymerization of indene and oxygen. Gutmann has k6 2 R 0 0 . --f non-radical products pointed out that in the absence of an added catalyst At steady state conditions the rate of initiation the latter reaction is about as fast a t 40" as the thermal polymerization a t 1 5 0 O . 9 Even in the presence (Ri) of a free radical initiator the rate of indene disapR ; = 2eki[AIBS] 2ki'[02] [RH] is equal to the rate of removal of radicals by ter- pearance is much greater for the copolymerization with oxygen than for polymerization. Thus, a t 50" mination (Rt) in the presence of 0.01 M AIBN and oxygen a t 750 Rt 2ke[R00.I2 rnm., indene is consumed a t a rate of 0.4% hr.-l Rate expression 3 is obtained by solving these ex- while it can be calculated from the data of Breitenpressions for [ROO.] and substituting the result bach and Schindler that the rate of polymerization into the expression for the rate of disappearance of of indene containing 0.016 M AIBN is about 0.06Yc indene hr.-I a t 50°.10 These results point out the error of the generality that oxygen inhibits vinyl polymeri-d[RH],ldt = (k, k3')ke-0.6[RH](eki[AIB~] zation since in this particular case oxygen does not ki'[Onl [RH] lo.' (3) From the slope and intercept of the straight line act as an inhibitor, but as an active copolymerizaobtained by plotting (rate)2 as a function of [A41- tion agent, and the rate of indene consumption is B N ] a t constant hydrocarbon concentration and greater in the presence than in the absence of oxyoxygen pressure, eki(k3 k3')?k6-' is calculated to gen. For some other vinyl monomers (e.g., methyl 1. mole-l hr.-2 and ki'(ky / ~ 3 ' ) ~ -methacrylate) the reverse effect has been noted.7 be 7.54 X Mechanism of the Thermal Initiation Reaction.k6-I to be 2.S6 X 1. mole-' hr.? mm.-l. The The product and mechanism of the initial reaction rate constant for the production of free radicals of oxygen with non-polymerizable olefins, where the from AIBK, 2ki, is known to be 1.9 X hr.-I a t 30' in benzene solution.jS6 Provided that the major oxidation product is a hydroperoxide, has rate of initiation is equivalent to the rate of produc- been widely discussed." Three distinct reactions ( 7 ) A . A. Miller, F. R. Mayo and G. A. Russell, unpublished results. tion of free radicals from AIBIC' ( e = I ) , it can be (8) J . L. Bolland, T r a n s . F Q V Q ~ Stc.. U ~46, 358 (1WO). calculated that (k3 k3')/k6"'' is 8.90 mole-O.j ( 0 ) 1'. G u t m a n n , J . Polymer Sci.. 3, 336 (1948). mm. hr. -I. hr. --Oiand ki' is 3.60 X (IO) J. W. Breitenbach and A. Schindler, M o n a f s h . , 33, 271 (1952). (11) (a) E. H Farmer, G . F . Bloomfield, A. Sundralingam and D . A . X s shown in Table I, columns 4 and 7, good agreeSutton, Trans. Faraday S O C .38, , 348 (1942); E . H. Farmer, ibid., 42, ment was found between the experimentally nieas- 228 (1946); (b) J. I,. Bolland and G. Gee, i b i d . , 42, 230, 244 (1040); ROO.
+ -CH~CHECH~-
k3
-,----+ -CH2CHCH,OOR
+
+
+
+
+
+
+
(.I) Aiialogoiis reaction sequciicei have been suggested for the oxidat i m of olefins (J. L. Bolland, O u a v f . Res.?. (London). 3, I (1949); I . . Rateman, i b i d , 8 , 147 ( I S , j l ) ) a n d aralkyl hydrocarbons (G. A . Russell, THISJ O U R N A L , 77, 4583 (1955)) a t "high" oxygen pressures (.i) C. E. H. Bawn and S. F. A'Sellish, T r a n s . Favaday .S'OC. , 41, 1216 ( I 93 I ) . ( 0 ) The rate of production of iree radicals from A I B N is not n e c e s ~ sarily twice t h e r a t e of decomposition of A I B N (G S. H a m m < C d . J S. Sen and C. E. Boozer, THISJ O U R N A L . 77, 3241 (19.55)).
(c) F . D. Gunstone and T. P. Hilditch, J. C h e m . S o c . , 1022 (1046); T. P. Hilditch, Natur.c, 166, 558 (1950); (d) K.A . K h a n , Cali. J . Chertz., 32, I149 (1954); J. Chem. P h y s . , 22, 2090 (l!A54): (e) R. S. Mortell a n d S. Marks, J . Oil and Cofour Chemists' Assoc.. 12, 183 ( 1 9 2 9 ) ; T. P. €Elditch, i b i d . , 30, 1 (1947); R . S. Morrell and S. Marks. J . .S Chcm. I n d . , 6 0 , 27T (1931); 66, 237T (1936); T. R. Bolam and W. Sirn, ibid. 6 0 , 50T (1941); (f) L. Bateman, H. Hrighes and A . J,. Morris, Disc.Faraday Soc.. 14, 190 (ln53): I,. Bateman, Quart. Revs (London), 8, 147 (1954); J . Chem. Phys.. 22, 2090 (1954).
THERMAL AND CATALYZED OXIDATION OF INDENE : KINETICS
March 5 , 1956
have been proposed. Farmerlla and Bollandllb have suggested the direct reaction of oxygen with the olefin t o yield a diradical or two radicals
+
RCH,CH=CHR 0 2 + RCH2CHCHRO0. (or HOO.
+
RCHCH=CHR)
Farmer also considered the possibility of direct formation of hydroperoxide in a one step reaction11a and HilditchlIc and Khan11d have supported this suggestion.
+
RCHzCH=CHR 0 2 + [RCH2CHCHR00.]
+ RCH=CHCHROOH
Another proposal has been that the olefin and oxygen react to form a cyclic peroxide.llcee H
H
1
1
-CH2-C=C-
H
+
0
I 2 +-CH2-C-CI
H
0-0
I I
1043
action of indene with oxygen to produce free radicals directly. Presumably a similar initiation reaction occurs in the thermal oxidation of non-polymerizable olefins such as methyl linoleate or linolenate. In fact, reported kinetic data support this conclusion. The oxidation of methyl linolenate a t 50' apparently follows (1) and not (2). The most careful workI3indicates that a plot of rate versus [AIBN]'.' involves a slight curvature, although rate measurements have not been reported a t very low AIBK concentrations where this effect would be more easily observed. Similarly, the benzoyl peroxidecatalyzed oxidation of methyl oleate, squalene and ethyl linoleate apparently follow (1) and not (2). Here again, the thermal rate of oxidation cannot be ignored and linear relationships are obtained when (ratel2is plotted as a function of [ B z ~ O ~ ] . ~ ~ "
Experimental Purification of Indene.-Barrett technical grade indene (1 kg.) was shaken with 6 N hydrochloric acid for 24 hours
three Of these reactions the autocatalysis observed in the oxidation of olefins since (to remove basic nitrogenous material) and refluxed with presumably all of these peroxidic products can dis- 40% sodium hydroxide for two hours (to remove benzosociate into free radicals. nitrile) . l 4 Rectification of the material thus treated through a Podbielniak column (
