INITIATION OF PHOTOOXIDATION BY CHARGE-TRANSFER EXCITATION
431.7
On the Possible Initiation of Photooxidation by Charge-Transfer Excitation
by J. C. W. Chien Hercules Research Center, Wilmington, Delaware
19899
(Received July 19, 1966)
The photooxidations of methylcyclohexane, cyclooctene, octene-1, benzene, toluene, cumene, and chlorobenzene have been investigated under conditions which minimized initiation by the photolysis of “impurities.” The dependences of the initial rate of oxidation upon the oxygen pressure, the substrate concentration, and the incident light intensity were obtained for some of the systems. The results are interpreted assuming two primary processes, the charge-transfer excitation and the oxygen-perturbed S -+ T excitation. The relative significance of these two processes in photooxidation is discussed. The over-all quantum yields o€ initiation are given.
Introduction The primary processes initiated by allowed transitions in the photooxidation of aromatic hydrocarbons have been reviewed112; the photooxidation of aliphatic hydrocarbons has not received much attention. The primary concern of this communication is the mechanism of initiation of photoxidation in the wave length regions where there are no allowed transition! : >2000 8. for aliphatic hydrocarbons and >3100 A. for aromatic and olefinic hydrocarbons. There are several possible primary processes. The first possibility (I) is the photolysis of “impurities” such as hydroperoxides
ROOH $. hv -+ RO* f OH*
(1)
According t o this mechanism, the induction period should increase and the rate of oxidationa should decrease with increasing purity of the substrate. At , ~ rate is independthe limit of high oxygen p r e ~ s u r ethe ent of oxygen pressure. The second possible process (11) is the excitation of oxygen via the Herzberg forbidden transition to the %+ state. This sta,te could react with the substrate by either a direct reaction or an energy-transfer proce m 5 Furthermore, it could cross over to the reactive IZg+or lAg state. The rate of oxidation should iiicrease with oxygen pressure. The reactions of O2 (lAg) are strongly dependent upon the nature of the substrate.6 From consideration of the occupational number of the antibonding orbitals, O2 (12,+) could be a more electrophilic and selective species than O2 (IAa,),
A third possible process (111)is the oxygen-perturbed S --t T transition which is known in olefinic and aromatic hydrocarbons.’ For aromaOic hydrocarbons, the triplet state is presumed to interact with oxygen to give directly the product peroxide2; it is not very reactive in hydrogen abstraction reactions.8-11 There is no simple relationship between the S -+ T absorption intensity and the oxygen concentration. Little or nothing is known about the triplet state energy and the S T transition for aliphatic hydrocarbons. Ultraviolet absorptions have been reported12 for -+
(1) R. M. Hochstrasser and G. B. Porter, Quart. Rev. (London), 14, 146 (1960). (2) W. A. Noyes, Jr., G. 5.Hammond, and J. N. Pitts, Jr., “Advances in Photochemistry,” Vol. 1, Interscience Publishers, Inc., New York, N. Y., 1963, p. 23. (3) .Unless otherwise stated, the rate of oxidation refers to the initial rate (aide infra). (4) L. Bateman, Quart. Rev. (London), 8 , 147 (1954); L. Bateman and A. L. Morris, Trans. Faraday SOC.,49, 1026 (1953). (5) Analogous to the mercury photosensitization. See ref. 2, p. 209. (6) E. J. Corey and W. C. Taylor, J . Am. Chem. SOC.,86, 3881 (1964); C. S. Foote, private communication. (7) D. F. Evans, J . Chem. Soc., 1735 (1960). (8) The ground triplet state of naphthalene does not react with alcohol or ether solvent but the excited triplet state does.9 The TT* triplet state of 1-naphthaldehyde is found t o be unreactive in photoreductions.10 The lower T K * triplet state of acridine is also said to be unreactive.” (9) S. Siege1 and K. Eisenthal, J . Chem. Phys., 42, 2494 (1965). (10) G. S. Hammond and P. A. Leermakers, J . Am. Chem. Soc., 84, 207 (1962). (11) A. Kellmann and J. T. Dubois, J . Chem. Phys., 42, 2518 (1965). (12) A. V. Munck and J. R. Scott, Nature, 177, 587 (1956).
Volume 69, Number 1.8 December 1966
4318
cyclohexane and for ethanol (both saturated with oxygen). Similar absorptions were also reported for aromatic compounds13 and for nitrogen-containing conip0unds.1~ These absorptions have been identified1* as contact charge-transfer absorptions. These excited 3CT states (process IV) could initiate photooxidation by several possible reactions (vide infra). The rate of oxidation may be expected to increase with both the concentrations of oxygen and of the substrate. The purpose of this communication is to examine the possible roles of these processes in the initiation of photooxidations of hydrocarbons. Experimental Section The alkanes used in this work are Phillips research grade chemicals. These are cyclohexane, methylcyclohexane, n-heptane, 2,3-dimethylpentane, 2,2-dimethylbutane, 2-methy1pentanel 2,4-dimethylpentane1 and 2,2,4-trimethylpentane (isooctane). I n various experiments, these alkanes were purified by any one of the following methods. I n method A, the compound was passed through a 0.6-m. column of freshly activaked alumina16g16immediately before use. I n method R, the compound was fractionally distilled in a 100plate Todd column. The middle third (b.p. range +0.05”) was used. Treatment with concentrated sulfuric acid was involved in method C, followed by drying, fractionation, and treatment with alumina. I n method D, the compound was refluxed over copper powder under a nitrogen atmosphere and then flashdistilled at reduced pressure. The following compounds were fractionated at reduced pressure and stored in darkness at 5’: diethyl ketone, di-t-butyl peroxide, cumene, octene-1, cyclooctene, and chlorobenzene. The procedure used to measure the ultraviolet absorption intensities of compounds saturated with oxygen was essentially identical with that described by Tsubomura and M~1liken.l~The dependence of absorption intensity upon oxygen concentration was determined at 1 atm. of oxygen and at 1 atm. of air; the applicability of Henry’s law was assumed. The dependence upon donor concentration was measured using suitable solvents which in the presence of oxygen do not absorb in the w?ve length region of interest ( i e . , CHzClz a t 22150 A.; heptane and isooctane at 22600 8.). To avoid concentration changes in these experiments, the oxygen was bubbled in at reduced temperature. Two light sources were usedo: a Hanovia S-100 lamp for the monochromatic 2537-A. radiation17 and a GE AH-6 lamp with a NiSO4-CoS04-Corning 0160 filter The Journal of Phgaical Chemistry
J. C. W. CHIEN
system for the 3130-A. radiation (the band width a t half-intensity is about 70 i.). The output of the S-100 lamp was unchanged after 300 hr. of operation. The output of the AH-6 lamp decreased by as much as a factor of 2 in some experiments; the arithmetic mean of intensity measurements1* made before and after an experiment was taken to be the average intensity. Photooxidation was carried out in a cylindrical cell, 10 cm. in length and 3 cni. in diameter, which was thermostated (~t0.25”)in a water bath constructed of optical grade quartz. All irradiations of saturated hydrocarbons, octene-1, benzene, toluene, cumene, and chlorobenzene, were on neat compounds. For cyclooctene, both the neat compound (7.62 M ) and solutions in isooctane were irradiated. Photooxidation of the solvent in this system is negligible.19v20 I n some experiments, the rate of oxidation was measured volumetrically by a gas buret operated automatically at constant pressure. The position of the mercuryleveling bulb was continuously recorded. The oxidation curve is characterized by: (1) an initial period of relatively slow and constant oxygen consumption, referred to as the initial rate; (2) an increase in the rate beginning about one-third to one-half way through the induction period; and (3) a period of “maximum rate.’121 Unless otherwise specified, the oxygen pressure was 1atm. I n other expcrinients, the substrate was distilled on a greaseless vacuum line through a column of alumina into the reaction cell. One atmosphere of an oxygen 4.6% 0236, and 6.5% Ar) was inmixture (81% 0232, troduced with stirring. Aliquots of gaseous samples (0.67 ml.) were taken during the photochemical oxidation for mass spectrometric analysis. No isotopic (13) D. F. Evans, J. Chem. SOC.,345 (1953). (14) H. Tsubomura and R. S. Mulliken, J . Am. Chem. SOC.,82,5966 (1960). (15) It was reported16 and substantiated in our laboratory that alumina quantitatively removes hydroperoxides. (16) K. U. Ingold and J. R. Morton, J. Am. Chem. SOC.,86, 3400 (1964). (17) The S-100 lamp also emits about 10% of 1850-A. radiation which was completely absorbed by the quartz cell and the quartz bath. (18) The intensity of the incident light was measured by uranyl oxalate actinometry: W. A. Noyes, Jr., and P. A. Leighton, “The Photochemistry of Gases,” Reinhold Publishing Corp., New York, N. Y., 1941, p. 78. (19) The charge-transfer absorption of oyclooctene-On at 3130 1. is about 600 times more intense than the corresponding absorption of isooctane and the allylic hydrogens of cyclooctene are much more active than the tertiary hydrogen of isooctane.28 (20) A. L. Williams, E. A. Oberright, and J. W. Brooks, J. Am. Chem. SOC.,78, 1190 (1954). (21) A. V. Tobolsky, D. J. Metz, and R. €3. Mesrobian, ibid., 72, 1942 (1950).
INITIATION OF PHOTOOXIDATION BY CHARGE-TRANSFER EXCITATION
mixing was detected when the gaseous oxygen mixture was irradiated by 3130-A. light for 15 hr. in the absence of a substrate. Only the compositions of the oxygen isotopes in an irradiated solution are reported here. A description of other products of oxidation will be given elsewhere. The ratios of rate constants, k2/k3'/', where kz and k3 are the rate constants of propagation and termination, respectively, are needed in calculations of quantum yields. Values of this ratio in the autoxidation of methylcyclohexane a t 62.5 and 72.5" were determined by the method of Hammond, et aLz2 The rate constant of decomposition (lcl) of azobis(isobutyronitrile) (AIBN) in methylcyclohexane was found to be 1.4 X and 5.2 X sec.-l a t 62.5 and 72.5", respectively. The efficiency of radical production ( a ) , was found to be 0.61 at 62.5" and 0.73 at 72.5". For coinparisonlzZvalues of 1.4 X sec.-l and 0.65 for kl and a, respectively, have been reported a t 62.5' in chlorobenzene. The values of kz/k3'/', calculated from the observed rates of oxidation by
(1 - a)[AIBN]kl are 3.95 X and 8.3 X lo-* l.'/' mole-'/' set.-'/' at 62.5 and 72.5') respectively. The rate is independent of oxygen pressure from 15 to 76 em. The values of kz/k3'/2for cumene oxidation, interpolated from the work of Melville and are 1.48 X 2.1 X and 2.94 X l.'/'mole- I' see. "'at 30,45, and 62.5", respectively. For octene-1, l.'/' mole-'/z this ratio was givenz4to be 5.47 X sec.'/'at 250.
ResuIts and Discussilon Minimum E$ectiz,e Concentration of "Impurity" in Photooxidation. In the preliminary experimen ts reported in this section, the measured induction period is used to estimate the effect of "impurity" and of purification. Unpurified alkane undergoes photooxidation after a relatively short induction period. Purified alkane photooxidizes after a longer induction period which does not increase with further purification. This limiting induction period appears to depend upon light intensity, wave length, and temperature. Table I summarizes some of the results, each entry representing the average of duplicate experiments. Results similar to those given in Table I were also obtained in the photooxidations of ethanol, 1-butanol, isopentyl alcohol, and tetrahydrofuran. When suffi-
4319
~~
~~~~
Table I: Effect of Purification on Photooxidation of Alkanes Io
x
108, einsteins
Compound
Methylcy clohexane Methylcyclohexane Methylcyclohexane Methylcyclohexane Methylcyclohexane Methylcyclohexane Isooctane Isooctane Isooctane Isooctane Isooctane Isooctane 2,4Dimethylpentane 2,4Dimethylpentane a
Methoda of puri- crn.-2 iioation aec.-1
A A A B C D
A A B D A C B A
0.46 0.46 2.52 2.33 3.07 2.33 0.46 0.46 0.46 1.05 2.32 2.32 2.60 2.53
tind
A,
Temp.,
A.
OC.
2537 2537 3130 3130 3130 3130 2537 2537 2537 2537 3130 3130 3130 3130
62.5 62.5 62.5 62.5 62.5 72.5 62.5 62.5 62.5 62.5 72.5 72.5 72.5 72.5
x
10-4, 880.
1.62 1.55 5.90 5.50 6.70 4.90 6.95 5.36 6.36 4.60 15.7 16.7 5.58 5.30
See Experimental Section.
ciently pure compounds are used, it appears to be meaningful to speak of a limiting induction period for specified experimental conditions. The minimum concentration of "impurities" necessary to show an effect in photooxidations was determined. Because in most photooxidations the oxidation products (peroxides and carbonyl compounds) can be photolyzed to initiate further oxidationsza (secondary initiations), the initial rates of oxidation were measured before the concentrations of oxidation products exceeded their respective minimum effective concentrations. The "impurities" chosen in this study are di-t-butyl peroxide (DTBP) and diethyl ketone (DEK). The photochemistry of both of these compounds has been studied extensively. When either of these compounds is introduced, the induction period is greatly reduced, depending upon the concentration of the added ((inipurity" (Table 11). Under these experimental conditions, the minimum effective concentration of DTBP in the photooxidation of cyclooctene is about 10-3 mole l.-l; it is about lov4mole 1.-l in the photooxida(22) G. 5. Hammond, J. N. Sen, and C. E. Boozer, J . Am. Chem. SOC.,77, 3244 (1955). (23) H. W. Melville and S. Richards, S. Chem. SOC.,944 (1954). (24) L. Bateman and G. Gee, Proc. Roy. SOC.(London), A195, 391 (1948). (25) For example, G. R. McMillan, J. Am. Chem. Soc., 84, 2514 (1982); R. D. Doepker and G. J . Mains, J . Phys. Chem., 66, 890 (1982).
Volume 6Q,Number 18 December 1966
J. C. W. CHIEN
4320
Table 11: Minimum Effective “Impurity” Concentration in Photooxidations
3.0
X 10-4, see. photooxidazion ofIsoCyclooctane“ octeneb tind
r
“Impurity”-
--in
-
Compound
Concn., mole 1.-1
DTBP‘ DTBP DTBP DTBP DTBP DTBP
10-1 10-2 10-3 10-4 10-5 10-6
0.13 2.52 5.54 7.55 11.9 10.85
DEK~
10-1 10-2 10-3 10-4 10-6
0.42
DEK DEK DEK DEK
0.26 0.46 0.76 1.25 1.33 1.28 TIME, MINUTES
5.70
Figure 1. Photooxidation of cyclooctene: I, = 1.61 X 10-6 einstein 1.-l see.+, temperature 25”, pol = 1 atm., path length, 10 cm.
6.50 12.0 11.2
s.;
Temperature 72.5”; A 3130 10 N 3 X 10-8 einstein sec.-1. * Temperature 25”; X 3130 10 1.6 X 10-8 einstein sec.-1. Di-t-butyl peroxide. Diethyl ketone. a
B.;
N
tion of isooctane. The minimum effective concentration of DEK in the photooxidation of isooctane is about mole l.-I. Since €3130 for DTBP is 0.93 and €3130 for DEK is 2.07, DTBP is about 20 times as active as DEK. This is consistent with the observedz6 low quantum yield of dissociation of DEK (-10-2) in perfluorodimethylcyclobutane solution. If the same amount of “impurity” is needed to have an effect on both the rate of oxidation and the induction period, the measurement of initial photooxidation rate is meaningful if the quantity of oxygen consumed is less than the minimum effective peroxide concentration. I n 50 ml. of cyclooctene, absorption of 1 ml. of oxygen (STP) corresponds to about mole of oxygen reacted. Experimentally, the rate of oxidation remained relatively constant up to about 0.5 ml. of oxygen reacted (Figure 1). This rate is reported in the following sections as the initial rate in 3130-A. photooxidatjons of cyclooctene, octene-1, and cumene, and 2537-A. pkotooxidation of methylcyclohexane. For the 3130-A. photooxidation of methylcyclohexane, the initial rate is calculated from the time required to consume 0.05 ml. of oxygen. These measurements were made with precision microburets. (b) Ultraviolet Absorption Spectra of RH-02 Systems. All of the alkanes studied in this work, when saturated with oxygen, possess ultraviolet absorptions (Figures 2 and 3). The absorption intensities are directly proportional to the optical path length, to the oxygen concentration, and to the substrate concentration. The relationship with substrate concentration is illustrated for the methylcyclohexane-02 system in The Journal of Physical Chemistry
1 .o
0.8
t ,zn
~i 0.6
-1
dE 0.4 0.2
0 22W
2404
2600
2800
3000
3200
3400
3600
3800
WAVELENGTH,
Figure 2. Ultraviolet absorption spectra (1-em. path length, 1 atm. of oxygen) of: (1) 2,2-dimethylbutane; (2) 2-methylpentane; (3) 2,3-dimethylbutane; (4)n-heptane, 2,2,4trimethylpentane (isooctane) and 2,4dimethylpentane; (5) 2,3-dimethylpentane; (6) cyclohexane and methylcyclohexane; ( 7 ) oetene-1; (8) chlorobenzene; (9) cyclohexene and cyclooctene; (10) benzene; (11) cumene and toluene.
Figure 4. I n this system, the absorption intensity increases by a factor of 2.7 as the temperature is lowered from25 to -60”. These absorptions are not believed to be perturbed S + T tra.nsitions because there is no indication of unperturbed 8 -+ T absorptions in the same wave length region. Our experimental conditions excluded the assignment of these absorptions to the “high pressure” bands of O2 discovered by Wulf.26 The Herzberg (26) 0. R.Wulf, Proc. Natl. Acad. Sci.,
U.S.,
14, 609 (1928).
4321
INITIATION OF PHOTOOXIDATION BY CHARGE-TRANSFER EXCITATION
0.9-
90,000
-
0.8 0.7
I
2 0.6-
.80,000
j;
zn
35
0.5
5
c
-
z +
B
0.4-
I
-
0
0.1 -
P
0.3
:y N -
0.2
0.02300
3100
3500
3900
i)
Figure 3. Ultraviolet abisorption spectra (at 1 atm. of oxygen; path length = 10 cm.) of: (1) isooctane, and (2) methylcyclohexane.
0.8.
2 z 0.6
z
2 0
0
60,000
2700
WAVELENGTH(
u
70,000
0.4
0.2
[METHYCYCLOHEXANEJ, MOLE LITER- 1
Figure 4. Variation of ultraviolet absorption intensity of methylcyclohexane-02 with methylcyclohexane concentration (solvent, C€12C12;path length, 1 cm.; poz = 1 atm.).
forbidden continuum hm a very low absorpotion eoefficient.* For example, the observed 21.50-A. ultraviolet absorption of isooctane a t 1 atm. pressure of oxygen has an optical density of 0.65 a t 1 em. of path length, whereas the corresponding absorption intensity of Herzberg bands for 1 atm. of gaseous oxygen is only 1.6 X 10-4 optical density unit. The same absorption in solution is expected to have much lower intensity because of the solubility of oxygen.28 To some extent, this reduction of absorption intensity may be counter-
14
J
30,000
40,000
FREQUENCY OF ABSORPTION, CM,
50 00 ”
Figure 5. Frequency of ultraviolet absorption vs. ionization potentials: (1) 2,2-dimethylbutane; (2) isooctane; (3) 2,4-dimethylpentane; (4)ethanol; ( 5 ) +heptane; (6) cyclohexane; (7) tetrahydrofuran; (8) diethyl ether; (9) benzene; (10) cumene; (11) toluene; (12) dialkyl sulfides; (13) xylene; (14) mesitylene.
acted by the perturbation of the forbidden transition by the interaction between oxygen and the solvent. A plot of the wave length for equal absorption intensity, corrected for the solubility of oxygen in these ionination potenc o r n p o ~ l l d ~us.~ ,their ~ ~ ~respective ~~ tial+ gives a reasonably straight line (Figure 5 ) . On the basis of these considerations, the observed ultraviolet absorptions for the alkane-02 systems are attributed primarily to the charge-transfer transitions. Comparison of the spectra of olefins and aromatic compounds saturated with oxygen with those of alkanes in Figure 2 showed that the former have more intense long wave length tail absorptions. I n particular, vibrational fine structures are clearly discernable in the spectrum of benzene. Both the unperturbeda2 (27) R. M. Langer, Phya. Rev., 85, 740 (1962). (28) In fad, compounds with high ionization potentials, such as CHsCN and Hs0, have no absorptions down to the short wave length limit of the Caw-14 spectrometer. (29) This treatment, similar to that employed by Tsubomura and Mulliken,l’ is necessitated by the lack of absorption maxima in these spectra. The solubilities were obtained from the 1iterature.m (30) A. B. McKeown and R. R. Hibbard, Anal. Chem., 28, 1490 (1966); C. B. Kretschrner, J. Nowakowka, and R. Wiebe, Ind. Eng. C h . , 38, 606 (1946) ; “International Critical Tables,” Vol. 111, McGraw-Hill Book Co., Inc., New York, N. Y., 1928, pp. 262, 263. (31) F. H. Field and J. 2. Franklin, “Electron Impact Phenomena,’! Academic Press Inc., New York, N. Y., 1967, pp. 243-309; G. Briegleb, “Elelrtronen-Donator-Acceptor-Komplexe,” Springer-Verlag, Berlin, 1961; W. C. Price, Chem. Rev., 41, 267 (1947); J. N. Murrell, Quart. Rev. (London), 15, 191 (1981). (32) C. Reid, S. C h m . Phys., 18, 1299 (1960).
Volume 60, Number 18 December 1966
J. C. W. CHIEN
4322
Table I11 : Ultraviolet Absorption Intensities" -Optical 3130
Compound
4.
density X 10+ 2537
0.045 1.5 0.5 7.0 16.0 10.0 5.7
Rlethylcyclohexane Cyclooctene Octene-1 Benzene Toluene Cumene Chlorobenzene
A.
2.5
a At 3130 8., the oxygen-free absorption intensities for aromatic compounds are less than 0.005 optical density unit; none is measurable for olefins and methylcyclohexane.
and the perturbed? S --t T absorptions have been reported at these wave lengths for olefins and aromatic compounds. However, the continuous increase of absorption intensity with decreasing wave length cannot be accounted for by S T transition. It is reasonable to consider that these ultraviolet absorptions represent the sum of charge-transfer and S + T absorptions. Without more quantitative measurement, the relG tive contributions of each transition to the over-all absorption intensities cannot be estimated. Tsubomura and I\iIullikenl* considered that charge-transfer interaction is largely responsible for the perturbation of S -+ T absorption by oxygen. I n Figure 5, the points representing xylene and mesitylene, and possibly toluene as well, appear to deviate from the line shown. This deviation is possibly due to the enhanced S 4 T absorptions in these compounds. The total absorption intensities of several compounds -+
v1 Y
' Iw
2
b
012
014
016
OI8
If0
lI2
114
116
1:,
INCIDENT LIGHT INTENSITY IO^), EINSTEINCM. - 2 SEC. - 1
Figure 7. Initial rate of photooxidation of cyclooctene as a function of incident light intensity (temperature 25'; P O , = 1 atm.; path length = 10 cm.; X 3130 8.; [cyclooctene] = 7.62 mole l.-I).
to be used in the calculation of absorbed light intensities are summarized in Table 111. (c) Photooxidation of Methylcyclohexane. Figure 6 shows the dependence of the rate of photooxidation of methylc@ohexane upon oxygen pressure a t 3130 and 2537 A. Since the rate of thermal oxidation of methylcyclohexane under otherwise similar conditions is independent of oxygen pressure, the observed dependence is assumed to arise from the changes in the absorbed light intensity with oxygen pressure. ( d ) Photooxidation of OZejins. The intense u l t m violet absorption intensity of the cyclooctene-02 system enables us to examine the dependence of initial oxidation rate upon oxygen pressure, cyclooctene concentration, and incident light intensity. The last relationship is shown in Figure 7. The other results are given in Table IV. A linear plot of -d[OZ]/dt. (Ia) vs. la-'/', where 1,is the absorbed light intensity, for the first four experiments in Table IV, is shown in Figure 8.
Table IV: Photooxidation of Cyclooctene a t 25 and 3130 A. 20
IO
z
2
s8 i;
Y '
I
I
I
40
60
80
I 100
I
I
120
140
OXYGEN PRESSURE, CM.
Figure 6. Variation of rate of photooxidation of methylfyclohexane with oxygen pressure a t 62.5': 0, X 3130 lo = 2.52 X einstein sec.-l; 0 , ?, 2537 A., 10 = 4.6 X einsteh see.-'
t.,
The Journal of Physical Chemistry
w
160
2
Oxygen pressure, cm.
[Cyolooctene], mole 1. -1
einsteins cm. -2 880. -1
d[Ozl/dt X lo', mole 1. -1 sec. -1
76 50 31 15.2 76 76 76 76
7.62 7.62 7.62 7.62 5.72 3.81 1.90 0.76
16.1 4.4 1.7 5.6 8.0 12.0 24.0 32.0
3.9 1.1 0.58 0.75 1.65 1.39 0.99 0.45
11X 109,
4323
INITIATION OF PHOTOOXIDATION BY CHARGE-TRANSFER EXCITATION
1;1/2
-
(10-3)
Figure 8. Photooxidation of cyclooctene a t 25’ and 3130 A. (both the oxygen pressure and incident light intensity were varied, [cyclooctene] = 7.62 mole
The results of the photooxidation of octene-1 and of cumene are summarized in Table V. (e) Photooxidation of Aromatic Compounds. The results in Table V allow a comparison between the photooxidations of loctene-1 and cumene. The maximum rate of oxidation,23given in the last column, is a function of the “steady-state” hydroperoxide concentration, of the quan tum yield of its decomposition, and of k2/k31’2. On the basis of consideration of the last and the most significant factor, cumene is expected to oxidize more rapidly than octene-1. Cumene’s more rapid oxidation is substantiated by the observed maximum rates. I n contrast, the initial rates of oxidation, as well as the induction periods, are nearly the same for both systems within the temperature range studied. Furthermore, the ultraviolet absorption intensity of curnene-02 is about seven times that
Table V: Photo2xidation of Octene-1 and of Curnene a t 3130 A. and 1 Atm. of 0 2
Compound
x 108, einstein cm.-2 sec. -1
Octene-1 Octene-1 Octene-1 Cumene Cumene Cumene
1.43 1.50 2.04 1.43 1.50 2.04
Io
d[Ozl/dt
OC.
mole 1.-1 8ec.-1
tind x 10-4, sec.
30 45 62.5 30 45 62.5
2.2 5.9 14.0 2.3 5.7 14.0
3.32 1.85 0.65 2.43 1.2 0.73
x
Tempsj
10’9
x
lo’, mole 1.-1 sec. -1
4.1 9.1 22 17 28 94
x
TIME, HOURS
Figure 9. Rates of oxidation of neat compounds a t 25’ and 1 atm. pressure of isotopic 0 2 : A, benzene; 0 , chlorobenzene; 0, toluene; A, cumene.
of the octene-l-02 system at 3130 8. (Table 111); the primary quantum processes in cumene appear to initiate photooxidation less efficiently than those in octene-1. The photooxidations of other aromatic compounds were also examined. The results of oxygen consumption and isotopic mixing33caused by photooxidations are summarized in Figure 9. I n benzene, chlorobenzene, and toluene, whose hydrogens are much less reactive than the benzylic hydrogen in cumene,20the initial rates of oxygen consumption and isotopic mixing are not significantly different. Possibly the photochemical reaction is primarily that of formation of peroxides. Only cyclic peroxides and not hydroperoxides are detected by potentiometric titration in these oxidizing mixtures sampled during the induction period. The initial rate of isotopic mixing in cumene is noticeably faster and it appears to reach a maximum near the end of the induction period. This isotopic mixing could result from the interaction of two peroxy radicals,assuggesting the occurrence of free-radical reactions as well as reaction 7. Semiquantitative potentio(33) T. C. Traylor and P. D. Bartlett, J. Am. Chem. Soc., 85,2407 (1963).
Volume 69, Number 13 December 1966
J. C. W. CHIEN
4324
The values of I$ for methylcyclohexane and olefins are not greatly difficult. On the other hand, 6 in cumene is significantly smaller. Because the S -+ T transition is more easily perturbed in aromatic compounds than in olefin~,~ it is possible that the low value of 4 may be associated with the chemistry of the state. Thus, in addition to reactions 2 to 5, one may add2
Table VI : Quantum Yields for Photooxidations Compound
A,
Methylcyclohexane Methylcy clohexane Cyclooctene Octene-1 Cumene
A.
2537 3130 3130 3130 3130
rb
0,023 0.02
0.018 0.01 0.002
3A
metric determinations showed the presence of both cyclic peroxides and hydroperoxides in this system; the former are present in larger amounts during the induction period. The results with chlorobenzene are particularly noteworthy. Chlorine is known34to exert a significant heavy atom effect on intersystem transitions. The lifetime of the benzene triplet is more than a thousandfold longer than that of the chlorobenzene triplet state. Since the ultraviolet absorption intensity of chlorobenzene-02 is no more intense than those of the other aromatic systems, it is suggestive that the primary processes in photooxidation are fast compared to the decay of the triplet state. (f) Quanlum Yields of Photooxidations. If we exclude either the photolysis of "impurities" or the direct absorption by oxygen molecules as important primary processes (vide inJra), then the most likely process is
RH
+ 0 2 + hv +2 R .
2$Ia (2) where the nature of R - is not specified and the reaction includes both charge-transfer and perturbed S --*. T excitations. Together with the other reactions commonly accepted in autoxidationa5
R.
+
0 2
+ROz.
+ R H +ROOH + R. 2R02---+products + RO2.
0 2
kl kz k3
(3)
(4) (5)
We obtained, for the initial rate of oxidation
Equation 6 takes into account the evolution of oxygenaa in the termination reaction ( 5 ) . This equation predicts us. Ia-'/', such as that a linear plot of (- d [02]/dt) shown in Figure 8 . The quantum yield can be obtained directly from the intercept. Alternatively, it can be calculated from the observed rate of oxidation using measured or literature values of k2/k3"z (vide supra). Average values of 6 are summarized in Table VI. The Journal of Physical Chemistry
+ O2 -+ peroxide
(7) That this reaction does not entirely exclude the other reactions, at least in the case of cumene, is evident from the temperature dependence of the photooxidation rate (Table V). Possibly also the 3A state has low intrinsic reactivity because of electron delocalization. From the slope of the plot in Figure 8, a value of 7.0 x 10-6 1.'12 mole-'/z set.-'" for kz/k3'/' in cyclooctene oxidation at 25" can be calculated. This value agrees well with that reported26for cyclohexene oxidation at 25", which is 8.5 X low5l.'/'mole-"/" sec. -'".
Conclusions The results presented above rather convincingly eliminated the photolysis of "impurities" (I) as an important primary process. Process 11, which involves the direct excitation of oxygen, is also unlikely. Using the upper limit for the Herzberg band absorption intensity, we estimate that the quantum yield ,Of radical formation must exceed 4, even a t 2537 A., to account for the observed oxidation rate. Initiation by the triplet state, process 111,is probably important in the photooxidations of olefins and aromatic compounds. We assume that this process is unimportant in the photooxidations of alkanes for reasons given above. I n process 111, reaction 7 probably predominates. Even though singlet oxygen may be produced in the S --*. T transition, i.e.
A
+ + h~ +aA + 0 2
0 2
('Ag)
(8)
reaction of O2 ('Ag) is probably of minor importance in these systems. The reactivity of O2 ('Ap) is highly selective,'jJ whereas the observed initial rates of oxidation of benzene, toluene, and chlorobenzene are not significantly different. However, if the singlet oxygen is vibrationally excited, the statement above may not be valid. The dependence of the oxidation rate upon O2 and substrate concentrations parallels the dependence of ultraviolet absorption intensity upon these same variables. Since these absorptions are either entirely (34) D. 8. McClure, J . Chem. Phgs., 17, 905 (1949). (35) C. Walling, "Free Radicals in Solution," John Wiley and Sons, Inc., New York, N. Y . , 1957, pp. 397-466.
4325
N.M.R.,THERMAL MAXIMUM, AND SCAVENGING TECHNIQUES FOR RATEMEASUREMENT
or partly attributed to charge-transfer transitions, the latter are assumed to contribute toward the initiation of oxidation of all the systems studied, particularly in the photooxidat ion of methylcyclohexane. The photochemistry following charge-transfer excitation of halide ions in aqueous solution has been The primary quantum process was postulated to be
+
XaQ- hv
3. eaq-)
-+ (Xaq
(9)
Those solvated electrons which had escaped cage recombination were observed by reaction with NzO. Photochemistry of charge-transfer excitation in organic systems has not been studied, to the author's knowledge, but an analogous reaction can be written RH
+
0 2
$- h~ .+
(RH+
+
02-)
(10)
Separation of ionic species in nonsolvating media has low probability. 37 However, since these photooxidations are low quan tum efficiency processes, reaction
of those ionic species which have escaped cage recombination cannot be entirely discounted as an initiating species. There are several other alternative possibilities. The cage recombination of RHf and Oz- could produce radicals or the charge-transfer absorption could be partly dissociative ; these two possibilities cannot be readily differentiated. Two other possible reactions are +ICT --+ 3RH 302 (11) 3CT +3RH ' 0 2 (12) The results obtained so far are insufficient to justify speculation about the relative importance of these reactions in photooxidation.
+
+
(36) J. Jortner, M. Ottolenghi, and G. Stein, J. Phys. Chem., 6 8 , 247 (1964). (37) In the absence of an electric field, irradiation of hexane by l.&Mev. y-rays gave less .than 0.09 separated ion pairs per 100 e.v. of energy absorbed; A. 0. Allen and A. Hummel, Discussions Faraday Soc., 36, 95 (1963).
Comparison of Nuclear Magnetic Resonance, Thermal Maximum, and Scavenging Techniques for Rate Measurement
by Peter G. Evans,la Gerald R. Miller,lb and Maurice M. Kreevoylo Physical Chemistry Laboratory, Oxford, England
(Receiged July 19, 1956)
Rates of dehydration of acetaldehyde hydrate have been studied by a scavenging method and an n.m.r. line-broadening technique. The latter has also been used to determine the hydration rate for the a1deh;yde. The agreement between the two methods and with the older, thermal maximum results is satisfactory and suggests that all of the methods involved are reasonably reliable.
The rate of the hydronium ion catalyzed approach t,o equilibrium for the reaction shown in eq. 1 has been measured by a method based on its evolution of heat.2 The quantity obtairled directly in such a measurement
CH3CH=O
+
+ HzO
CH,CH(OH)Z
(1)
is kz k-2, where k 2 , is the second-order rate constant for the acid-catalyzed hydration (first order in sub-
strate, first order in H+) and k-2 is the second-order rate c0nstan.t for dehydration. The individual rate (1) {a) U. K. Gas Counoil Research Scholar, 1962-1964; (b) U. 8. National Science Foundation Postdoctoral Fellow 1961-1963; (0) U. S. National Science Foundation Senior Postdoctoral Fellow 1962-1963; Department of Chemistry, University of Minnesota, Minneapolis, Minnesota. ( 2 ) (a) R. P. Bell, M. N. Rand, and K. M. A. Wynne-Jones, Trans. Faraday Soc., 52, 1093 (1956); (b) L. C. Gruen and P. T. McTigue, J . C h m . sot., 5224 (1963).
Volume 69, Number 18 December 1956
