LIQUIDPHASE PHOTOLYSIS OF THE DIETHYL KETONE-ISOPROPYLBENZENE SYSTEM
2557
The Liquid Phase Photolysis of the Diethyl Ketone-Isopropylbenzene System
by John M. Jarvie and Allan H. Laufer Gulf Research & Development Company, Pittsburgh, Pennsyluania
(Received April 2, 1964)
The photolysis of solutions of diethyl ketone in isopropylbenzene has been investigated. The effect of temperature and diethyl ketone concentration on the product distribution has been studied. The gaseous products are exclusively ethane and carbon monoxide. The major liquid products are 2,3-dimethyl-2,3-diphenylbutane, 2-methyl-2-phenylbutane, 4,5dimethyl-5-phenylhexan-3-one, 2-methyl-2-phenylpentan-3-one, plus small amounts of propionaldehyde. The photolysis of diethyl ketone prduces ethyl and propanoyl (CZH~CO) radicals and is sensitized by isopropylbenzene. The formation of the reaction products can be explained by radical combination reactions. An 85-90% material balance was obtained for ethyl radicals. The absence of butane and ethylene as products suggests that the abstraction of hydrogen from isopropylbenzene by ethyl radicals has a sufficiently low activation energy to suppress ethyl radical combination and disproportionation reactions.
Ethyl radicals can abstract hydrogen by two routes, vix., CzH6 kr
+
+
+ C6H6C3H7
ka ---f
C2H, CsH5C3H6and C2N6 -I- CzH&OCzH5 -+ CzHa C ~ H ~ C O C Z HThe ~ . results indicate k s / k 4 = 0.23 f 0.02 at 75". A comparison of the gaseous products from diethyl ketone photolysis in cyclohexane, benzene, and isopropylbenzene has been made.
Introduction The vapor phase photolysis of diethyl ketone has been studied extensively.' At temperatures above looo, the decompogition mechanism is well understoood,2-S but at temperatures below looo, the stability of the propanoyl radical creates complication^^-^ which restrict understanding of the photochemistry of the system. The vapor pressure of diethyl ketone interferes with the liquid phase photolysis being performed at temperatures in excess of looo. Bamford and Norrish6 briefly studied the photolysis of diethyl ketone in medicinal paraffin and determined that a t 100' about 87% of the decomposition occurs by a free radical process, the remainder via a molecular path. Ausloos7 investigated the pure ketone as well as its mixtures with normal alkanes; addition of the latter was reported to increase the quantuni yield of volatile products. The photolysis of diethyl ketone in an inert solvent (perfluorodimethylcyclobutane) has been recently reported.8 With allowance for diffusion effects, the normal vapor phase mechanism satisfactorily explained the results. The study has two purposes: (a) to determine the
role of a relatively reactive hydrocarbon solvent, such as isopropylbenzene, in the photolysis of the diethyl ketone, and (b) to elucidate the reactions of the intermediate radicals.
Experimental All photolyses were performed in a cylindrical quartz cell, 4 cm. in diameter and 7 cm. long (about 75 ml.). The cell was connected by a graded seal to a cold-finger condenser, at the top of which was attached a standard taper joint (T19/38 inner). The complete cell could be attached to a vacuum system for either degassing or ~
(1) (a) W. A. Noyes, Jr., G. B. Porter, and J. E. Jolley, Chem. Rev., 56, 49 (1956); (b) D. S. Weir, J . A m . Chem. Soc., 83, 2629 (1961). (2) L. M. Dorfman and Z . D. Sheldon, J . Chem. Phys., 17, 511 (1949). (3) K. 0. Kutschke, M. H. J. Wijnen, and E. W. R. Steacie, J . Am. Chem. Soc., 74, 714 (1952). (4) R. K. Brinton and E. W. R. Steacie, Can. J . Chem., 33, 1840 (1955). (5) P. Ausloos and E. W. R. Steacie, ibid., 32,593 (1954). (6) C. H . Bamford and R. G. W. Norrish, J . Chem. Soc., I531 (1938). (7) P. Ausloos, Can. J . Chem., 36, 400 (1958). (8) R. D. Doepker and G. J. Mains, J . Phus. Chem., 66, 690 (1962).
Volume 68, Number 9
September, 1964
2558
analysis by means of an adapter containing a conventional high-vacuum stopcock. The quartz tubing above the level of the liquid was wrapped in aluminum foil to prevent any vapor phase photolysis. The reaction cell was placed in the center of a helixtype 3-kw. niediuiii pressure lamp (Hanovia Type UA15). A Vycor jacket, 6.5 cm. in diameter, surrounded the cell and served the dual purpose of a water jacket for temperature control and as a filter for light below 2200 8. Temperatures between 40 and 95' (h0.2') were readily maintained with this arrangement. The intensity of the light source was determined with the uranyl oxalate actidonieter.Q Assuming an average quantum yield of 0.57 for decomposition a t wave lengths transmitted by Vycor, 2.34 X 1019 quanta/sec. were absorbed by the actinometer under the experimental conditions. No attempt was made to use monochromatic light. Matsrials. Diethyl ketone (Matheson Coleman and Bell) was used without further purification. Gas chromatographic analysis of this material indicated that small amounts of impurities were present. As chromatographic peak areas attributable to these impurities did not change on photolysis, no further attempt a t purification was made. Isopropylbenzene (Gulf Oil Corp.) was purified by distillation a t atmospheric pressure. The middle third of the distillate was collected and stored in the dark prior to use. No detectable impurities were present as determined by chromatographic analysis. Gulf Oil cyclohexane (minimum purity is 99.9%) was analyzed chromatographically. No impurities were detectable; hence, it was used without further purification. Benzene (J. T. Baker Co., reagent grade) was used without further purification. Procedure and Analysis. Samples were prepared by weight and degassed by conventional freeze-pump-thaw techniques a t - 196' on a mercury-free vacuum systeni prior to photolysis. After photolysis, the cell was attached to the analysis system, the noncondensable gases a t -78' were removed, and the amounts measured in a combination Toepler pump-gas buret. The gas samples were collected and subsequently analyzed with a Consolidated Electrodynamics Nodel 21-103C mass spectrometer. Liquid products were analyzed on an F &: R4 Model 720 dual column gas chromatograph with a 1.2-m. column of 20% silicone gum rubber (GE SE-30) on 60-80 mesh Chroiiiosorb P. The liquid products were identified by marking as well as by collection of selective effluents from the column with subsequent analysis The Journal of Physical Chemistry
JOHN N I . JARVIE AND ALLANH. LAUFEH
by mass spectrometric and infrared spectroscopic techniques. Long exposures (e.g., 300 min.) produced copious quantities of gas. Photolyses in these cases were performed at atmospheric pressure. After removal of dissolved gases by conventional degassing techniques, the photolysis cell was filled to 1 atm. with prepurified nitrogen (Air Reduction Co.) which had been passed over copper a t 500' to remove any oxygen impurities. The amount of gas evolved (0.7-2.0 1. STP) during the photolysis was continuously measured by a calibrated wet test meter. Liquid products were analyzed as before.
Results All of the results reported are derived from experiments performed a t 50, 75, or 90", the great majority being done at 75'. The photolysis products are ethane, carbon monoxide, 2,3-dimethyl-2,3-diphenylbutane (DMPB) , 2-methyl-2-phenylbutane (MPB) , 4,5-dimethyl-5-phenyihexan-3-one (DMPH) , and 2-methyl2-phenylpentan-2-one (MPP). The chromatographic analysis also indicated a peak with a retention time which might be expected for propionaldehyde. However, this represented a quantity of material which was small in comparison to the other major liquid products. It is entirely possible that small amounts of ethylene and butane were produced but the sum of these products amounted to at most 3% of the total gases measured. Hydrogen and methane, arising from primary processes involving isopropylbenzene, could not be detected. The absence of these gases, in the presence of ketone, indicated that decomposition of the solvent was of negligible importance although the latter exhibits a broad absorption spectrum in the ultraviolet. lo Likewise, photolysis of isopropylbenzene, without added ketone, gave no detectable amount of gas. Hydrogen and methane, in low yields, have been reported to be the main gaseous products of the y-irradiation'l and photolytic decomposition a t 2637 &I2 of liquid isopropylbenzene. Sworski, et al. , reported quantum yields of the order of 10-4 to 10-5 for hydrogen and methane production. Small quantities of gas associated with such quantum yields would be a t the lower limit of detectability of the analytical system and would not be observed. (9) W. G. Leighton and G. S. Forbes, J . Am. Chem. Soc., 5 2 , 3139
(1930). (10) American Petroleum Institute Research Project 44, National Bureau of Standards, Catalog of Ultraviolet Absorption Sgectra, Serial No. 160. (11) R. R. Henta, J . Phys. Chem., 66, 1622 (1962). (12) T. J. Sworski, R. R. Henta, and M . Burton, J. Am. Chem. Soc., 73, 1998 (1951).
LIQUIDPHASE PHOTOLYS~S OF THE DIETHYLKETOS-E-ISOPROPYLBENZESE SYSTEM
Preliminary experiments showed that the rate of formation of gaseous products was independent of time for the first 20 min. In subsequent experiments, all exposures were made for 10 min. This produced sufficient gas for convenient analysis at a relatively low ketone conversion ( ~ 5 % ) . The rate of forniation of ethane and carbon monoxide was not proportional to the initial ketone concentration (Fig. 1). Even at the lowest concentration of diethylketone (0.0412 mole/l.), 100% of the light is absorbed, and thus the decrease in gas yields is not due to any change in absorption. Samples of the liquid products from the same experiments were analyzed chromatographically. As in the case of gases, the rate of product formation decreases with increasing ketone concentration (Fig. 2 ) . It should be noted that the rate of gas evolution is greater than the sum of the rates of forination of liquid products.
2559
I .4
1.2
i
(d
-
-E
1.0 -
I
I
I
I
0 X
0.8
-1
I I
z
\
cn W
-d
‘ s
0.6
W
0.4
0.2
0.4
Figure 2.
---,sum of
0.8 DEK CONC ( M O L E S / L )
1.2
Rate of formation of liquid products: liquid products; V, DMPB; 0 , MPB; A, DMPH;
0, MPP.
- 2.0
q0
X c
3 I \
u)
w
-I
g
1.0
c
w I-
d
0:
1 1 0.4 0.8 DEK CONC (MOLES/L)
1
1
1.2
Figure 1. Rate of formation of ethane and carbon monoxide: A, CnH8; 0, CO.
Chromatographic analysis of liquid products indicated that about 20 minor peaks were present. The amounts, however, were too small to be individually identified and measured. The contribution by these peaks to the total product yield is not known but is considered less than 10%. The material balance is defined as CaH6
+ 2DiiIPH + LlPP + MPB = co + DRIPH + lllPP
2
Experimental values were between 1.6 and 1.7, which accounts for 80--85yeof all the ethyl radicals produced in
the decomposition. Since the ethane and carbon monoxide yields are large compared to the liquid products, a slight error in gas yields would markedly affect the material balance. Conversely, a small error in the amount of liquid products, such as that arising from the omission of minor, unidentifiable chromatographic peaks, is of little consequence. Effect of Other Parameters. Stirring. Several experiments were performed to determine the effect of stirring; no change in product distribution was noted. Even with stirring, concentration gradients may exist in solution during photolysis. These would be in the form of “zebra stripes” due to the helical nature of the lamp. The effect of this, however, is expected to be small. l 3 Temperature. Temperature effects are not easily interpreted in the liquid phase due to (a) vapor phase photolysis caused by boiling and (b) the temperature coefficient of viscosity. The effect of temperature, over a narrow range, is shown in Table I. All product rates have been expressed relative to the rate of CO formation a t 75’. The rates increase with temperature but the increase in CO yield is greater than that of the other products. Time. The over-all rates of formation of products in the 300-min. run at 75’ are smaller than those of the 10-min. run at the same temperature. The decrease ~~~~
(13) R.M. Noyes. J . Am. Chem. SOC., 81, 566 (1959).
Volume 68, Number B
September, 196‘4
2560
JOHN 14.JARVIEA N D ALLANH. LAUFER
Table I : Effect of Temperature on Relative Product Yields"
Photolysis time, min.
Temp.. "C.
co
CzHe
MPBb
MPPC
DMPHd
DMPBe
Ketone conversion, % '
300 300 10 300
50 75 75 90
0.27 1 .oo 1.73 1.34
0.62 1.34 2.64 1.81
0.12 0.18 0.37 0.25
0.05 0.06 0.18 0.07
0.08 0.14 0.30 0.17
0.13 0.23 0.48 0.30
11.7 34.7 2.1 45.3
a Initial ketone concentration was -1.3 moles/l. * MPB is 2-methyl-2-phenylbutane. MPP is 2-methyl-2-phenylpentan-3-one DMPH is 4,5-dimethyl-5-phenylhexan-3-one.e DMPB is 2,3-dimethyl-2,3-diphenylbutane.
can be attributed to a t least two effects, viz.,the reduction of ketone with photolysis time and the photolysis of liquid products as their concentrations increase. Effects of Other Solvents. Experiments using either cyclohexane or benzene as a solvent gave, in addition to ethane and carbon monoxide, ethylene and butane. A valid comparison of the total gas yields can only be made when the relative absorption by the solvent is considered. Cyclohexane does not absorb radiation in the wave length region emitted by the lamp. At the concentrations of ketone used in this study, absorption of light by isopropylbenzene and benzene can be as great as 100 times that of the ketone. Based on the total amount of light absorbed by the system, the absolute yields of both the gaseous ethyl radical products and carbon monoxide increase in the solvent order : isopropylbenzene, benzene, cyclohexane. Based on the quantity of radiation absorbed by the ketone, the order is reversed.
Discussion A mechanism which can account for the reaction products found is RH RH"
+ hv
--+
RH*
(18)
+ CtHsCOCzHs +
+ CzHsCOCzHs" CzHsCOCzHs* + CzHs + CzHsCO RH* + RH 2RH CO + CzHs CzHsCO CzHs + RH +CzHs + R RH
-
--+
CZH,
+ CzHsCOCzHj
+
CzHs CzHsCO CzHjCO
+ RH
--+
CzHsCHO
+ C2HsCOCaHs + CZHSCHO
The Journal of Physical Chemistry
+ CzH4COCzHs +R
+ CzHdCOCzHs
(lb) (IC) (Id) (2)
R
R R
+ CzHs +RCzH5
+ CzHsCO +RCOCZH,
+ CzH4COCzHs +RCzH4COCzHs R+R+Rz
(7) (8) (9)
(10)
where R is C6H5G(CH&. Evidence for Photosensitization. The propanoyl radical (C2H5CO) produced in reaction IC can either react with the substrate or decompose, the latter being a function of temperature'. Although reaction with the solvent ( 5 ) will depend, in large part, upon the ease of removal of a hydrogen atom from the solvent itself, the yield of CO should be relatively independent of solvent if sensitization does not occur. The CO yield from the isopropylbenzene-diethyl ketone system is 50 times greater than that from cyclohexane-diethyl ketone when absorption phenomena are considered. Since polychromatic radiation was used, it is difficult to say emphatically that sensitization occurs (see reactions l a and lb) ; however, since there is an increase in the CO yield from the isopropylbenzene-diethyl ketone system compared to the cyclohexane-diethyl ketone system when absorption by isopropylbenzene is taken into consideration, there is strong evidence that sensitization from isopropylbenzene to diethyl ketone probably takes place.14 Dubois and Wilkinsonlb have shown that benzene, an aromatic system with similar absorption properties to isopropylbenzene, can sensitize the fluorescence and phosphorescence of biacetyl. Whether energy transfer from isopropylbenzene to the ketone results in excitation of the latter (as in reaction
(3)
(4) (5)
(6)
(14) One of the referees has pointed out that it is difficult to show energy transfer with polychromatic radiation. Some preliminary photolysis experiments on the diethyl ketone-isopropylbenaene system with 2537-A. light from a low pressure mercury lamp gave higher CO yields than were expected from absorption considerations of the ketone alone. However, further work is needed to establish whether the primary process involves energy transfer from isopropylbenzene to diethyl ketone. (15) J. T . Dubois and F. Wilkinson, J . Chem. Phys., 38, 2541 (1963).
2561
LIQUIDPI-IASEPHOTOLYSIS OF THE DIETHYL KETONE-ISOPROPYLBENZENE SYSTEM
lb) or in the direct decomposition into radicals could not be determined from product analyses. Weirlb has shown that a t 3130 d. ('hot" propionyl radicals are formed which decompose to an ethyl radical and CO. It has been suggested, however, that excited acetone molecules can abstract H to form an alcohol.16 The complete lack of alcoholic products in the present system indicates that such a process is unimportant. Discussion of Mechanism. Diethyl ketone does not decompose directly into two ethyl radicals but rather into an ethyl and propanoyl radical. This latter radical subsequently decomposes into another ethyl radical and carbon monoxide. It is, therefore, most probable that two ethyl radicals are not formed within the same cage of solvent molecules. The absence of cage combination'? cannot be the complete explanation for lack of ethylene and butane among the products, since the results using cyclohexane instead of isopropylbenzene gave relatively large amounts of products resulting from ethyl-ethyl radical reactions (i.e., butane and ethylene). The effect, therefore, cannot be totally explained on the physical effect of molecular screening. Based on product yield, ethyl radicals abstract hydrogen more efficiently from isopropylbenzene than from cyclohexane. It is known that the abstraction of hydrogen by methyl radicals from isopropylbenzene and cyclohexane, in the gas phase, proceeds with activation energies of 6.4 f 0.5 and 8.0 kcal./mole, respectively.18 Although the energies required for reaction in the liquid phase would not be equal to that for gas phase reaction, the relative difference would not be large since the contribution of viscosity to the activation energy for the two solvents is similar, about 2.6 kcal./mole.'Y Although the benzene ring is considered to be a very effective radical scavenger in the liquid phase,2o the CzH5 or CzH5C0 radicals produced in the initial processes preferentially abstract the tertiary hydrogen from the isopropyl side chain giving the l-methyl-l-phenylethyl (phenylisopropyl) radical denoted as R (reactions 3 and 5). Hardwicklzl however, has presented evidence that for reaction of hydrogen atoms with isopropylbenzene, the rate constant ratio for abstraction relative to ring addition is about 0.05 at 23'. Hentzl' has shown that methyl radicals abstract hydrogen from isopropylbenzene rather than add to the ring and that the small activation energy for abstraction is less than that for addition to the ring. Addition of ethyl radicals does occur to a small degree. Infrared analysis of the CIlH16 product (2-methyl-2-phenylbutane) shows 2-3y0 of disubstituted aromatics, the great majority in the meta and para position. In Fig. 2 the'product resulting from reaction 10, the combination of two phenylisopropyl radicals, shows
a marked dependence upon the concentration of ketone. The simplest explanation for this decrease is the effective competition for radical R by the ketonated radicals (reactions 8 and 9). The total rate of liquid product formation remains relatively constant over the concentration range from 200 to 1300 mmoles/l., indicating that the phenylisopropyl radical concentration is constant. The only difference in product distribution is then due to the relative concentrations of the other reacting radicals. Kinetic Treatment. Treatment of the data, assuming that the concentration of the propanoyl radical is less than that of the ethyl radical and hence reaction 6 is small compared to reaction 4, predicts an expression
The plot, however, gives an intercept greater than unity implying that there is yet another route by which ethyl and CzH&OCzHs radicals are being removed from the system. A reaction such as CzHs
+ CzH4COCzHs +C4H9COCzHj
(11)
could account for the discrepancy. The product which
2.0
I-
0
20
10
[RH]/[DEK]
Figure 3.
Plot of eq. B t o obtain k3/k4.
(16) (a) E. J. Bowen and E. de la Praudiere, J . Chem. Soc., 1503 (1934); (b) P. E. Frankenburg and W. A. Noyes, Jr., J . Am. Chem. Soc., 75, 2847 (1953). (17) J. Franck and E. Rabinowitch, Trans. Faraday Soc., 30, 120 (1934). (18) 1. B. Burkley and R. E. Rebbert, J . Phys. Chem., 67, 168 (1963). (19) The values were calculated from American Petroleum Institute Research Project 44, National Bureau of Standards, Viscosity Data. (20) J. G. Burr and J. M. Scarborough, J . Phys. Chem., 64, 1367 (1960). (21) T. J. Hardwick, ibid., 66, 117 (1962).
Volume 68, Number 9 September, I964
2562
CONWAY PIERCE AND BLAND EWING
results from this reaction has a boiling point similar to isopropylbenzene and any chromatographic signal due to this niaterial would be swamped by the solvent peak. It was, therefore, not possible to obtain a quantitative result for the yield of such a product. This product, C4H9COC2Hs,was not included in the material balance; the amount formed would be relatively small compared to ethane and carbon monoxide, and thus there would be a small effect on the over-all material balance. The only other fate of CzH4COCzHsradicals is by reaction 9. From steady-state derivations, it can be shown that
RMP P R C ~ H ~- kske [RH] ksk4 RMPBRco[DEE(] kzk7jDEKI -k kzlc7
()'
A plot of the left-hand side us. the ratio [RH]/ [DEK] should give a straight line if the mechanism is correct. Confirmation is shown in Fig. 3.
The ratio of the slope (k8k3/k21c,) to the intercept (k8k4/k2k7)of this line gives the value lc3/k4.22 From our data Ica/lcr = 0.23 f 0.02, in excellent agreement with previous value^.^ Refinement of this figure requires both a more detailed mechanism and a more complete product analysis.
Acknowledgment. We wish to thank Mr. E. L. Brozek for performing the mass spectrometric analyses. Thanks are also due to Dr. J. R. Tomlinson for helpful discussions. (22) Another steady-state expression was suggested by one of the referees, viz.
giving ka/k4 = 0.27 f 0.05, in good agreement with the value obtained from eq. B.
Areas of Uniform Graphite Surfaces
by Conway Pierce and Bland Ewing Department of Chemistry, University of California, Riverside, California
(Received April 5, 1964)
The previous suggestion that conventional nitrogen areas of uniform surface graphites are too low is confirmed. Area measurements by benzene, n-hexane, and ethyl chloride agree with nitrogen areas if the cross section is taken as 20 instead of 16.2 8.%. A proposed explanation is that nitrogen molecules are localized a t graphite lattice sites, so that each one fills four of the unit hexagons, an area of 21 Isotherms of similar size molecules, oxygen and cazbon monoxide, are in agreement with this model. General limitations of surface area measurements by gas adsorption are discussed.
Carbon blacks heated to about 3000" possess extremely uniforin surfaces, consisting chiefly of basal graphite planes. Nitrogen isotherins on such graphite surfaces are characterized by an unusual step-like rise in the relative pressure region 0.2-0.4, first noted by Joyner and Eniniett. It has been shown2that when V , is coniputed by the V / n ratio in the multilayer region for such isotherm^,^ the value obtained is 1.2-1.25 tiilirs larger than the R.E.T. or point B value for V,, whweas for isotherms on other surfaces the V , estiThe Joirrnnl of Physical Chemistry
mates froin V / n are in good agreement with B.E.T. values. G. Joyner and P. H. Emmett, J . Am. Chem. Soc., 70, 2353 (1948). (2) C. Pierce and B. Ewing, ibid., 84, 4070 (1962). (3) This is done by dividing V adsorbed at a given relative pressure by the number of statistical layers, n, normally adsorbed at that pressure. The computation of V , is conveniently done by dividing V at 0 . 5 ~ by 0 1.70, a figure established from nitrogen isotherms for many different surfaces. This n ( 0 . 5 ) is riot exact for isotherms of other adsorbates, but may be used as a good approximation for many. (1) L.
