2285
PHOTOLYSIS OF 3-PENTANONE-2,2,4,4-d4
Photolysis of 3-Pentanone=2,2,4,4-d4 by Charles I. Bartal and Alvin S. Gordon Research Department, Chemistry Division, Naval Weapons Center, China Lake, California 93666 (Received August 62,1969)
The photolysis of CH3CD2COCD&H3has been studied over the temperature range from 20 to 407'. The ethyl-dz radicals produced in the primary-primary process can combine, disproportion, and/or abstract an a-D or a p-H from the starting ketone. The disproportionation combination ratio was studied in the range 20-150' and a small negative temperature coefficient was noted for the ratio, confirming the observations of other workers and extending the effect to higher temperatures. The abstraction processes were studied in the range 150407O. The specific rate constants for the abstraction of an a-D and 0-H by ethyl radical were k,D
kaH
= 108.6*0.0610 - 1Ot300*l70/4.68TM-1 sec-l = 108.7i0.40~0-12,400f1,600/4.67~ M-1 sec-l
The two pentanonyl radicals formed in the abstraction reactions, CH2CD2COCD&H3 and CH&DCOCDzCH,, can decompose to give ethene-dz and -dl, respectively, and ethyl-dz radicals. By studying the rates of formation of the ethenes, it was concluded that the radical CH3CDCOCDzCHa undergoes an internal rearrangement in comDetition with its decomposition at temperatures above 300'. This rearrangement product decomposes t i give ethene-dzand ethyl-dl radicals. A possible rearrangement mechanism 6 discussed.
Introduction The photolysis of 3-pentanone-2,2,4,4-d4 provides a convenient source of partially deuterated ethyl radicals for studying the kinetics of ethyl radical reactions with molecules R-H. For this purpose, however, one must know the kinetics of the reactions of the radical with the parent molecule. The only previous study of this system was reported by Wijnen and Steacie.2 Due to relatively low experimental accuracy available with analytical techniques and purity of deuterated compounds at that time, the parameters they report for the ethyl radical abstraction reactions are questionable. We therefore decided to reinvestigate the system. The following are important elementary reactions in this system. CHaCDzCOCD2CH3 52CH3CDz
+ CO
2CH3CDz .---t CH3CD2CDZCH3 L-, CHsCDzH
CHaCDz
+ CHzCDz
+ CHaCD2COCD2CHa * CHaCDa + CH3CDCOCDZCH3 and CH3CD2H + CHzCDzCOCDzCHs
The interesting chain propagation steps involving the reactions of the pentanonyl radicals are studied in lesser detail in this work.
Results and Discussion The disproportionation-combination rates for the ethyl-& radicals were determined from the relative rates of formation of butane-& ethane-dz, and ethene-
dz in the photolysis of 3-pentanone-2,2,4,4-d2 (DEK) at temperatures from 20 to 150". The reactions involved are
2CH3CDz + CH3CDZCDzCH3 +CHaCDzH
+ CHzCDz
Between 20 and 88' inclusive, pure ethane-& with an equal amount of ethene-& is formed. Since ethane-d3 would be the predominant product from ethyl radical abstraction, abstraction reactions are not important for our experimental conditions; the disproportionationcombination ratio was calculated from the ratio of the amounts of ethane-d2 and butane-& formed. When the temperature range is extended to 150', ethane-& is found in the products, and ethene-& rather than ethanedz must be used as the monitor for disproportionation reaction of the ethyl radicals. A plot of the disproportionation combination ratios as a function of temperature is shown in Figure 1. A slight inverse temperature effect on the ratio is noted.3 This may be interpreted as a negative temperature coefficient for the disproportionation rate constant. An explanation involving activation barriers is not suitable because if any barrier exists, the disproportionation barrier involving H atom transfer is almost certainly higher than the combination barrier and would result in a temperature effectopposite to that observed. To determine the abstraction reactions above 150', it (1) National Research Council Postdoctoral Research Associate. (2) M. H. J. Wijnen and E. W. R. Steacie, Can. J. Chem., 29, 1092 (1951). (3) P. S. Dixon, A. P. Stefani, and M.Szwarc, J . Amer. Chem. Soc., 85, 2551 (1963), observe the same effect.
The Journal of Physical Chemistry, Vol. 74,hro. 11, 1970
2286
CHARLES I. BARTAAND ALVINS. GORDON ~~
Table I: The Rates of Formation of Products and Product Isotope Composition in the Photolysis of 3-Pentanone-2,2,4,4-d4 Rates of formation----
7--
Temp, OC
150 153 153 153 180 183 207 209 234 234 269 274 307 314 357 357 406 405 a
Time, min
3.0 15.0 3.3 5.0 3.3 13.5 16.1 10.0 10.5 2.0 15.0 2.0 15.0 5.0 3.3 6.3 6.3 2.0
(DEW M x 10'
7.9 7.9 7.9 7.9 7.4 7.4 7.0 7.0 6.7 6.7 6.2 6.2 5.8 5.8 5.3 5.3 5.0 5.0
Ethene
0.70 0.73 0.68 0.71 n.m.
0.57 0.71 0.65 0.83 0.75 0.81 1.02 1.16 1,25 2.39 4.07 9.10 12.58
Ethane
0.94 0.97 1.03 1 .oo 0.96 0.99 1.51 1.51 2.04 1.92 2.96 2.35 4.44 4.20 4.88 6.17 10.91 15.24
Butane
5.8 6.9 6.5 6.3 4.5 4.8 6.3 5.9 5.2 4.9 4.2 3.7 3.1 3.1 1.4 2.1 2.5 2.8
dl
di
100 100 100 100 100 100 100 100 100 100 96.7 96.5 89.0
... 81.7 78.9 80.6 82.0
-----
Isotope composition ( 70)-----------Ethane--Butane------.
I
------Ethene-----ds
..*
... ...
...
... .,.
...
... * , .
... 3.3 3.5 11.0 ... 18.3 21.1 19.4 18.0
dz
24.2 26.0 25.9 29.0 37.6 42.5 53.6 51.7 64.8 64.0 72.8 76.0 76.9 74.4 70.8 70.3 57.0 51.0
dia
75.8 74.0 74.1 71.0 62.4 57.5 46.4 48.3 35.2 36.0 27.2 24.0 23.1 25.6 27.0 26.9 38.0 41.1
... ... ...
... ... ...
...
,..
,..
...
...
...
... ... 2.2 2.8 5.0 7.9
7
dr
ds
daa
100 100 100 100 100 100 100 100 100 100 100 100 100 100 80.6 75.5 63.8 44.6
...
... ...
.
I
.
... ...
...
...
... ...
...
... ...
.,,
...
...
...
...
... ...
...
...
...
...
...
...
18.3 22.8 32.2 44.2
1.1 1.7 4.0 11 -2
...
...
As calculated.
was necessary to correct for the contribution of ethanedz formed by disproportionation. The amount of disproportionation ethane-& was calculated from the kdis/koombratio taken as 0.10, A t temperatures above 300°,the situation is further complicated by the fact that ethyl-dl radicals are present in the system and disproportionate to form ethane-&. Fortunately, at the temperatures where ethyl-& reactions are important, the rate of ethane formed by disproportionation is negligible compared to that by abstraction and can be ignored. Ethyl Radical Abstraction Reactions. The rate constants for ethyl abstraction reactions in the range 150407" were determined using the rates of formation of the various ethanes and butanes. The rates of formation of total ethane, ethene, and butane are reported in Table I. The rates at any one temperature depend on the light flux through the reaction vessel but the calculated rate constants are intensity independent, as was shown by Wijnen and Steacie.2 The deuterium contents of the ethene, ethane, and butane products were determined for the photolyses from 150 to 406'. At temperatures up to 314", the butane product consisted entirely of butane-&. At 357" and above, butane-& was observed. This product is formed by the combination ethyl-d2 and ethyl-& radicals. The presence of ethyl-& radicals predicts butane-dz to be a product of the photolysis. The direct mass spectral quantitative determination of this product is not feasible because the butane-& parent peak (m/e 60) is contaminated by the fragment peaks of butaned3 and -d4. Even if a butane-& mass spectral standard had been available, the parent of butane-d2 accounts for only a very small percentage of the total m/e 60 peak T h e Journal of Physical Chemistry, Vol. 74,No. 11, 1070
,130
r-----7
.loot
1
20
I
I
I
I
I
40
60
80
100
120
1 140
I
T 'C
Figure 1. Plot of the ratio of ethyl-& radical disproportionation to combination us. temperature.
and small measurement errors would result in a very large relative error in the butane-cL determination. For similar reasons, the amount of ethane-& could not be accurately determined. We found it expedient to calculate the amounts of butane-& and ethane-& from the easily measured values of butane-& and -da and ethane-ds and -dz as determined by mass spectrometry. From the rate expressions for the formation of butane-&, -da, and -d2 = kd,(ethyl-dd2 R-da
= kd,(ethyl-d,)( e t h y l 4
R-dl
=
kd,(ethyl-dl)2
we can determine R-dz in terms of R-drand R-d6, since it has been shown4 that kq = ICZ = 1/&3
2287
PHOTOLYSIS OF 3-PENTANONE-2,2,4,4-d4
R-dn
=
If the rates of formation of b u t a n e 4 and -da are expressed in mass spectral peak heights/cm3 sec, then the peak height/cma sec of butane-& is easily calculated. The rate of formation of ethane-& can now be calculated from the relative concentrations of the two ethyl radicals ( 2 R-d4 /R-d and the measured values of the rates of formation of ethane-& and -dz in mass spectral units/cm3 sec as outline below. Ethane-& is formed only by bn ethyl-& abstracting an a-D atom from DEK. Ethane-&, however, is formed in two abstraction processes; the abstraction of a @-Hatom from DEK by eth~7l-d~ and the abstraction of an a-D atom by ethyl-&. The ratio of a to /3 abstraction rates should be the same for ethyl-& and ethyl-dl. The following relationship then exists
- -- k D abstraction -x RCW~CD~H - x y kHabstraction
I
I
(l/4) (R2-da/R-d
I
3.0 2.4
2.2
I
I
I
I
I
I
2.0
1.8
1.6
I/T x 103
RCHsCDa
where X is the rate of formation, in mass spectral units, of ethane-& formed by an a-D atom abstraction by an ethyl-& and Y is the rate of ethane-dl formation. Another relation between X and Y may be determined from the ratio of the sum of the rate of products arising from ethyl-& abstraction, RCH~CD~R C H ~ C D ~ H- X, to the sum of the rates of formation of ethyl-& products, X Y. With the reasonable assumption that k~ and k~ are the same for the two kinds of ethyl radicals, the above ratio is seen to be equal to the ratio of the two radicals. Thus
I
Figure 2. Arrhenius plot of the rate constants for or-D abstraction from 3-pentanone-2,2,4,4,-d4by ethyl-& radicals (in 1. mol-' sec-1).
I
I
I
+
+
RCHzCDa
-I- RCH~CD~H - X -- _(CHsCD2) - X X+Y (CHICDH) - Y
From the two expressions in X and Y , both quantities may be calculated. Since the total ethane and butane are known in conventional units from the measured areas of the calibrated gas chromomatograph, the consistent arbitrary units of the various ethanes and butanes permit an easy conversion to conventional units. The deuterium content of the ethene product was measured directly by mass spectrometry. The label contents for all products are given in Table I. At temperatures up to 314", R b u t a n e - d 4 = R b u t a n c total, and from the relationships between the rate of formation of ethane-da = kD(ethyl-&) (DEK) and the rate of formation of butane = kd4(ethyl-d2)2,the rate constant for a-D abstraction may be formulated
t
I 1.7
Figure 3. Arrhenius plot for the rate constants for p-H abstraction from 3-pentanone-2,2,4,4,-d4 by ethyl-& radicals (in 1. mol-' sec-1).
At temperat,ures of 357" and above, it was necessary to separate the ethane-& into that formed by 0-H abstraction by ethyl-& and ethane-& formed by a-D abstraction by an ethyl-dl. This is readily done using the relationship we had derived before RCHaCDa
X
RCH~CD~H - X - RCH~CD~H
The rate constant for the combination of two ethyl-dz
radicals was taken to be the same as that for two ethyl radicalsI52 X 1Olo M-l sec-I. Similarly, the rate constant for @-Habstraction is
1.5
1.6 I/T x 103
Arrhenius plots (Figures 2 and 3) and kinetic parameters for the rate constants k~ and k~ were obtained (4) J. A.
Kerr and A. F. Trotman-Dickenson, Progr. React. Kinet.,
1, 105 (1961).
(5) A. Shepp and K. 0. Kutschke, J. Chem. Phgs., 26, 1020 (1957). The Journal of Physical Chemistry, Vol. 74, N o . 11, 1970
CHARLES I. BARTAAND ALVINS. GORDON
2288
using a least-squares program on a UMIVAC 1108, leading to kn = 108.6+ 0.0610-(10,300 rt 170)/4.58T 1w-1 set-I and .i kH = 108.1 & 0.410-(12,400 f I2600)/4.58T&f-I set-1, Rhere the reported error is for one standard deviation. Ethyl-& radicals could conceivably be formed by a secondary reaction
+ CHsCDzCOCDzCHa CHaCDHCOCD2CH3 + CH2CD2COCD2CHa
CHaCDCOCD2CHa
----t
As the d2 ketone builds up during the course of reaction, it photolyzes to give ethyl-dl radical. Examination of the ketone marking after 6.3 min of the experiment at 357" showed that the d2 ketone concentration increased by 2% over the original. That is an average over the time of reaction of 1%,and since half the ethyl radicals formed by the photolysis of da ketone are ethyl-d3, the ethyl-dl to ethyl-& radical content increases by 0.501,This extraneous source of ethyl-dl will account for butane-& in the product being present as 1% the butane-&. The original 2.4% ketone d2 impurity r d l not appear as butane-& because of the b u t a n e 4 standard, as explained below. Since there are 4 a atoms, the 2.401, d3 ketone impurity results in (1/4)2.4 = 0.6% of the a positions marked H. Taking the ABactfor abstraction as 1.5 lical/mol less for H than D and equal preexponential factors, in the temperature range of abstraction, 307-405', the ethane-d2 formed from this source would be 5-801, of the ethane-d3. Since the effect on the temperature coefficient for the ethane-d3:ethane-dz ratio is quite small, the correction would have a negligible effect on the Eact and would appear mostly in the A factor, lowering it something over 5%. The correction was not made since the spread of the data was so much greater than the correction. All of our rate constant calculations were corrected for the change in DEK concentration during the photolysis (always less than 10%). The Decomposition and Rearrangements of Pentanonyl Radicals. Although the primary purpose of this work has been to determine the abstraction parameters for ethyl radicals from DEK, the results do allow us to make some qualitative statements about the reactions of the pentanonyl radicals formed in the abstraction processes. Ethene produced in the photolyses to 209' can be accounted for by ethyl radical disproportionation. Above this temperature, there is more ethene than can be accounted for by disproportionation. In the photolysis at 234", all the excess ethene formed is -dz. Two possibilities for its formation are the decomposition of an ethyl radical or the decomposition of a pentanonyl radical. Simple kinetic calculations show clearly that ethyl radical decomposition cannot be an important source of ethene. The alternative explanation is that pentanonyl radicals decompose to give ethene. The Journal of Phyeical Chemistry, Vol. 74,No. 11, 1970
Two pentanonyl radicals are formed when ethyl radical abstracts from the ketone parent CH3CDCOCD2CHh
I
CHzCDzCOCD2CHa I1
The rates of formation of these radicals are simply the iates of a and /3 abstractions, respectively. Radical I , which has some resonance stabilization, should be fairly stable to decomposition at these temperatures by analogy with the acetonyl radicaln6 The radical formed by p abstraction, 11, would decompose more readily to give ethene-d2 and ethyl-dz radicals. No intraradical isomerization seems to occur since no ethene-d3or ethane-& are found in the products CHzCDzCOCDzCHa +CHzCDz
+ CO + CD2CH3
If we compare the rates of formation of ethene-d2 (by nondisproportionation) to that of formation of ethaned2 by nondisproportionation at 234', we find they are very similar. This would suggest that the predominant fate of radical I1 is decomposition. As radical I concentration cannot be determined from our data, no conclusions can be drawn about the energetics of this process. At temperatures from 269 to 314", ethene-& in addition to ethene-d2 is formed. By examining the rates involved, we find again that the ethene-d2 can be rationalized as arising from the decomposition of 11. Ethene-& cannot be explained by this process nor conveniently by a direct decomposition of radical I. A possible explanation is the isomerization of radical I by intraradical H(D) abstraction, CH,CDCOCD2CH, -+ CH2CHDCOCDzCH3. The radical formed by this path is analogous to radical I1 and would decompose to give ethene-& and ethyl-d2 radicals. The path we believe to be most reasonable' for this isomerization is CHsCDCOCD2CH3 --3 CH&D=COHCD&H3
wall --f
decompose
CH2CHDCOCDZCH3 -----f CHzCHD
+ CO + CHSCD2
The ketonization of the enol intermediate probably takes place on a wall because of orientation requirements for the process. I n experiments with undeuterated 3-pentanone, in which the vessel walls were pretreated with D20,we determined that exchange does (6) J. R.McNesby and A. S. Gordon, J . Amer. Chern. Soc., 7 6 , 1416 (1954). (7) The formation of ethyl-& radicals can be explained either by a path involving the carbonyl group and the hydrogen of the other methyl group or by a simple hydrogen abstraction for radical isomerization CHsCDCOCDzCHs -c CHaCHDCOCDzCHz -+ CHsCHD CO CHzCDz. However, to form ethene-& by a hydrogen atom migration on the carbon skeleton is needed. CHsCDCOCDzCHa -.t CH2CHDCOCDzCHs -.t CHzCHD CO CDzCHs and such a 1,2gas-phase shift is not very probable.
+
+
+
+
PHOTOLYSIS OF
2289
3-PENTANONE-2,2,4,4-d4
not take place with surface deuterium atoms in the ethene forming process. While we a priori might have expected the enol intermediate to exchange with DzO on the surface, the lack of such exchange does not rule out the above mechanism, since exchange would depend on the OD content of the silica and competitive rates of exchange and ketonization at this temperature (300"). The formation of ethyl-& radicalss in the experiments at 357 and 405" can also be explained by the rearrangement of I. A possible path for such a process would be CH~CDCOCDZCHI---f CH3CD=COHCD$H2
+
CHaCHDCOCDzCHz + CH3CHD
+ CO + CHzCDz
Any mechanism involving the formation of ethyl-& from radical I predicts that an amount of ethene-& equal to ethyl-dl is also formed. By subtracting the ethene-& rate formed by disproportionation and by the decomposition of I1 from the total rate of formation of ethene-&, the remainder corresponds very closely to the rate of ethane products formed by the reactions of eth yl-4.
Experimental Section Materials. The 3-pentanone-2,2,4,4-d4 was prepared in an acid-catalyzed exchange of 3-pentanone (Aldrich Chemical) with DzO following the procedure of Siebl and Gausmann.$ The isotope composition of the ketone after three exchanges was determined by mass spectrometry to be 97.6%-&, 2.4%-d3. The label was found by nmr to be exclusively in the 2 and 4 positions. The ethane-d3 used as a mass spectral standard was research grade obtained from Merck Sharpe and Dohm of Canada, Ltd. It was used without further purification. The ethene-& and butane-& used as mass spectral standards were prepared by photolyzing the DEK at room temperature and collecting the ethene and butane fractions. At room temperature the ethene and butane are formed only by recombination and disproportionation. The butane actually contains about 2% butane-& due to the starting material impurity. However, if we used this material as our butane-& standard, we are still able to determine if the butane deuterium content has changed in any photolysis. Thus, in all the runs in Table I which show 100% butane-& the butane label content was identical with that of the room temperature photolysis; Le., our results are effectivelycorrected for the starting material isotope
impurity. Similarly, the isotopic impurity in the ethene-& is corrected by the mass spectrometer standard. Analysis. Product yields were determined by gas chromatography using an 8 ft X '/4 in. 0.d. Porapak Q (Waters Associates) dual copper column operated with 196 to +230". temperature programming from Samples were introduced onto the column at -196" using a stainless steel flask inserted in the helium carrier stream ahead of the column. One-third of the effluent from the column was passed through a Carle thermal conductivity detector maintained at 60". The remaining effluent was passed through a collection system containing packed glass U-traps cooled to liquid nitrogen temperature. Individual components were diverted through such traps which were then evacuated and sealed. Samples obtained for label analysis were introduced from the U traps into the mass spectrometer with a CEC gas sampling system. A Hitachi-Perkin-Elmer RMU6D double-focussing mass spectrometer was used. The isotopic composition of the ethene, ethane, and butane products were determined. Label analysis of the recovered ketone could not be accomplished by collecting it from the gas chromatograph because Porapak Q is an efficient exchange surface for deuterated ketones. lo Ketone analyses were performed by introducing the photolysate directly into the mass spectrometer. Photolysis. The 3-pentanone-2,2,4,4-d4 was photolyzed in a quartz vessel at a pressure of 21 Torr with the full arc of a medium pressure Hanovia SC-100 mercury lamp. The photolysis temperature was varied from 20 to 406" using a furnace. The volume of the sample vessel was 88 cm3. The time of photolysis was varied but always kept short enough to ensure that less than 10% of the ketone was reacted. I n a control experiment to determine surface effects, the photolysis vessel was thoroughly rinsed with D,O and evacuated. A sample of undeuterated 3-pentanone was photolyzed at 300" for a long period of time and the ethane and butane products analyzed for deuterium content.
-
(8) The possibility that CHaCDCOCDzCHa could cause the parent purity to be changed by abstraction on H from the parent to give CHsCHDCOCDzCHs, which would become a source of ethyl-d, by subsequent photolysis was eliminated by examining the purity of the ketone before and after reaction at 357O. At the end of 6.3 minutes, the ketone-& had changed from 2.4% d3 to at most 6.4% da. The average increase of 1% increase in da content would only contribute 0.6% increase in ethyl dl content, since the ketone-da photodecomposes equally t o ethyl-& and ethyl-dl radicals. (9) J. Siebl and H. Gausmann, X e h . Chim. Acta, 46, 2858 (1963). (10) C. I. Barta and A. S. Gordon, accepted by J. Chromatogr. Sci.
The Journal of Physical Chemistry, Vol. 74, N o . 11, 1970
