PENTAFLUOROETHYL ETHYL KETONE
2307
Photochemistry of the Fluoro Ketones. Pentafluoroethyl Ethyl Ketone'
by R. L. Thommarson and G. 0. Pritchard Department of Chemistry, University of California, Santa Barbara, California 93106 (Received January 31, 1966)
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The photochemistry of the title ketone was investigated at 3130 A and its behavior is compared to that of other fluoroalkyl ketones. Quantum yield data as functions of temperature and pressure are determined, and it is concluded that the primary decomposition mode is C2F5COC2Hs hv -t C2FS COC2H5. The disproportionation/combination ratio for C2F5 and C2H5 radicals is 0.56, and the cross-combination ratio is about 2, both almost independent of the temperature. The activation energies for H-atom abstraction from the ketone are 5.6 and 6.8 kea1 mole-' for C2F5 and C2H5 radicals, respectively.
+
Introduction In our previous investigation2 on the photolysis of C3F7COC2H5,we obtained some surprisingly strong temperature dependencies for the disproportionation/ combination and cross-combination ratios for C3F7 and C2H5 radicals. This investigation represents a determination of these ratios for C2Fs and C2H5 radicals; also quantum yield data as functions of the temperature and pressure for the photodecomposition of the ketone are presented. Experimental Seetion The apparatus was identical with that used in the C S F ~ C O C ~ experiments. H~ The C2F5COC2H5was obtained from Columbia Organic; it appeared to be pure from vpc and mass spectrometric analysis. The extinction coefficient at 3130 A is 7.3 1. mole-' cm-l. Photolysis temperatures varied from room temperature to over 300", and times varied from 2 min to 4 hr. Runs of short duration and higher intensities were employed when radical-radical interactions were being investigated. Longer times were employed when quantum yield determinations were being made and solution filters were used.2 Under these conditions the incident intensity was either 2.2 or 2.8 X l O l 3 quanta/cc sec, depending on the filter combination. The absorbing intensity without the filters was -2030 times greater. The pressure of the ketone used was maintained a t 4.5 f 0.3 em, except when the pressure ~ being determined. dependence of 4 %was Product analysis was effected by low-temperature fractionation and vpc. CO was separated at -210"
+
and occasionally checked on the mass spectrometer. C2Heand C2H4were collected at -175" and analyzed on a 1-m 3% squalane-on-silica gel column. CZFsH, C4Hl0, C4F10, and C2F5C2H5, together with some C2FaCOC2Ha,were collected at - 110" and analyzed on a 0.5-m 3% squalane-on-alumina column followed by a 2-m 15% silicone oil-on-Chromosorb W column. The ketone was retained by the alumina column, and the other components were identified by mass spectrometric analysis and vpc retention times. C ~ F S C ~ H S was prepared from the reduction of pentafluoro-lbutene, obtained from Peninsular ChemResearch.
Results and Discussion
Radical-Radical Interactions. The following reactions are of interest.
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CzF5 C2Hs +C2F5H (32% CzFs CzHs +CzF5CzHs CzF5 CzFs +C4Fio
(1) (2) (3)
C2H5
(4)
+
+
-
+ C2H5 C2H6 + C2H4 C2Hs + C2H5 C4Hio ----f
(5)
C2F5H and C2H6 are also formed in the abstraction reactions
+ CzHsCOCzFs +C~FSH+ C2H4COC2Fs (6) FS CzH5 + CzH5COCzFs +C2H6 + C ~ H ~ C O C ~ (7)
C2F5
~
(1) This work waa supported by a grant from the National Science
Foundation. (2) G. 0. Pritchard and R. L. Thommarson, J . Phys. Chem., 69, 1001 (1965).
Voolume 70,Number 7 J u l y 1966
R. L.THOMMARSON AND G.0. PRITCHARD
2308
At higher temperatures, the radical formed in reactions 6 and 7 may decompose into c2H4* (from rearrangement of CHaCH), CO, and C2F5, or, as a referee has suggested, * into CH3CHC0 and c2F5. We did not distinguish 1 0.0 . 0 *O between these two possibilities. As we previously B 0 P 0 0 0 assumed,2 the C2H4formed in reaction 1 was found n s e by subtracting 0.14 of the CJL0 yield from the total 0 O0' M 0 0 ethylene formed. The Arrhenius plot of k1/k2 for a 2 -0.5 '1 number of representative runs is shown in Figure 1. I 0 I The results are very scattered, due to difficulties that 2.0 2.5 3.0 lOZ/T(OK). we encountered in obtaining accurate ethylene analyses. The total ethylene formed varied between -35 and Figure 1. Arrhenius plots of CzF5 and CzH5 radical interactions: I C ~ / I C ~ ~ 0, ; / *kl/kz. ; 5% (at high temperatures) of the - 1 7 5 " fraction, and any error in the butane analysis is reflected in the k&2 values. The line drawn represents the average value of k1/k2 = 0.56, independent of temperature, for the 14 runs recorded. A line of positive slope could also very well represent most of the points, yielding a value of El - E2 -1.5 kcal mole-', in good agreement with the comparable value that we obtained for C3F7and C2H5 radicalsa2 In view of the uncertainties involved, it would appear simpler to conclude that these relatively large activation energy differences are not established, although it is reasonably certain that Edisproportionation < E e o m b i n a t i o n for alkyl radical interactions,s the difference being of the order of 300 cal mole- l. The cross-combination ratio k2/k31/'k51/' = R C ~ F ~ C ~ H J where R represents mean rate of R~,~lo1"RC,~lo"2, formation, is also plotted in the Arrhenius form in Figure 1 (all the values are close to the theoretical value of 2). A least-squares line through the points 2.0 2.5 3.0 given yields k2/k31/'k51/2 = 4.7e(-750*400)/RT. This is 10'/T(eK). a similar but lesser temperature effect than we obtained Figure 2. Arrhenius plots for H-atom abstractions: 0, ks/k;/' mole-'/' ccI/' sec-'/'; 0, kT/k&l/' mole-'/' ccl/z in the C3F7 C2Hs system.2 If we include the dissec-Ila; (3, high-intensity] low-temperature runs. proportionation reactions in the cross-combination ratio, a correction factor of 1.56/(1.14)'/' is needed, assuming that the disproportionation/combination elimination from the vibrationally excited C2F5C2Hs* C2H5 and 2C2H6 radicals are inderatios for 62F5 species formed in reaction 2. No evidence was found pendent of temperature; the temperature dependence for such a reaction owing to the large number of of the cross combination will remain unaffected. available degrees of freedom in the excited molecule.2 The present data suggest very small, 1 a t 570°K may be a valid observation, as in the photolysis of CF3COCH3Sieger and Calvert6found that CH4 was formed by a chain process a t this temperature, owing to a reaction such as CFa
+ CFBCOCHS
+
1.5
I
1
~ H ~
a
1.0
3
'A c1
I
8 0.5
(CF3)2COCH3+CHs
+ CF3COCF3
An analogous process could occur with CzFSand CZFsCOC2H5. The quantum yield data for the radical containing products in the two systems are very similar, except for the comparison between @ c ~ Hand ~ @c,H~~. The high values for @cZHb (0.7 at 570°K in CF3COCH3 photolysis) were thought to be due to the reaction6 CH3
the conversion of t.he original electronic excitation into vibrational energy, which becomes distributed throughout the mole~ule,~ the dissociative fate of the molecule will depend upon the nature of the energy distribution and the relative C-C bond strengths. The generation of CF3 radicals in the systems CHa CF3COCF310
+ CH3COCF3 Jc
+ CF3CO
0 300
500
400
600
T,OK.
Figure 3. Quantum yields for C2FsCOC;Hs photolysis ( p = 4.5 f 0.3 cm) vs. temperature: 0, CO; 0, C2Hs; 9, C2F5H; @, C4HHlo; B, C2H4; 8, C4Hia or C2H4.
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and CH3 CF3N2CF311has been known for some timelOa$lla and has recently been subjected to careful A comparable mode of formation of butane is not reinvestigation.* o b ~ c ~ l l bFrom these observations it evident in our system. The decomposition of the may be rationalized*lb that the CF3-N and CF3-C CRCO radical thus formed leads to (PCO > 1 at tembonds are at most equal to or more likely weaker peratures greater than 470°K. In the C~FSCOCZHS than the corresponding CH3-N and CH3-C bonds. l 2 system, (PCO only approaches unity, within experimental (6) R. A. Sieger and J. G. Calvert, J. Am. Chem.Soc., 76,5197(1954). error, a t 570'K. (7) G. 0.Pritchard and J. T. Bryant, J . Phys. Chem., 70, 1441 The temperature dependence of the h0is typical (1966). of that for simple aliphatic ketones, other than (CFs(8) E. A. Dawidowicz and C. R. Patrick, J. Chem. Soc., 4250 (1964). H)2C0,7where after the initial electronic excitation (9) P.Seybold and M. Gouterman, Chem. Rev., 65, 413 (1965). @CO will depend upon the extent to which radiative (10) (a) 0.0.Pritchard and E. W. R. Steacie, Can. J . Chem., 35, 1216 (1957); (b) W. G. Alcock and E. Whittle, Trans.Faraday SOC. and radiationless transitions occur, and the lifetimes and possible reactions of C2HSCO and/or CZFSCO 61,244 (1965);(0) R.D. Giles and E. Whittle, ibid., 61, 1425 (1965): (11) (a) G. 0.Pritchard and J. R. Dacey, unpublished data, 1961; radicals. (b) L. Batt and J. M. Pearson, Chem. Commun., 575 (1965). Primay Process. We may consider the two possible (12) The effect of fluorination upon C-C bond strengths is uncertain. A recent tabulation (E. Tschuikow-Roux, J . Phys. Chem., primary processes 69, 1075 (1965))puts D(CFa-CF3) in the range 65 to 95 koa1 mole-', (CHJ2COCF3 +C2He
CzFsCOCzHs
+ hv +C2FsCO + CzHs
+ CZF5
+CZHSCO
(A) (B)
Calvert and Siegers favored a type-A decomposition, but in a recent reinvestigation of CF3COCH3 photolysis6 the identification of biacetyl in the reaction products certainly indicates the importance of a typeB process
compared t o D(CHa-CHa) 84 kcal mole-'. Recent evidence (I. P. Fisher, J. B. Homer, and F. P. Lossing, J. Am. C h a . Soc., 87, 957 (1965); J. P. Simons, Nature, 205, 1308 (1965)) puts D(CFz= CFZ)at 70-80 kcal mole-1, and one would expect the single bond in C2F6 t o be weaker than the double bond in CZF4, which indicates a low value for D(CFa-CF3). Cottrell (T. L. Cottrell, "The Strengths of Chemical Bonds," Butterworth and Co. Ltd., London, 1958) quotes the average C-C internuclear separation in the ground state as 1.31 and 1.52 A for ClF4 and CZF6, respectively. Chlorination of ethane leads t o a weakening of the C-C bond; D(CClS-CCla) has recently been quoted as 68.4 kcal mole-' (G.J. Martens and G. H. Huybrechts, J . Chem. Phys., 43, 1845 (1965)).
Volume 70,Number 7 July 1966
R. L. TIIOMMARSON AND G. 0. PRITCHARD
2310
The rupture of a particular C-C bond requires the instantaneous localization of sufEcient energy in the bond. The increased vibrational energy capacity of perfluoroalkyl groups over alkyl groups can be imagined to cause an excess of vibrational energy adjacent to the perfluoroalkyl groupcarbon bond, facilitating the localization of suEcient energy for decomposition in that bond. This would add to the probability of the type-B ketone photodecomposition and the CF3 generation reactionslo*llthat have been observed. It should be noted that our ethane yields exceeded the ethylene yields (see Figure 3) even a t room temperature, when reaction 7 is very slow. Moreover, the Arrhenius plot for kT/ki/' showed distinct upward curvature (dotted line, Figure 2) below about 100" at high intensities. This presents strong evidence for the finite existence of the c2H6co radical and the occurence of the reaction" C&
+ CzHSCO +C2He + C2H4CO
10
20
30 40 P, mm.
50
60
70
Figure 4. *CO us. ketone pressure (mm): 0, 90'; @, 45'; 0,27".
of C Z F ~ C O C ~ a tH3130 ~ A. We may write a sequence of primary eventP A
+ hv +lA*
(a)
lA* +dissociation A lA* -+ 'Ao -I- A
(b)
A ' 0 +3AO 'Ao +A hvr
(d)
1AO -+ A* 3A0+A hv,
(f)
+-
(9)
Experiments conducted a t low relative intensities (when quantum yield determinations were being made) did not exhibit such curvature. The radical-radical disproportionation (reaction 9) would be expected to be more important at high inten~ities.'~ @GO us. Pressure. This is shown in Figure 4 for the temperatures 27, 45, and 90". The decrease in @CO with increasing pressure at a given temperature may be correlated with increased collisional quenching rates of excited ketone molecules C~FSCOC~H,* and also, a t low temperatures, C2HsCO*, which is formed containing excess vibrational energy. The reaction probability of this radical is also increased at higher pressures. Before a more complete interpretation can be made, the radiative processes need to be examined. Ausloos and Murad16 have observed both fluorescence and phosphorescence in CF3COCH3 vapor following photochemical excitation at 3130 A. The participation of both singlet and triplet molecules is also evident in the photochemical excitation of C2HsCOCzH6 at 3130 A.I7 The photochemistry of the three perfluoro ketones, CFICOCF~,C Z F ~ C O C ~ and F , C3F&OC3F7, have been investigated in some detail by Steacie and his co-workers ; their photochemical behavior at 3130 A is very similar.'* Fluorescence yields were originally obtained for CF3COCF3,and, although phosphorescence was not observedl19the formation of triplet state molecules was suggested.20 More recently, phosphorescence yields have been obtained a t 3130 A It would seem that both singlet and for CFSCOCFI.~~ triplet processes will be involved in the photochemistry The Journal of Physical Chemistry
01 0
+
3A0
(4 (e>
+
(€9
dissociation
(h)
3AO +A*
(9
where IA* is a ketone molecule in a high vibrational level of the upper singlet state (A is a ground-state molecule and A* a vibrationally excited ground-state molecule), 'Ao is a molecule in a low-lying, nondissociating level of that singlet-state, and *Aois a molecule in a low-lying vibrational level of an upper triplet state from which dissociation can occur with a small activation energy.22
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(13) CZFS CtHsCO + CtFsH CzHEO will also occur, but curvature would be less evident in the ks/ka'/z plot aa ES< E7. The possibility of curvature is evident in the ks/ka'/z plot; seeref 1Oc for an observation of this nature in the CFs CHaCO reaction. (14) P. Ausloos and E. W. R. Steacie, Can. J . Chem., 33,47 (1955). (15) The participation of the CzHaCO radical in CzHsCOCzHa photolysis appears to be significantly less than in CzHsCOCzFs photolysis: see K. 0. Kutschke, M. H. J. Wijnen, and E. W. R. Steacie, J . Am. C h m . SOC.,74, 714 (1952). (16) P. Ausloos and E. Murad, J . Phys. Chem., 65, 1519 (1961). (17) D.S. Weir, J . Am. C h m . Soc., 83, 2629 (1961). (18) G. Giacometti, H. Okabe, S. J. Price, and E. W. R. Steacie, Can. J . C h m . , 38, 104 (1960). (19) H. Okabe and E. W. R. Steacie, ibid., 36, 137 (1958). (20) G. Giacometti, H. Okabe, and E. W. R. Steacie, Proc. Roy. SOC.(London), A250, 287 (1959). (21) P. G. Bowers and G. B. Porter, J . Phys. Chem., 68,2982 (1964).
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PENTAFLUOROETHYL ETHYL KETONE
2311
1
3.0 I
(OK) leading to an apparent negative activation energy of 7.5 1.5 kcal mole-', which is the energy barrier mainly associated with decomposition from the triplet state,I8 reaction h. The values of a are very similar to those given for the three perfluoroketones, CFsCOCFI, C Z F ~ C O C ~and F ~ ,CaF7COCaF7,over the same temperature range a t 3130 A, leading to apparent negative activation energies of 9.2, 8.9, and 7.8 kcal mole-', respectively.'8
*
. 2.0 -
Acknowledgment. We are indebted to Dr. Bernard Kirtman for helpful discussions. 0
1.0 2.0 lo-(/[ A], co mole-1.
3.0
4.0
-
@CO) vs. reciprocal of ketone Figure 5. @.co/(l concentration: 0, 90'; 8, 45'; 0, 27'.
Assuming that k,[A] >> k b , the following equation is obtained from the above mechanism? %0/(1 %o) = h,/k0[A](1 - a) a/(l - a), where cy = kdkh/(kd ke kf)(kg kh ki), the quantum yield a t infinite concentration.18*20Plots of %o/ (1 - @CO) vs. the reciprocal of ketone concentration are given in Figure 5. Good straight lines are obtained similar to those obtained for CzF5COCzF5at 3130 A.18 The values of (Y which may be obtained from the intercepts (0.11 at 27O, 0.26 a t 45",and 0.54 at 90") can be plotted in the of log [ ( l / a ) - 13 vs. 1/T
+ +
+
+ +
(22) This is not necessarily the correct mechanism, but it has been chosen so that we may compare directly our data with the perfluoro ketones. CFsCOCHa is presumably a better comparative model, but OCO v8. pressure data a t different temperatures are not available in the literature. (The data6 on Oco v8. pressure a t 117' show the expected quenching effect.) It is not clear that the intersystem crossover should be represented solely by reaction d, or necessarily by 'A* + SA* aa Ausloos and Murad suggest for CFsCOCHa." Presumably the singlet-triplet "leak" may occur more readily a t some other level in the vibrational cascade. Ausloos and Murad observed16 an increase in the phosphorescence with increasing pressure, compatible with the step aA* A + aA' A, but a t still higher pressures (at 3130 A) the phosphorescence decreased. They further. observed no pressure effect on the fluorescence yield, which was difficult to visualise.l6 (23) The assumption that primary quantum yield = @CO neglects the fraction of CzHaCO radicals that do not dissociate; this will be more important a t lower temperatures. This will not, however, affect the nature of the pressure dependencies as depicted in Figure 4, which are identical with those observed for CzFsCOCzFs, where there is no evidence of a nondissociative fate for the CzFaCO radical.1
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Volume 70,Number 7 July 1066
