F. H. DORERAND B. S. RABINOVITCH
1964
Unimolecular Reactions of Chemically Activated Species Produced in Systems of Methylene with Propene, Butene-1, 3,3,3=Trifluoropropene,
and 4,4,4-Trifluorobutene-11
by F. H. Dorer2and B. S. Rabinovitch Department of Chemistry, Universdy of Washington, Seattle, Washington 981 06
(Received December 68,1964)
Dilute mixtures of ketene in propene, in butene-1, in 3,3,3-trifluoropropene, and in. 4,4,4trifluorobutene-1 were photolyzed at 3200 A. and 25". The rates were measured for some isomerization processes of the chemically activated alkyl cyclopropanes that arise. Experiments were performed both in the presence and absence of 3 4 % added oxygen. The effect of oxygen on product composition is explained in terms of an earlier suggestion that the proportion of triplet methylene reaction is pressure dependent and that there is approximately 29% triplet methylene in these systems at higher pressures. Some mechanistic effects of the trifluoromethyl group on the relative rates of methylene reaction with olefins, and on various competitive rates connected with alternative modes of hot alkylcyclopropane isomerization, are noted.
Introduction The preceding paper3s,described the decomposition of chemically activated alkene products formed in methylene systems by photolysis of diazomethanealkene mixtures. The present paper compares the rates of some primary and secondary processes that occur after methylene produced by ketene photolysis has reacted with propene, butene-1, 3,3,3-trifluoropropene, and 4,4,4-trifluorobutene-l. The systems ostensibly follow the scheme CH2(lAJ
+ RCH=CH, kE
3- R--C-C~H~* + olefins* O_ olefins -% R-c-C3H5
3 olefins*
O_ olefins
where R is CH3, CzH5, CF3, and CFICHZ. The vibrationally excited cyclopropanes are formed with 100105 kcal. mole-l of energy and can undergo structural isomerization to excited olefins with a critical energy Eo 61 kcal. m01e-l.~ The vibrationally excited olefins, whether formed by isomerization or insertion (some are formed uniquely by insertion), have approxi-
-
Th,e Journal of Physical Chemistry
mately 8 kcal. mole-l more energy than the cyclopropanes; unless collisionally stabilized, they may undergo decomposition by allylic C-C bond rupture with critical energy of approximately 70 kcal. mole-'. The relative amounts of the olefin and cyclopropane products have been measured over a range of pressures in each system, both in the presence and in the absence of 3 to 8% added oxygen.
Experimental Materials. Ketene was purified by gas chromatography at -78" with a Fluoropak column.4b The hydrocarbons were Phillips research grade and were further purified by gas chromatography when necessary. The 3,3,3-trifluoropropene was prepared by two methods. First,5 an equimolar gaseous mixture of (1) Work supported by the National Science Foundation. (2) National Science Foundation Predoctoral Fellow, 1962-1964. (3) (a) F. H. Dorer and B. S. Rabinovitch, J.Phys. C h m . , 69, 1952 (1966) : (b) F. H. Dorer, Ph.D. Thesis, University of Washington. (4) (a) D. W. Setser and B. S. Rabinovitch, Can. J . Chem., 40, 1425 (1962) : (b) J. W. Simons and B. S. Rabinovitch, J.Phys. Chem., 68, 1322 (1964) ; (c) J. W. Simons, B. S. Rabinovitch, and D. W. Setser, J . Chem. Phys., 41, 800 (1964). (5) R. N. Haszeldine, J . Chem. SOC.,2856 (1949).
1965
UNIMOLECULAR REACTIONS OF CHEMICALLY ACTIVATED SPECIES
trifluoromethyl iodide and ethylene was photolyzed with a G.E. AH-6 high pressure lamp in a Pyrex reactor having a bottom collection tip at 0" which was shielded from radiation. The product was dehydroiodated with solid KOH. The propene was also synthesized by the reaction of SF4with acrylic acid in a stainless steel vessel at approximately 20 atm. and 100".'j After purification by gas chromatography, the principal product from both methods had the same infrared spectra and a mass spectral pattern with a parent peak at mass 96. 4,4,4-Trifluorobutene-l was prepared by SF, reaction with 1-butenoic acid. Yields were approximately 50% of the starting acid. The gaseous product was purified by gas chromatography. Its infrared spectra showed the vinylic CH stretch and C-C double bond stretch frequencies reported by Haszeldine.? The product had a mass spectral parent peak at mass 110. Apparatus and Procedures. Pyrex reactors, ranging in sizes from 1 to 5000 c n ~ . were ~ , pumped to a t least mm. on a conventional vacuum apparatus before being loaded with a 1: 10 ketene-olefin mixture. Reactants were photolyzed for 1 to 2 hr. with an AH-6 lamp fitted with a quartz water jacket and a filter of a NiS04-CoS04.7H20 solution circulating through a Corex D glass envelope. The average wave length absorbed by the ketene was 3200 Product Analysis. Condensable products were analyzed by gas chromatography. The methylene-propene products were separated on a column of silver nitrate-ethylene glycol on firebrick. This column did not separate isobutene from the propene so that aliquots of the products were also run on a hexamethylphosphoramide firebrick column. The methylenebutene-1 products were analyzed with the silver nitrateglycol column followed by a 2,4-dimethylsulfolane firebrick column. The fluorocarbon systems were analyzed on the 2,4-dimethylsulfolane column followed by a dibutyl phthalate column. The hydrocarbon analyses were calibrated with known samples of each product. Peak area calibrations for several compounds were used as the basis of the measurement of the amounts of fluorocarbons.
Results Ketene-Propene
Systems. The isomerization of chemically activated methylcyclopropane in oxygenfree systems has previously been studied by Butler and Kistiakowsky8 (BK). The present results for the 3200-A. photolysis of ketene in propene, with and without 3 to 8% added oxygen, are illustrated in Figure 1.
The mechanism and notation of the preceding paper
40
20
o X 10-8, sec.-1. 60 80 100
120
140
160
h
3
0.3 A
0.2 0.1
10
20
30
40
50
70
60
80
90
P, cm.
Figure 1. The variation of the C4H8product composition with total pressure for the ketene-propene photolysis system. Open points are pure system results and filled points are results for 5 to 10% oxygen added to the photolysis mixture: methylcyclopropane, 0; butene-1, A; trans-butene-2, 0; cis-butene-2, 0;isobutene, V.
are used. If olefin decomposition were unimportant, the steady-state assumption applied to the hot species would lead to total butenes =y l methylcyclopropane w (butene), inethylcyclopropane -
+
I.(& + w ka
2)+
ki ka
2) 2 +
(1)
(2)
Here ki,/kd is the rate of each type of CH insertion relative to double bond addition by the methylene, and 3
ki
=
Ck
ij.
The ratio k,,/k, represents the fractional
j= 1
rates of formation of the butene isomers by methyl cyclo3
propane isomerization, and ka =
k,,.
The same
j=1
olefins are formed by both insertion and isomerization : butene-1, butene-2, and isobutene ( j = 1, 2, 3, respectively). Figure 2 presents plots of [total butenes/ methylcyclopropane] vs. u-l for both the pure and oxygen systems. From eq. 1, the slopes of these lines ~~~
(6) The sulfur tetrafluoride and some helpful literature were generously supplied by the DuPont Chemical Co.; W. C. Smith, Angew. Chem. Intern. Ed. Engl., 1,467 (1962); W.R.Hasek, W. C. Smith, and V. A. Engelhardt, J . Am. Chem. Soc., 82, 543 (1960). (7) R.N.Haszeldine, J. Chem. SOC.,2040 (1954). (8) J. N.Butler and G. B. Kistiakowsky, J . Am. Chem. SOC.,8 2 , 759 (1960).
Volume 69,Number 6
June 1966
F. H. DORER AND B. S. RABINOVITCH
1966
Table I : The Rate Constants and Rate Ratios for the Ketene-Propene System -1someriaationa Pure system Pure system (BK) Oxygen system
Insertion’
ka>see. -1
ka/L
kadka
kadks
kil/kd
kdkd
kia/kd
3 . 4 x lose 8 . 2 X 10sd 7 . 2 X lo8’
0.30 0.53 0.27
0.47 0.45 0.46
0.23 0.18 0.28
0.063 0.060 0.097
0.039 0.035 0.070
0.035 0.029 0.070
Results of least-squares analysis of [(b~tene)~/methylcyclopropane] us. w - l curves.% Low pressure scatter of the oxygen data introduces an estimated uncertainty of &25% in those results. Experimentally measured (per bond) ratios at high pressure. c k, calculated by setting butene-l equal to the measured butene-2 from isomerization; the change incurred, relative to value based on the measured butene-l, averaged +5%. Value of BK has been converted to the collision diameters used here.
’
The rate constant, ka, included in Table I was calculated by correcting the butene-l count for the amount that has decomposed, assuming that kal/ka = kadka.
0.08
0.16 w-1
x
0.24
0.32
0.40
108, 880.
Figure 2. The variation of the butene/methylcyclopropane ratio with for ketene photolysis in propene. The open circles are pure system results; the closed circles are results for 5 to 10% oxygen added.
are proportional to k,. Table I tabulates the results obtained from Figure 2 and plots of [(butene)j/ methylcyclopropane] vs. u-l plots3b(not shown). The results obtained by BK from the 3100-A. photolysis of ketene in propene are included for comparison. The disagreement between the total k, values found here and by BK for the pure system cannot be reconciled by the difference in photolysis energy. (It may be noted that the present value at 3200 A. accords better with the k, value determined at 2600 8. by BK than does their value a t 3100 8.) However, the discrepancy between the two measurements of kal/ka is due a t least in part to the process CH&HZCH=CHz
+ CH3 *
+ .CH&H=CHz
Butene-1 and butene-2 should probably arise in nearly equal amounts from methylcyclopropane, as suggested by the thermal isomerization. The present measurements included lower pressures than BK, and some butene-1 decomposition occurs. The observed rate of isomerization to butene-1, ka, compared to butene-2, ke2 (Table I): may reflect preferential loss of butene-1. The Journal of Physical Chemistry
The oxygen results for the kaj/ka are reliable only to *25%, due to the excessive scatter and sparsity of the data for [ (b~tene)~/methylcyclopropane]ratios below 60 mm. The relative amount of insertion to addition at higher pressures was found to change upon the addition of oxygen (Table I). Similar behavior has been noted previously a t higher pressures in a cis-butene-2-ketene system4b; the effect of added oxygen in that system was attributed to the presence of 29% triplet methylene. Ketene-Butene-1 Systems. Figure 3 gives the pressure dependence of the percentage of each of the principal
2.0
o X 10-7, see.-]. 4.0
x’,-.0.2 Gw
’-
o‘2 1.0
2.0
200
600
P, mm.
Figure 3. The variation of the C~HIO product composition with total pressure for the ketene-butene-1 photolysis system. Bottom figure gives the pure system results and the top figure gives the results for 5 to 10% oxygen added to the photolysis mixture: ethylcyclopropane, 0; pentene-1, A; trans-pentene-2, 0;cis-pentene-2, n; 3-methylbutene-1, X ; 2-methylbutene-1, V. (9) (a) D. W. Setser and B. S. Rabinovitch, J . Am. Chem. SOC.,86, 564 (1964); (b) J. P. Chesick, ibid., 82, 3277 (1960).
UNIMOLECULAR REACTIONS O F CHEMICALLY ACTIVATED SPECIES
1967
Table 11: The Rate Constants and Rate Ratios for the Ketene-Butene-1 System
-1somerizationc--. k,, aec.
--la
4 . 1 X 106 3 . 6 X lob
Pure system Oxygen system
Insertiond
r
7
kap aec. - I b
kdka
kadka
kadka
kidkd
kidkd
kdkd
kidkd
5.8 X 106 6 . 4 X 106
0.13 0.00
0.70 0.85
0.15 0.15
0.077 0.120
0.075 0.070
0.045 0.070
0.095 0.125
Rate constants from least-squares analysis of the curves of Figure 4 which ignores all olefin decomposition. * Rate constants based on the most stable olefin, pentene-2, and calculated from the [pentene-2/ethylcyclopropane]us. w - l curve.3b This estimate of ka can These results are from least-squares analysis of the still be as much as a factor of 1.6 too low because of pentene-2 decomposition. [(pentene)j/ethylcyclopropane]8s. w - 1 curves.sb These numerical results serve t o indicate that there is extensive olefin decomposition These are experiin this system; they do not reflect the true relative rates for olefin formation by ethylcyclopropane isomerization. mentally measured (per bond) quantities a t high pressure.
is being formed by some reaction in addition to free radical processes. Similar behavior was displayed by these products in the diazomethane-butene-1 system.3 Unfortunately, explicit values of koa, could not be obtained from the data. The [total pentene/ethylcyclopropane] 8s. ~ 3 - l curve (Figure 4) is roughly linear throughout the pressure region of the measurements in both the pure and oxygen systems. Only those olefins that can arise from both the insertion reaction and the isomerization of ethylcyclopropane are (or need be) included in the figure. The slopes of the curves in Figure 4 are naive measures of k , (eq. l), which neglect the complication of olefin decomposition; ie., they yield an apparent (too low) k, value.
CsHlo products of the total CSHI0products for the oxygen-free system, and for the system with 4 to 8% oxygen added. It reveals that a t low pressures olefin decomposition and isomerization also occur in ketene systems. This behavior is shifted to lower pressures than for diazomethane, corresponding to the lower energy of methylene as produced by ketene phot~lysis.~ At high pressure, oxygen again has the effect (Table 11) of altering the [insertion/addition] ratios from the pure system values. A more detailed analysis of the effect of oxygen in the propene and butene-1 systems is postponed to the Discussion. The general mechanism of the reaction of methylene with butene-1 has been outlined in the preceding paper3; the same notation is continued here
CH2
+ CH3CH2CH=CHz
2.4
5 CH~CH~CH~CH=CHZ*
"
3
-% CH3CH2CH=CHCH3*
2 CH~CHZ(CH~)C=CH~* -%
(CH3)2CHCH=CH2*
kd
+ C~H~-C-C~H~*
The products of hot ethylcyclopropane isomerization are I
CzH5-c-C3H5* -% CH3CH2CH2CH=CH2*
-%
CH3CH2CH=CHCH3*
kaJ_ CH3CH2(CH3)C=CH2* Figure 3 shows that although the pentene-2 qualitatively conforms t o the simple mechanism, the pentene-1 appears to be decomposing as fast as it is formed a t low pressures. The increase of 3-methylbutene-1 a t low pressures in the pure system, as well as the (reduced) increase in the oxygen system, indicates that it
0.4
0.8
1.2 1.6 X 107,seo.
2.0
2.4
2.8
3.2
Figure 4. The variation of the pentene/ethylcyclopropane ratio with w - 1 where only those pentenes that arise by both insertion and ethylcyclopropane structural isomerization are included. The open circles are pure system results and tmhe closed circles are results for 5 to 10% oxygen added.
The experimentally determined rate constants and rate ratios from these data are tabulated in Table 11. Since pentene-2 decomposes more slowly than the other pentene products, a rate constant, k,, was calculated Volume 69, Number 6
June 1966
F. H. DORER AND B. S. RABINOVITCH
1968
from the [pentene-2/ethylcyclopropane] pressure dependence assuming that the pentene-2 is produced by the ethylcyclopropane in the same ratio as the butene2 is produced by chemically activated methylcyclopropane isomerization.6 This value is also included in Table I1 for both systems. It is a lower limit to the true rate. Ketene-S,S,S-Tri$uoropropene Systems. Figure 5 shows the pressure dependence of the five major products of the two series of runs over the pressure range 0.4 to 360 mm. The principal insertion reactions of methylene on trifluoropropene are simplified in that methylene apparently does not insert across C F bonds,lo an observation that was confirmed here. The insertion and addition reactions are CH2
+ CF,CH=CH2 kiC
x
10-7,B B C . - ~ . 30
20
10
200 400 600 I/
0.8
0.6
0.4
-6
0.2
4
G
Gw
a # 2
'_I
1.0
0.8 0.6
0.4
CF3CH=CHCH3* (cis and trans)
-4
0.2
CF3(CH3)C=CH2* '-
kd, CF3--c-C3Hs* The isomerization reactions of the trifluoromethylcyclopropane are CF~-C-C~H~*
k.c kat k"C
CF3CH&H=CH2* CF3CH=CHCH3* (cis and trans) CF3(CHa)C=CHz*
Each of the five products gave a mass spectra1 parent peak at mass 110. The 2-trifluoromethylpropene and 4,4,4-trifluorobutene-l were further identified by comparison of their g.1.p.c. retention times with known samples. The major product a t high pressure was unaffected by bromine (in contrast to the other products) and was identified as trifluoromethylcyclopropane. The two remaining unidentified products, the butene-2 pair, could be isomerized to each other by the technique'l of addition of -1% HzS and heating to 450" for 3 min. Their identity as the butene-2 isomers was confirmed by comparison of their g.1.p.c. retention times with those of the gaseous products resulting from dehydroiodation of the liquid produced by trifluoromethyliodide photolysis in propene. The pyrolysis of trifluoromethylcyclopropane supported the above product assignment.12 The reaction of methylene with trifluoropropene appeared very clean a t all pressures. Only one extra product in the C4 to Cg hydrocarbon range was obThe Journal of physical Chemistry
10
20
200
400
P, mm.
Figure 5. Variation of the CJ?sHs product composition with total pressure in the ketene-3,3,3-trifluoropropenesystem. The bottom figure gives the pure system results where, for several runs, only the total of the butene-1 and trans-butene-2 were measured; the top figure gives the results for 5 to 10% oxygen added: trifluoromethylcyclopropane, 0 ; trifluorobutene-1, A ; trans-triffuorobutene-2, 0; cis-triiluorobutene-2, 0 ; 2-trifluoromethylpropane, V.
served and at most it amounted to only 3% of the olefin products. No CZFSwas observed a t any pressure. Olefin decomposition is therefore not important for this system in the pressure region of these measurements. Figure 6 presents the [total butene/trifluoromethylcyclopropane] vs. w-l curve for the two systems; the measurements in the oxygen-free system extend to lower pressures. Data obtained by least-squares analysis of these and the [ (butene),/cyclopropane] os. 0-l plots3b (not shown) are tabulated in Table 111. The sparsity of the data limit the accuracy of the ka,/ k. value for the pure system. The effect of oxygen, as measured a t high pressure, was again to increase the relative importance of the insertion reaction a little, although it had little effect on the k, values here be(10) B. A. Grzybowska, J. H. Knox, and A. F. Trotman-Dickenson, J . C h a . SOC.,746 (1963). ( 1 1 ) F. S. Looney and B. S. Rabinovitch, unpublished work. (12) D. W.Placzeckand B. S. Rabinovitch, J . Phys. Chem., in press.
UNIMOLECULAR REACTIONS OF CHEMICALLY ACTIVATED SPECIES
1969
Table III: Rate Constants and Rate Ratios for the Ketene-Triiuoropropene System
Pure syetem Oxygen system
k.,aeo.-t
h/k8
kadk
Wk.
Wka
kidkd'
ha/h
x x
0.26 0.28
0.57
0.05 0.03
0.00 0.00
0.026 0.033
0.015
0.68
3.6
4.1
107
107
0.030
These are high pressure measured ratios per CH bond.
X 101,sec.-1. 0.8 1.2 1.6
w
0.4
0
0.8
0.4 0-1
1.2
1.6
cause of the relatively small amounts of insertion that occur in these systems in any case. K e t e n e - 4 , 4 , 4 - T ~ ~ ~ u o ~ o bSystems. u ~ e - l Figure 7 illustrates the pressure dependence of the principal products produced in both the oxygen and oxygenfree systems. Only the total of trifluoropentene-1 and 2-methyltrifluorobutene-1 was determined. The principal addition and insertion products of methylene here are
+ C F ~ C H Z C H = C H ZCF&HzCH=CHCHa* ~ -% CF3CH2(CH3)C=CH2* kit CF3(CH3)CHCH=CHz kd, CF3CH-eC3H6*
The isomerization reactions of the trifluoroethylcyclopropane are CF&Hz-c-C&s*
1200
X 107, see.
Figure 6. Variation of the trifluorobutene/ trifluoromethylcyclopropaneratio with 0-1 for the ketene-tdluoropropene photolysis system. Open circles are pure system results and closed circles are results for 5 to 10% oxygen added.
CHZ
400
k.t CF3CH2CHzCH=CH2* -k CF&H&H=CHCH3 5 CF3CH2(CHJC=CHZ*
0.2
0.2
0.4
0.6
0.8
200
600
P, mm.
Figure 7. Variation of the CPaH7 composition with pressure
in the ketene-4,4,4trifluorobutone-lphotolysis system. The top figuregives the pure system results and the bottom figure gives results for 5 to 10% oxygen added: trifluoroethylcyclopropane, e ; 5,5,5-tdluoropentene-l Zmethyl-4,4,4tdluorobutene-l, A ;truns-5,5,5trifluoropentene-2, 0;cis-5,5,5-t~uoropentene-2,0 ; 3-methy1-4,4,4trifluorobutene-1,X
+
.
Identification of the principal products was made by comparison of g.1.p.c. retention times of similar and known compounds, their mass spectra, and their behavior with bromine. Hot olefin decomposition does occur in these systems although not as extensively as in the analogous hydrocarbon systems. A run at 0.28 111111. with diazomethane as the methylene source (S/D for trifluoroethylcyclopropane was -l),I8 and analyzed on a 91.5-m. squalane capillary column, revealed ten products besides the principal products in the Cd to Ce hydrocarbon range, (13) F. H. Dorer and
B. S. Rabinovitch, J. Phys. Chem., 69, 1973
(1965).
Volume 69, Number 6 June 1966
F. H. DORERAND B. S. RABINOVITCH
1970
Table IV : Rate Constants and Rate Ratios for the Ketene-Trifluorobutene-1 System
Pure system Oxygen system
1 . 5 X 106 2 . 0 x 106
0.46 0.44
0.52 0.50
0.00 0.00
0.040
0.016
0.050
0.023
0.013 0.015
These values are too low by -20% due to neglect of olefin decomposition. These ratios are from the respective [(trifluoropenpentene)j/trifluoroethylcyclopropane] va. w -l curves. Their numerical values are only of qualitative significance because of olefin These are ratios per CH bond measured at high pressure. decomposition.
and no other side products through C,. The primary products of interest constituted 7 0 4 0 % of the total products. This is in contrast to the 28 extra (higher) products that constituted 50% of the total in the diazomethane-butene-1 system at these pressures a t which olefin decomposition was extensive3; side reaction went essentially to zero at high pressures. Butene-1 plus butadiene (not separated by analysis) was the major side product in this fluorocarbon system and it constituted up to 7% of the total products. Hexafluoroethane, trifluoropropene, and a small amount of a product suspected of being hexafluorobutane (CF3CH2CH2CF3) were observed. It is difficult to ascertain if the processes
+ .CH~-C-C~H~ CF3CH2. + *-c-C~H~
1.6
1.2
e
1-I
CF~CH~-C-C~HS* + CF3. 4
take place a t all. Figure 8 shows the [total olefin/cyclopropane] vs. W-1 plots for these systems. Detailed plots of [(olefin),/cyclopropane ] are given elsewhere.rb The data obtained from these plots using the measured high pressure limits are recorded in Table IV for both the pure and oxygen systems. These k, values are probably low by a factor of -1.2 due to olefin decomposition.
Discussion E j e c t o j Oxygen. I n all systems studied here, the relative rate of insertion to addition by the methylene generally increased (the exact amount depending on the bond type) when 5-10% oxygen was added to a reaction mixture (Tables I to IV). The effect of oxygen on the high pressure product composition in the ketene-cis-butene-2 and ketene-cis-butene-2-ds systems has been attributed earlier to the presence of -29% triplet methylene.4b The high pressure product composition resulting from triplet methylene (produced by mercury-photosensitization of ketene) reaction with butene-114 is compared in Table V to the measured high pressure ( p > 170 mm.) product composition of the ketene-butene-1 (3200 A.) photolysis The Journal of Physical Chemistry
0 w-1
2.0 X 107, 880.
4.0
6.0
Figure 8. Variation of the trifluoropentene/ trifluoroethylcyclopropane ratio with 0-1 where only those trifluoropentenes that arise by both insertion and trifhoroethylcyclopropane structural isomerization are included. Open circles are pure system results, closed circles are results for 5 to 10% oxygen added.
system. Not unlike an earlier comparison for ketenecis-butene-2 agreement here between the oxygen results and the pure system results as corrected for the presence of -29% triplet methylene is striking (even to the extent that although over-all insertion becomes more important in a pure singlet system, the percentage of pentene-2 product decreases). Earlier, we suggested3 that since the percentage of apparent "insertion" products decrease in the triplet methylene-olefin systems,14-16at least part of the pressure dependence of the insertion/addition ratio found in the diazomethane-olefin photolysis systems is due to an increasing concentration of triplet methylene with increase of pressure a t high pressures. Indications are (14) F. J. Duncan and R. J. Cvetanovi6, J . Am.. Chem. SOC.,84, 3593 (1962). (15) F.J. Duncan and R. J. Cvetanovi6, private communication. (16) H. M. Frey, J . Am. Chem. SOC.,82, 5947 (1960).
UNIMOLECULAR REACTIONS OF CHEMICALLY ACTIVATED SPECIES
Table V : Product Composition (yo)for Reaction of Singlet and Triplet Methylene Radicals with Butene-1
Ethylcyclopropane Pentene-1 cis-Pentene-2 trans-Pentene-2 2-Methylbutene-1 3-Methylbutene-1
Tripleta
Exptl.*
79.2 4.8 4.1 7.3
61.3 14.0 3.4 5.6 3.2 11.8
...
4.8
CorrectedC singlet Oz resultsd
54.5 18.0 3.1 5.0 4.6 14.8
54.8 19.7 2.7 4.8 4.0 13.7
Triplet methylene reaction with butene-1 data is from the mercury-photosensitization of ketene in butene-l (ref. 14 and private communication). * Experimental results measured for the pure system a t high pressures. Data for pure system corrected assuming the presence of 29y0triplet methylene by using column 1. Experimental results from this work for system with 5-1070 oxygen added.
that the proportion of triplet methylene is pressure dependent in the ketene-cis-butene-2 systems also4b; similar dependence, although not unexpected, could not be demonstrated as explicitly in the present systems because of the uncertainty in the true values of the relevant k , and k", quantities. Another manifestation of the pressure dependence of a triplet methylene component is that the rate constants for structural isomerization of cyclopropane,17 and of methylcyclopropane studied here, are enhanced by a factor of at least 2 when 5 to 10% oxygen is added to the system. These systems exhibit relatively high k , values and were studied at pressures above 10 mm. in general. By contrast, the k , values for the slower structural isomerization reactions of trifluoromethylcyclopropane and of dimethylcy~lopropane,~~ and studied below 10 mm., are unaffected (or even decreased by -30%) by the presence of oxygen. Thus, in the lower pressure region triplet methylene effects seem again to be substantially decreased. The quantum yield of photolytic decomposition of pure ketene is pressure dependent even at 3130 A,18919; this suggests that some of the present behavior could conceivably be due to changing average energy of the formed methylene with total pressure of the system due to collisional vibrational deactivation of excited singlet ketene.lg However, the pressure effectsin our systems cannot be adequately explained by a possible variation of the average energy of the reacting methylene and therefore these results support the possibility of a collision-induced singlet-triplet transition of the excited ketene followed by triplet methylene formation, as suggested earlier.4b Trifluoromethyl Influence on Insertion/Additwn Ratios. It has been reported that fluorine substitution
I971
in ethylene makes the double bond and the remaining CH bonds less susceptible to reaction with methylene. 2o The data of ref. 13 have shown that a trifluoromethyl group adjacent to a double bond does not cause a decreased rate of methylene addition. However, the present data (Tables I to IV) reveal that the presence of a trifluoromethyl group does decrease the rate of methylene insertion into CH bonds relative to double bond addition. The extent of deactivation of the CH bond depends on its proximity to the triflu0romet)hyl group. The closer it is to the trifluoromethyl group the greater is its deactivation; this becomes as great as a factor of 7 to 8 in the pure and oxygen systems for the relative rate of allylic CH insertion to addition for trifluorobutene-1, as compared to the allylic CH insertion to addition ratio for butene-1, i e . , (kil/kd)H/(kiJ k d ) N ~ 7-8. Such deactivation is plausible in terms of the electrophilicity of methylene. The Relative Rates of OleJin Formation by A l k y l Cyclopropane Isomerization. Table VI gives the ratios for the relative rates of olefin formation by methylcyclopropane and trifluoromethylcyclopropane. Due to the sparsity of the data in several cases and the experimental complications, these results are only of a semiquantitative nature. They do indicate, however, that the relative probability for formation of 2-trifluoromethylpropene by trifluoromethylcyclopropane isomerization is less than the relative probability of isobutene formation by methylcyclopropane, i.e., (ka3/ ka)R/(kaB/ka)F = -4-8 for pure and oxygen systems. The activation energy for isobutene formation from methylcyclopropane isomerization is greater than that for the formation of the other butene isomers9 and these data suggest that the activation energy difference is greater yet for trifluoromethylcyclopropane isomerization.
Table VI: Comparison of the Relative Rates of Olefin Formation by Methyl Cyclopropane" and Trifluoromethylcyclopropane
Pure system Oxygen system
0.7 0.6
1.5 1.4
4 8
' Corrected for butene-l decomposition.
~~
(17) J. W. Simons, Ph.D. Thesis, University of Washington, 1964. (18) B. T. Connelly and G. B. Porter, Can. J . Chem., 36,1640 (1958). (19) G.B.Porter and B. T. Connelly, J . Chem. Phys., 33,81 (1960). (20) A. F. Trotman-Dickenson, Proc. Chem. SOC.,249 (1964).
Volume 69,Number 6 June 1966
F. H. DORERAND B. S. RABINOVITCH
1972
The data of Table I11 show that the probability of formation of the trifluorobutene-2 is more than twice as great as the formation of trifiuorobutene-1, i e . , (ka2/k& 2. This reflects a tendency for H atoms to transfer to the C atom farthest from the trifluoromethyl group (both butene isomers arise by rupture of the same C-CJ bonds and involve H transfer to different C atoms with the same reaction path degeneracy). The corresponding ratio is unitya for methylcyclopropane. Decomposition of the pentene products makes a comparison like that shown in Table VI impossible for the ethyloyclopropane and trifluoroethylcyclopropane systems. However, the ratios kaj/k,, in the trifluoroethylcyclopropane system (Table IV), even though uncorrected for olefin decomposition, still show that the relative rate of formation of the trifluoropentenes by the isomerization reaction is more statistical (the statistical ratio (kal k&)/ka2 is 1.5, experimental is 0.9) than the analogous trifluorobutene isomer formation by trifluoromethylcyclopropane isomerization for which (kal k,3)/ka2 = 0.5 (Table 111)). Removal of the trifluoromethyl from the reaction site by one -CH2 group thus reduces its mechanistic influence by a factor of almost 2.
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The Journal of Physicel Chemistru
Total Cyclopropane Structural Isomerization Rate Constants. Unfortunately, the use of a naive measure of olefin formation (D)in order to obtain the total rate constant, k,, does not yield quantitative results for molecules larger than the methylcyclopropanes because of the “hot” olefin decomposition. Other methods that do not depend on a direct count, of D have been used to obtain the k, quantities for the larger cyclopropanes and are reported in a later paper.13 Since butene-1 decomposition is small and can be corrected in the methylcyclopropane system, while fluorobutene decomposition is negligible in the pressure region of the measurements for trifluoromethylcyclopropane, their measured rate constants, k , ~and k . ~ ,are fairly reliable. The rate constants measured here for these two systems (without oxygen) are a factor of -3 lower than those obtained for these molecules when diazomethane is the methylene precursor’3; this is about the difference expected due to the differA ence in the methyIene energy for the two s o ~ r c e s . ~ more quantitative experimental investigation and a theoretical treatment of the rate constants for alkyl and trifluoroalkylcyclopropane structural isomerization reactions is given in the following paper.13
