1076
J. W. SIMONS, B. S. RABINOVITCH, AND E’. H. DORER
The Rates of Decomposition of Chemically Activated Propylene1
by J. W. Simons, Department of Chemistry, N e w Mexico State University, University Park, N e w Mexico
88070
B. S. Rabinovitch, and F. H. Dorer Department of Chemistry, University of Washington, Seattle, Washington 98105
(Received September
Isv1965)
The decomposition of chemically activated propylene formed by the addition of methylene to ethylene has been studied. Decomposition into H allyl is characterized by a looser allyl. complex than that found earlier for decomposition of RCH2CH=CH2 into R This behavior is compared with that in related systems. Decomposition into CHs vinyl also occurs as a minor reaction. The combination rate of allyl radical with H atoms is intermediate between those of alkyl radicals and of alkenes with H atoms.
+
Introduction The study of the reaction kinetics of chemically activated species2 has been extended recently to the decomposition of alkenes.3 In the latter work it was shown that the decomposition into allyl plus alkyl radicals by butene-1, pentene-1, hexene-1, and several other higher alkenes, produced by the isomerization of various substituted cyclopropanes, proceeded at a rate suggestive of only a semiloose activated complex. To express it otherwise, the recombination rate of allyl and alkyl radicals does not involve as loose a complex as that for recombinations of alkyl radicals of compar0.12, able size4 which proceed with a specific rate, K where z is the specific collision rate; instead, the value 5 X lo-% W R S found. In a recent study of the collisional deactivation of vibrationally excited cy~lopropane,~ it was found that the decomposition of the resultant chemically activated isomerization product, propylene, became significant at low pressures. It seemed of considerable interest to compare this decomposition reaction, CH3CH= .CH2CH=CH2, with that of the higher CH2 -+ H alkenes; as is well known,6 the rate of association of H plus alkene (or the reverse decomposition) differs considerably in character from the analogous reaction of methyl, or other alkyl radicals, with alkenes. I n the present work, more-detailed low-pressure data and a quantitative treatment of the decomposition of chemically activated propylene produced by the reactions of methylene radicals with ethylene are re-
-
+
The Journal of Physical Chemistry
+
+
ported. Both ketene and diazomethane have been used as sources of methylene radicals.
Experimental Section The experimental procedure and analysis was the same as that described previo~sly.~”Product identification and analysis was made by glpc and mass spectrometry. Toward the end of the work, it was discovered that the pentene-1 analyses in both the ketene and diazomethane systems included a contribution from an unidentified compound having the same retention time as pentene-1 on the analytical column employed. Calibration runs ruled out allene, 1,3-butadiene, and other pentene isomers as the unknown. The glpc peak cor(1) This work was supported by the National Science Foundation and Office of Naval Research through the University of Washington. (2) (a) B. 5. Rabinovitch and R. W. Diesen, J . Chem. Phys., 30, 735 (1959); (b) B. S.Rabinovitch and M. C. Flowers, Quart. Rev. (London), 18, 122 (1964); (c) B. S. Rabinovitch and D. W. Setser, Advan. Photochem., 3, 1 (1964). (3) F. H. Dorer and B. S. Rabinovitch, J . P h y s . Chem., 69, 1952 (1965). (4) (a) A. Shepp and K. 0. Kutschke, J . Chem. Phys., 26, 1020 (1957); (b) G. Z. Whitten and B. S. Rabinovitch, J . P h y s . Chem., 69,4348(1965). ( 5 ) (a) J. W. Simons, B. S. Rabinovitch, and D. W. Setser, J . Chem. P h y s . , 41, 800 (1964); (b) D. W. Setser, B. S. Rabinovitch, and J. W. Simons, ibid., 40, 1751 (1964). (6) E. W. R. Steacie, “Atomic and Free Radical Reaction,” Reinhold Publishing Corp., New York, N. Y.,1954. (7) J. W. Simons and B. S. Rabinovitch, J . P h y s . Chem., 68, 1322 (1964).
RATESOF DECOMPOSITION OF CHEMICALLY ACTIVATED PROPYLENE
responding to pentene-1 was collected from a number of lower pressure runs and was reanalyzed on a boiling point column which resolved the two peaks. This analysis showed that at least two-thirds of the original peak was pentene-l.
Results and Discussion Analytical and Other Errors. The maximum error in the rate constant for propylene decomposition, due to the error in pentene-1 analysis, corresponds to only -20% if all correction is omitted. Small amounts of ethane, propane, and n-butane impurities in the ketene and, especially, diazomethane precursors could cause an overestimate of the rate, particularly at higher pressures, but becoming less important at low pressures. In fact, a very high value of the ratio, ethane/butane, in the diasomethane system is most easily explained as due to ethane impurity. It is felt that the most reliable measure of the experimental rat(: was obtained at limitingly low pressures; the higher pressure rate constants are considered unreliable. Ketene-Ethylene System. In Figure 1 the variation of the product composition with pressure is shown for ketene photolysis at -3200 A in -1 : 10 admixture with ethylene. The observed products and their variations with pressure arc1 adequately explained by the mechanism scheme CHZCO 1CH2
3200 A
'CHz
+ C2H4
----f
propylene*
ka v* -+ propylene*
(D)
(SI
W ' V
-% H + allyl O_ propylene
(Dh) (Sh)
followed by H
+ CzH4
+ allyl
CzH5
+CzHs
----f
pentene-1
+ C3H4 CzH4 + C3Ha
+ CzHe +
CH8
+ allyl
butene-1
2CzHs ----) n-butane --+
CHs
-
CzHs 4- CzH4
+ CzHs
-
CH4
+ CzK
(8b)
2CH3 +CzHs
(9)
2(allyl) -+-CeHlo
(10)
where the asterisk indicates internal vibrational energy in excess of that required for a decomposition reaction and w is assumed to be equal to the collision frequency (LO= 1.52 X 10sp (cm) sec-'; see ref 5). The possible origin of the methyl radicals (eq 6c, 8, and 9) in methylene radical systems has been considered previously.*b This source is not important in ethylene systems and reactions 6c, 8, and 9 are insignificant here; they will be considered again later.
ao
I
+ CO
+v*
propylene*
1077
C3Hs
Figure 1. Variation of product percentages with pressure in the ketene-ethylene system: 0, propylene; 0 , cyclopropane; ,. pentene-1; A, n-butane; 0,butene-1; A, ethane.
Reactions 3a and 3b are more correctly represented by multistep processes but simple, single-step representations are sufficient for present purposes. Reactions 1-3b and 4b have been discussed in detail previously for this system. Apart from complications due to the presence of triplet methylene, they quantitatively explain the V and propylene variations in Figure 1 down to a pressure of -10 cm.6'8 At lower pressures, propylene decomposition by eq 4a becomes important and explains, via reactions 5-7b, the decreasing propylene and increasing pentene-1, ethane, and nbutane yields with decreasing pressure as indicated in Figure 1. The small, relatively constant percentage of butene-1 is presumably due to secondary reaction of methylene radicals with propylene, mainly. Reaction 5 scavenges H atoms in this system. The (8) H. M. Frey and G. B. Kistiakowsky, J . Am. Chem. SOC.,79, 6373 (1957); B. S. Rabinovitch, E. Tschiukow-Roux, and E. W. Schlag, ibid., 81, 1081 (1959).
Volume 70, Number 4
A p r i l 1966
J. W. SIMONS, B. S. RABINOVITCH, AND F. H. DORER
1078
activation energy for addition to the double bond is low (-2 kcal m01e-l)~ and the ethylene is present in large excess over other species in the system. The allyl, is unimreverse recombination reaction, H portant because of the small steady-state concentration of allyl radicals relative to ethylene. Reactions 6 and 7 are the logical subsequent reactions of ethyl radicals in view of the relatively high activation energy (-8 kcallmole) for addition of ethyl radicals to ethylene.1° The disproportionation reactions, 6b and 6b’, amount to only 14% of the recombination reaction 6a.l’ The ratio of ethane to n-butane, as shown in Figure 2 for this system, is virtually pressure independent and has the average value 0.12 f. 0.03. This ratio is quite close to the accepted values of the disproportionation/recombination ratio for ethyl radicals. This lends support to the proposed mechanism and, together with negligible propane production, to the absence of reactions 8 and 9. The ratio of n-butanelpentene-1 as a function of pressure, shown in Figure 3, is virtually pressure independent. It is known that a small amount of the pentene-1 may be formed on photolysis of methylene precursors. Unfortunately, no search was made for hexadiene (reaction 10) in the products. Estimation of the amount of decomposition may be made either on the basis of H atom-derived products, or of allyl radicalderived products. The former basis is used immediately below, and for this reason other disproportionation reactions of allyl need not be listed in the scheme. On the basis of the mechanism, the specific rate of decomposition of propylene* in this system is
+
k~
=
WDh/sh
(1)
where the amount of decomposition is Dh = 2(ethane 4n-butane) 4- 1.14(penter-e-1); the amount of stabilization is S = propylene. Values of IGH are given in Table I. Part of the variation of k~ with pressure is due to the energy dispersion of the activated molec u l e ~ . The ~ lowest pressure results are considered as more reliable for the reasons described earlier. I n addition, the perturbation of the system due to the concomitant formation of triplet methylene3!’ tends to zero at low pressure. The listed value for p = 0, k m , is the zero-pressure extrapolated value. Ketene-Ethylene-Helium Systems. The product composition as a function of pressure for ketene photolysis (-3200 A) in ketene : ethylene :helium mixtures of 1:10:167, on a collision basis, is shown in Figure 4. The ratios of ethaneln-butane and n-butanelpentenel are shown in Figures 2 and 3, respectively. These The Journal of Physical Chemistry
c
0.8
O Q
p41
w y0.2
,,Ips 0
0
Q 0
L
I
I
0
20
4 0 P(cm)
1
I
I
60
80
10
Figure 2. Variation of the ethaneln-butane ratio with pressure in the various systems: 0, diazomethane-ethylene; 0, ketene-ethylene; 0, ketene-ethylene-helium (latter pressures are reduced by one-third for graphing convenience).
1
1.4
6 %
B
0.2
A
20
A
A
00
8 0
O I
0
A
0 0
I
I
p(cm)
60
I
80
I
I
100
Figure 3. Variation of the ratio, propaneln-butane, and of n-butanelpentene-1 with pressure in the various systems; open symbols are n-butanelpentene-1 ratios; 0, ketene-ethylene; A, ketene-ethylene-helium (pressures are reduced by one-third for graphing convenience); 0, diazomethane-ethylene; 0, propanel butane ratio in diazomethane-ethylene system.
results are similar in all respects to those for the ketene-ethylene system except that decomposition of propylene* in the present system occurs a t much higher total pressures. This result is consistent with an interpretation of inefficient collisional deactivation of propylene* by helium relative to ethylene; similar behavior was obtained earlier for V* deactivation where a total inefficiency of 0.21 was found.6a (9) K. R. Jennings and R. J. Cvetanovib, J . Chem. Phys., 35, 1233 65, 375 (1961);
(1961); R.Klein and M. S. Scheer, J . Phys. Chem., K.Yang, J. Am. Chem. SOC.,84, 719 (1962).
(10) M. Miyoshi and R. K. Brinton, J . Chem. Phys., 36, 3019 (1962); D.G.L. James and E. W. R. Steacie, Proc. Roy. Soe. (London), A244, 289 (1958). (11) D. G. L. James and G. E. Troughton, Chem. Commun., 94 (1965).
RATESOF DECOMPOSITION OF CHEMICALLY ACTIVATEDPROPYLENE
1079
~~
~
~~
Table I : Experimental Rate Constants (sec-1) for Propylene Decomposition in the Lower Pressure Region (25O)
PI cm k~ X 10-8
10.9 11.9
P, cm P, cm (cor) kH
x lo-'
k~ X 10-8 (cor) P, cm kH
x
lo-'
k, x 10-8
10.8 10.5
6.6 9.3
6.6 7.8
57 11.9 I23 25.9 22 12.1 4.9
Ketene-ethylene system 6.0 5.4 5.0 6.3 6.5 6.5
2.3 3.5
Ketene-ethylene-helium system 34 23 7.1 4.8 70 49 14.7 10.3 Diaeomethane-ethylene system 22 3.4 11.6 6.8 5.4 2.2
2.1 3.9 15 3.2 53 11.1 3.4 7.3 2.6
propyIene* CzHs
1.8 3.9
1.8 3.9
0.64 2.8
(0) 2.3
9.5 2.0 32 6.7
13 2.7
1.3 4.4 1.9
(0) 3 1.5
(0)
2C H ~+ vinyl
(4c)
+ vinyl +butene-1
(6d)
*CzHe + CZHZor 2C2H4
PFm) Figure 4. Variation of product percentages with pressure in the ketene-ethylene-helium system; symbols are identical with those in Figure 1.
Values of k~ calculated for this system by eq I are given in Table I. A rather large dependence of k~ on pressure is observed, and probably for the same reason as in the ketene-ethylene system. The zero pressure value, only, is accepted and the nominal value 13 X 108 sec-' becomes 2.7 X 108 sec-1, with use of 0. The agreement with the pure system is good. Some formation of triplet methylene due to the presence of the inert gas helium occurs, but a t this dilution amounts to only Diazomethane-Ethylene System. The product composition as a function of pressure for diazomethane photolysis (-4300 A) in 1:20 admixture with ethylene is shown in Figure 5. An important difference between these results and those for the ketene systems is the inversion of the amount of butene-1 relative to butane with decreasing pressure. It is believed that this results from propylene* decomposition to give vinyl radicals, in addition to allylic C-H rupture, in this higher energy system. The following additional reactions are proposed to explain the products and their variation with pressure
(6e)
The rise in propane product by reaction 8 a t lower pressures is consistent with reaction 4c. The ratio of ethane/%-butane (Figure 2) is larger than that for the ketene systems. This is consistent with an increased importance of reaction 9 which foIIows reaction 4c. However, the increase also reflects some ethane impurity in the diazomethane. The data in this system are the most inaccurate and least reliable. The ratio of n-butanelpentene-1 shown in Figure 3 is not greatly different from that for the ketene systems. This is as expected since the introduction of methyl
Figure 5. Variation of product percentages with pressure in the diazomethane-ethylene-system; symbols are the same as in Figure 1 with V, for propane, in addition.
(12)
H. M. Frey, J . Am. Chem. Soc.,
82, 5947 (1960);
R. F. W.
Bader and J. I. Generosa, Can. J . Chem., 43, 1631 (1965).
Volume 70,Number l+ April 1966
1080
J. W. SIMONS, B. S. RABINOVITCH, AND F. H. DORER
and vinyl radicals from reaction 4c would not affect this ratio greatly. The ratio of propaneln-butane, given in Figure 3, increases markedly with decreasing pressure, as expected from the increasing relative importance a t lower pressures of reaction 4c followed by reaction 8. Experimental values of k~ and k, were calculated by eq I, where DH = 1.14(pentene-1) 2.24(n-butane) 0.5(butene-l) 1.06(propane), and D, = 2 [ethane0.12(n-butane)] 0.5(butene-1) l:06(propane), and are given in Table I for various pressures. In the above expressions the ethane from disproportionation of ethyl radicals was taken to be 0.12 of the n-butane and the butene-1 was assumed to result equally from ethyl vinyl and from methyl allyl. The ethane from methyl radical recombination (reaction 9) was taken to be the measured excess over that resulting from ethyl radical disproportionation (0.12 X nbutane). Removal of ethyl by (6e) was simply ignored relative to reaction 6d which is itself an estimate. The data in this system are very sparse at low pressures and kHOis only semiquantitative in nature; also, D, is based on Ci13-derived products, only, and various reactions of vinyl and allyl are not listed. Thermochemistq and Energetics. The relevant thermochemical quantities for the following discussion are given in Table 11. The value of the critical energy, EO,for reaction 4a can be obtained within reasonable limits from the values in Table I1 and the resonance energy of the allyl radical. It is assumed that the activation energy
+
+ +
+
+
+
+
Table 11: Gaseous Heats of Formation and Bond Dissociation Energies at 0°K (kcal mole-’)” AHPa
H Methane Propane Ethylene Propylene Cyclopropane
51.7 -16.0 14.5 8.5 16.7e
DoO(R-H)
102.5 =k 0 . 5 b 96 i 1 (prim)” 104 zt 2d
a Unless otherwise specified, values are taken from F. D. Rossini, .4PI, “Selected Values of Physical and Thermodynamic Properties,’’ 1953. A median of the values given in a and by J. A. Kerr and A. F. Trotman-Dickenson, “Strengths of Chemical Bonds,” “Handbook of Chemistry and Physics,” 46th ed, Chemical Rubber Publishing Co., Cleveland, Ohio, 1965. A median of the values in b for propane and ethane a t 0°K. An average of the values determined by A. G. Harrison and F. P. Lossing, J . Am. Chem. SOC.,82,519 (1960), and by A. F. TrotmanDickenson and G. J. 0. T’erberke, J . Chem. Soc., 2580 (1961). e The 298°K value [J. W. Knowlton and F. D. Rossini, J. Res. 1Vatl. Bur. Std., 43, 113 (1949)l converted to 0°K.
The J O U T d of Physical Chemistry
for the combination reaction is zero. Although this may not be correct, it is certainly not in error by more than -1 kcal mole-’. Estimates of the allylic resonance energy have been discussed recently13 and the more reliable of these are encompassed by the limits of 12-14 kcal mole-’. This range of allylic resonance energies, coupled with the range of 95-97 kcal mole-’ for the primary C-H dissociation energy in propane, gives values ranging from 81 to 85 kcal mole-’ for Doo(C-H) in propylene; the mean of 83 kcal should be representative. A value of Eofor reaction 4c can be obtained within reasonable limits from the data in Table 11, again assuming zero activation energy for the recombintltion reaction for simplicity. Now Doo(CH3-CH=CH2) is given by AHrOo(CH3)
+ AHroo(CH=CH2)
AHroo(CH3CH=CH2)
where AHrOo(CH3) = Doo(CH,-H)
+ AHroo(CHs) -
AHfoo(H) = 34.3-35.3 kcal mole-’ and AHfoo(CH=CH2) = Doo(CH2=CH-H)
+
AHr0o(C2H4)- AHfoo(H)= 64.8-67.8 kcal mole-’ which gives a range 90.6-94.6 kcal mole-’ for Eo of reaction 4c; the mean value of 93 will be accepted as representative; From the mean value, it is seen that vinylic C-C rupture requires about 10 kea1 mole-’ more energy than allylic C-H rupture. The average energies of excitation, (E), of propylene* are 8.2 kcal mole-1 greater in these systems than those of cyclopropane* (Table 11), which previous estimates have placed at -103 kcal mole-’ for ketene photolysis a t 3200 A5a,l4 and -110 kcal mole-’ for diazomethane photolysis a t 4300 A.l4*l5 (These values are appropriate for use with a strong collision value for w and allow for the actual collisional ineffi~iency.)~Then ( E ) are 111 and 118 kcal, respectively, and the average excess energy (E+) for allylic C-H rupture in propylene* has the representative value of 28 lical mole-’ for the ketene system and 35 kcal mole-’ for the diazomethane system. For vinylic C-C rupture, (E+) is (13) (a) H. M. Frey and D. C. Marshall, J . Chem. SOC.,3891 (1962);
(b) D. M. Golden, K. W. Egger, and S. W. Benson, J . Am. Chem.
SOC.,86, 5416 (1964),and references therein.
(14) D. W. Setser and B. S. Rabinovitch, Can. J . Chem., 40, 1425 (1962). (15) F. H. Dorer and B. S. Rabinovitch, J . Phys. Chem., 69, 1973 (1965).
RATESOF DECOMPOSITION OF CHEMICALLY ACTIVATED PROPYLENE
1081
~~~~~
~~~~
) for Various Complexes and Values of EO Table 111: Theoretical Values of k ~ p (sec-1)
Eo, kcal mole-’ (E),kcal mole-’ Model I 11000 11150
81 111 2 . 4 X lo8 1 . 2 x 108 4 . 6 X lo8
I175 I11
1 . 6 X 1Ol0
84 111 8 . 5 x 107 4 . 7 x 107 1 . 6 X lo8 6 . 0 X 108 6 . 2 X 109
...
81 118 9 . 3 x 108 5 . 2 X lo8 1 . 8 x 109
84 118 3 . 7 x 108 2.1 x 108 7 . 1 x 108
...
...
...
...
Aa
... ...
1.8 x 6.0 x 2.4 X 7.6 X
1014 1014 10l6 10l6
L
2 . 5 X lo8
Expt a
3 . 0 X lo8
Calculated frequency factor a t 750°K for the relation k = Ae-Eo/RT; Le., this is not the “Arrhenius” factor.
Table IV : Frequency Bssignments for Propene Decomposition by C H Rupture &lotiona
C-H str
HCH bend
Molecule
3090 3013 2954 (2) 2992 2933
14741 1378 1443 1419
C=C str HC=C bend C-C=C bend C-C str CHBwag HCC bend
CH, rot
1652 1298 1172 428 920 912 578 963 991 1045 225
I
IIsaob
II*sob
* . .
...
..
300 (2)
150 (2)
150 ( 2 )
I176
IIIC
.. 75 (2)
allyl rot (2)
1400
1400
1400
1400
1400
600 1200
375 1300
375 1300
375 1300
375 1300
700 963 450
375 900 700 925 500
375 900 700 925 500
375 900 700 925 500
375 900 700 925 500
’ Only the changes made in the molecule assignment t o form the complexes are indicated. * The subscript indicates the frequency Assuming a 120” angle for symmetrical allyl radical (r = 2). assignment for the rupturing HCH bends,
18 kcal mole-’ for ketene and 25 kcal mole-’ for diazomethane. The excess energies for H rupture are less than the 40 and 47 kcal mole-‘ excess for cyclopropane isomeri0 a t i o n . ~ , 1In ~ the latter case, it has been shown that the experimental values of k , (eq 2b) can be adequately because the compared to the theoretical value of system is relatively monoenergetic; the same assumption is still adequate here. In the comparison of k ( E ) with k E O we may keep in mind the fact that the value of E appropriate for the low-pressure regime is smaller than ( E ) ,although still within the limits of experimental uncertainty.
R R K M Rate Calculations Accurate theoretical calculations of k H , the rate of decomposition by H rupture, have been made in the manner described previously. The frequency assignment for the propylene molecule used was that of S v e r d l ~ v ; however, ~~ the methyl torsion frequency was lowered from his assignment to 225 cm-l, in accord with similar assignments made earlier” (Table IV), (16) I. M. Sverdlov, Proc. Acad. Sci., U.S.S.R. (Chem. Sec.), 106, 80 (1956). (17) M. J. Pearson and B. S. Rabinovitch, J . Chem. Phus., 42, 1624 (1965).
Volume 70, Number 4
A p r i l 1966
J. W. SIMONS, B. S. RABINOVITCH, AND E”. H. DORER
1082
Table V
c-c=c
c=c=c
Ea, kcal mole
1.5”
. H addition 1. 5 ” , k
PC
~10-8”*~
Ea,kcal mole-’
7-ge
Po
-10-3e.h
9 r - 3 d 8 k *
c-c-c Very small or zerob k0.015d
-Ob
-0, 25b’d
CH, addition 5.7’ 10-3’4
