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Hydrogen-atom attack on 1-phenylpropene at high temperatures
Twenty-FifthSymposium(International)on Combustion/TheCombustionInstitute,1994/pp.867474
H Y D R O G E N - A T O M ATTACK O N 1-PHENYLPROPENE AT H I G H TEMPERATURES WING TSANG ANDJAMES A. WALKER Chemical Kinetics and Thermodynamics Division National Institute of Standards and Technology Gaithersburg, MD 20899, USA
The reactions of hydrogen atom with 1-phenylpropene (1PP) have been studied in single-pulse shocktube experiments. Contribution from a large number of the possible reactive channels has been observed. The prime channels are the displacement of the methyl group resulting from addition to the olefinic structure and abstraction of a hydrogen from the methyl group followed by internal displacement of the hydrogen, forming indene. Displacement from the ring is less favored. The following rate expressions at temperatures of 990-1100 K and pressures of 1.4-2.0 atm have been determined. k(H + 1-phenylpropene ~ styrene + methyl) = 10la.6-+~ exp(-1200 --- 1000/T) k(H + 1-phenylpropene-' propene + phenyl) = 1014.~176exp(-5300 + 1500/T) k(H + 1-phenylpropene ~ propenyl + benzene) = 1014.~§176exp(-6300 -+ 1500/T) k(H + 1-phenylpropene-* 3-phenylallyl + H2 + Ha)<~ 10t42-+~ exp(-5600 _+ 1000/T) in units of cma tool 1 s- 1, where the relative and absolute uncertainties in the rate constants are factors of 1.10 and 1.25, respectively. An attempt has been made to relate these rate expressions to basic thermal processes, and similarities to those for the appropriate smaller olefins have been noted. Displacement rate constants are made smaller by the reversibility of the hydrogen addition reaction, and together with possible isomerizations, they represent an additional complicating feature,
Introduction Hydrogen-atom reactions with unsaturated compounds are of great importance in combustion. This paper reports on single-pulse shock-tube experiments involving hydrogen-atom attack on 1-phenylpropene (1PP). A summary of earlier work on simpler compounds and pertinent lower-temperature data can be found in Table 1 [1-9]. Quantitative determination of mechanisms and rates for these reactions are difficult because there are numerous channels for reaction: addition to the sites of unsaturation, followed by decomposition of the radical and hydrogen-atom abstraction. The latter will lead to the formation of a large radical, which if not oxidized, can grow while the former may lead to the breakdown of the molecule to smaller fragments. The significance of such branching ratios to combustion chemistry is clear. 1-Phenylpropene is a more complicated molecule than those which have been studied. It contains both aromatic and olefinic structures. The large number of such species present during combustion creates the need for defining how information on simpler molecules can be extended. Figure 1 contains possible reaction pathways. The numbers are the chan-
nels of primary interest. The letters refer to subsequent fast reactions that may lead to the observed products. Although the mechanism is complex, the conditions in single-pulse shock-tube studies lead to simplifying features. Kinetics and thermodynamics drive reactions to eompletion, and many other possible pathways and products are eliminated. The experimental approach is to use the decomposition reactions of a very labile organic compound, hexamethylethane (HME), as a source of hydrogen atoms. The processes are [10] HME = 2 t-butyl = isobutene + H
(1)
with k 1 = 3 • 1016 exp(-34,500/T) s -1. The subsequent breakdown oft-butyl is rapid under the hightemperature conditions of single-pulse shock-tube studies. Other reaction possibilities are unimportant. The general instability of nearly all large organic radicals (certain resonance stabilized radicals are exceptions to this rule) under the conditions of these studies (high temperatures and high dilutions of molecule undergoing unimolecular decomposition) facilitates the interpretation of the data. Referring to Fig. 1, one sees the possibility of associating reaction pathways with particular unsaturated stable products.
867
868
REACTION KINETICS TABLE 1 Past work on hydrogen-atom attack on organics at high and low temperatures Rate expression Rate constant (1100 K) (cm mo1-1 s 1) Displacement [group]
Abstraction/H
1.7 x 1013exp(-- 1808/T)[CH3] 3.3 • 1012 6.7 • 1013exp(-3255/T)[CHa] 1.2 X 10WCH 3 3.5 X 1013exp(-3710/T)[CHa] 1.2 • 1012 4.8 • 10 la exp(-4795/T)[C1] 0.6 • 10lz 1.~ X 1013exp(-257S/T)[CHa] 1.2 • 1012 2.2 • 1013exp(-3990/T)[OH] 0.6 • 1012 ~9 • 10la exp(-4531/T)[Cl] 0.4 • 1012
1.7 X 1014exp(-4024/T) 0.7 X 1012 3.7 • 10TM exp(-4342/T) 0.8 • 10 ~2 9.6 • 10la exp(-4265/T) 0.6 • 10 lz
Compound attacked by H [Ref.] A. High Temperature Results Isobutene [2] Mesitylene [3] p-Chlorotoluene [4]
Toluene [1] Phenol [5] Chlorobenzene [6]
1.2 • 1014exp(-4138/T) 0.9 • 1012 1.1 • 1014exp(-6240/T) 0.4 • 1012
B. Lower-temperature results: rate constants are derived from rate expression and refer to addition without displacement. gutadiene [8] 4.1 X 1013exp(-655/T) (2 sites) 300 4.5 X 10~z 1100 2.3 • 10'z t-Butene-2 [9] 2.1 • 1013exp(- 1046/T) 300 6.3 • 10n 1100 0.8 • 1013 Isobutene [9] 3.7 • 1013exp(-849/T) (terminal addition) 300 2.2 X 1012 1100 1.7 • 1013 Benzene [7] 4 • 1013exp(-2170/T) ( 6 sites) 300 2.9 • 101~ 1100 5.6 X 10lz
Every isobutene that is generated implies a hydrogen released into the system. By carrying out the reactions in a vast excess of 1PP and mesitylene(1,3,5trimethylbenzene) (MES) in comparison to H M E (ratio of these two compounds to H M E is in excess of 100 to 1), with the latter present in concentrations near 100 ppm, practically all the hydrogen atoms will react with 1PP and MES. For MES [3], the result of hydrogen-atom attack are displacement of methyl leading to meta-xylene formation, H + MES = meta-xylene + CH3
(2)
and abstraction of benzylic hydrogens,
and MES are m u c h more stable than H M E (in the unimolecular sense), rate constants for products from 1PP and MES can be determined from the following relation: [product~/1PP]/[m-xylene/MES] =
Substitution of the rate expression for k2 will then yield rate constants for the process initiated by hydrogen-atom attack on 1PP. The yields of isobutene also serve as an internal t h e r m o m e t e r for the determination of the temperature of the system. T h e relations are
1/T
H + MES = 3,5dimethylbenzyl + H2 with the rate expressions given in Table 1. Since 1PP
k(product~)/kz.
with
= [38 - In(k,)]/34,500
HYDROGEN-ATOM ATTACKON 1-PHENYLPROPENE AT HIGH TEMPERATURES
H
.~
+ C3He
G
+H*
869
+ CH=CHCH 3
9
g. C2H2+CH3 *
+H*
FIG. 1. Mechanismfor the hydrogen atom-induced decompositionof 1PP.
k 1 = t 1.1n[1 - (1.03-isobutene/HMEi) where t is the heating time of 500/is and the factor of 1.03 refers to the propene formed during isobutene decomposition. The use of the internal temperature removes the chief uncertainty in such experiments. It depends on HME being present in small amounts relative to other reactants and short chain lengths for reactions involvingthe main components. Despite the large number of studies on hydrogenatom reactions with unsaturated organic compounds, there are no data on hydrogen-atom attack on 1PP or even related molecules. The most pertinent results are lower-temperature data on hydrogen-atom attack on the C4 olefins and benzene. These are summarized in Table 1. The values at 1100 K from the lowtemperature data are extrapolations. At room temperature, the main process is addition. Displacement does not occur, since the low temperatures prevent radical decomposition. There are large differences in rate constants. These can arise from small differences in activation energy. The data in Table 1 suggest that rate constants for addition to the aromatic ring may be an order of magnitude or more smaller than addition to an olefinic sti~lcture under the present reaction conditions. An important thrust of this work is to determine this branching ratio for 1PP. For olefins, relative rate constants for addition to a molecule are affected by the stability of the radicals [It] that are formed. Hence, the propensity for terminal addition. Terminal addition to butadiene will lead to a radical that is stabilized by allylic resonance, and one would expect that this will be the preferred mode. There are no data as to whether the full effect
of the allylic resonance energy, of the order of 50 kJ/ tool [12], is reflected in the rate constant. The data on butadiene and the butenes demonstrate that such effects do not occur between molecules. The same issue arises in the present system, except that benzylic resonance is substituted for allylic resonance. The values of the two resonance energies are similar. The differences in rate constants between trans butene-2 and the cis compound at room temperature are small. Fahr and Stein [13] have reported on the rate constants for phenyl and vinyl addition to benzene and ethylene at temperatures very close to those used here. Their data can be related to some of the present results through the thermodynamics, It will also be used to obtain information on the specificity of addition to the olefinic portion of 1PP,
Experimental Experiments are carried out in a heated singlepulse shock tube. This permits studying the reactions of high-boilingorganic compounds. Details of the experimental procedures have been given in earlier publications [1]. Analysis of reactants and products was by gas chromatography with flame ionization detection. A 6-ft Poropak N column was used to analyze all substances up to propene and was operated at 90 ~ All substances with boiling points higher than isobutene were analyzed with a 30-m dimethylsiloxane wide bore capillary column operated in the programmed temperature mode. Trans and c i s - l P P and
870
REACTION KINETICS 0.5
I::l
"'" *
0.0
=lib
,.o
" ;;';i " ; ;
o
"~.-0.5 o
OI o b-I
V v
-1.5
-2,0
~ 1020
= 1040
~ 1060
' 1080
TEMPERATURE (K) MES were purchased from Chemical Samples;* HME was from Aldrich Chemicals and, except for vigorous degassing, were used without further purification. Argon was ultrapure grade from Matheson. The only impurity was trace amounts of methane and was corrected for in the subsequent analysis. The major lower boiling point impurity in 1PP was allylbenzene. There are also some unidentified higher-boiling impurities in much smaller amounts than the allylbenzene in trans-lPP. The concentration of allylbenzene in the trans compound was about 0.2% in comparison to 1% in the cis mixture. A majority of the data and all the quantitative results on rate constants were derived from experiments with the trans compound. The results on the c/s compound served as a convenient check of the effect of impurities on the integrity of the results. Four sets of experiments were carried out. The compositions of the mixtures were as follows: 1. 100 ppm HME in 1% trans-lPP in argon; 2. 100 ppm HME in 1% cis-lPP in argon; 3. 136 ppm HME in 0.5% trans-lPP and 1.2% MES in argon; and 4. 100 ppm HME in 0.5% cis-lPP and 1.0% MES in argon. The invariance of the experimental results with changes of relative concentrations of HME, 1PP, and MES testifies to the absence of contributions of 1PP and MES decomposition in these studies and the validity of the postulated mechanism. This is a powerful test. The experiments were carried out at 1.4-2.0 atm pressure and in the temperature range 990-1100 K. *Certain commercial materials and equipment are identified in this paper in order to specify adequately the experimental procedure. In no case does such identification imply recommendation of endorsement by the national Bureau of Standards, nor does it imply that the material or equipment is necessarily the best availablefor the purpose.
11 O0
120
FIG. 2. Product distribution from the hydrogen atom-induced decomposition of 1PP. Large symbols are for reactions with trans-lPP. Results from mixture A. Small symbols are for reactions with cis-lPP. Results from mixture B: (#) styrene; ([-1) methane, ( i ) indene; (0) ethane; (C)) benzene; (0) propene; and (V) acetylene. Pressures range from 1.4-2 atm argon. Results
Data from studies with the first two mixtures can be found in Fig. 2. The yields have been normalized in terms of per hydrogen atom introduced into the system. There is very little difference in the data from the trans- and cis-lPP mixtures, except that at the lower temperatures, there are more propene and benzene from the cis results. This is probably due to the presence of impurities. Otherwise, it is not necessary to draw a distinction between the two sets of results. Trans ** cis isomerization is observed. This has no consequence in this work. Styrene is the main product. Indene is the next product of importance. All other products are in much smaller concentrations. Quantitative interpretation of the latter will have larger errors than that for the major products since there may be contributions from labile impurities or small side reactions that have been neglected in the analysis. They are a reflection of the "noise" in such studies. Uncertainties in the relative concentrations of the major products are expected to be in the range for standard gas chromatographic analysis or of the order of ---3%. For the minor products, this should be increased by a factor of 2. The presence of allylbenzene as an impurity prevented determination of contributions from this channel. The propene concentrations used here have been corrected for its formation from t-butyl decomposition. Large yields of methane and ethane are found. These are from fragments formed during styrene formation. The sum of methane plus twice the ethane is slightly larger than the styrene and other compounds, acetylene and propene from isobutene, whose presence imply simultaneous ejection of methyl. If the system is behaving as assumed, these should be exactly equal. This may be a reflection of small side reactions that have been neglected. From the propene yields, it is clear that only small amounts
HYDROGEN-ATOM ATTACK ON 1-PHENYLPROPENE AT HIGH TEMPERATURES
871
1.0
A
-~
0.5f 0
_ ~
. 00[ [0
~
n~
~
~O'v
1~
[]
g, f
o 9
_..
_
A A
O w
A
A
0
---
cP
o 9
o~
o
v
9
-1.0
-1.5
0.88
I
I
I
I
I
i
0.90
0.92
0.94
0.96
0.98
1.00
IO00/T(K)
FIG. 3. Ratio of rate constants for the production of styrene (i~), indene (11), propene (O), and acetylene (V), respectively, vs that for meta-xylene generation during the reaction of hydrogen atoms vs 1PP. Filled symbols are data for trans-lPP and mesitylene, mixture C. Open symbols are data for c/s-lPP and mesitylene, mixture D. Pressures range from 1.4-2 atm argon.
TABLE 2 Summary of experimental rate expressions k(Product channel) Product channel
Styrene Indene (from 3-phenylallyl) Acetylene (from propeny]) Propene Styrene + propene
k(Mesitylene)
k(Product channel) (cm3 mol -t s-t)
1100 K
0.65 x exp(2000 4-_ 190/T)
4.4 X 10~3exp(- 1200 _+ 1000/T)
1.4 • 10~*
12.6 • exp(-2400 + 300/T)
8.5 • 10~4exp(-5600 -2_ 1000/T)
5.2 X 10~~
2.2 x exp(-3100 _+ 510/T) 1.6 • exp(-2000 + 750/T) 0.85 X exp(1800 _+ 200/T)
1.5 x 10~4exp(-6300 _+ 1500/T) 1.1 X 10~4exp(-5300 -+ 1500/7") 5.7 X 10~3exp(- 1400 4- 1000/T)
4.7 • 10~ 9.1 • 10n 1.5 X 10~3
Uncertainties in column 2 are least-squares values, while those in column 3 are estimates and include 4.2 kJ/mol uncertainty in the rate expression for displacement of methyl from mesitylene. The uncertainties in the relative and absolute rate constants are factors of 1.1 and 1.25, respectively. Variations in the A factors are commensurate with these values. See text for further details. of phenyl radicals are formed. Together with its high reactivity, this is the reason for the absence of addition products such as toluene. The normalized yields of styrene are greater than 1. When all other products directly derivable from hydrogen reactions with 1PP are summed, the chain length is near 2.5. When MES is added, the chain length is reduced to 1.5. Figure 3 contains rate data pertaining to the ratio of rate constants for the formarion of the appropriate product from 1PP and the rate constants for the displacement of the methyl group from mesitylene-forming meta-xylene. Mso included are data fi'om the cis-compound. They track that of the trans mixture except for propene where, as noted earlier, there appears to be an extraneous sonrce.
Table 2 contains a summary of the rate expressions and constants derived from this study. The rate expressions in the second column in Table 2 are given in terms of the production of particular compound(s) compared to that of meta-xylene from MES and are a quantitative summary of the experimental results. The results in the third column are from multiplying those in the second column by k2. Also given in Table 2 are rate constants at 1100 K. This serves as a convenient reference point for comparisons. The value of the rate constants cover a wide range. For the less important channels, the rate constants, instead of rate expressions, are probably more meaningful. Estimated uncertainties for the relative rate expressions are 4.2 kJ/mol in the activation energy and a factor of 1.6 in the pre-exponential factor for styrene and
872
REACTION KINETICS 2.0
"d
w
1.5 0 k, n.
db
9
9
~ 1.0
~ 0
0.5 i 0.0 0.9
ill I
I
0.92
0.94
1000/T
(K)
indene and 8.4 kJ/mol in the activation energy and a factor of 2.5 in the A factor for acetylene and propene. In absolute terms, the uncertainty in the rate expression of the methyl displacement reaction from MES leads to an additional uncertainty of 4.2 kJ/mol in the activation energy and a factor of 1.6 in the A factor. These uncertainties are larger than that derived from the statistical scatter. They are based on experience from the consistency of earlier studies [10,14]. The relative and absolute uncertainties in the rate constants should be factors of 1.10 and 1.25, respectively. Figlwe 4 contains data on the ratio of products from hydrogen-atom attack on 1PP as compared to that producing styrene and demonstrate the effect of MES addition. For propene and acetylene, the results match. In the case of indene, the inhibiting effect of mesitylene leads to the observed lower yields. The additional mode for indene production in the absence of mesitylene is probably the abstraction by methyl of a hydrogen from 1PP, leading to the 3-phenylallyl radical, Only hydrogen-atom attack and the consequences were considered in Fig. 1. Methyl addition processes are reversed and will not make any contributions.
Discussion
The observations summarized in the third column of Table 2 will now be correlated with the thermal processes given in Fig. 1. Under the present conditions, the thermal instability of the larger radicals formed from hydrogen reactions simplify mechanistic possibilities. The only reaction they can undergo is unimolecular decomposition. Figure i lists four reaction channels. Two of these involve addition (channels 3 and 4) to the olefinic portion of 1PP. Another process is addition to the propenyl site (channel 5) on the ring and the last is hydrogen abstraction
0.96
FIG. 4. Ratio of rate constants for the production of indene (1), propene (O), and acetylene (y), respectively,vs styrene generation during the hydrogen atom-induced decomposition of 1PP in the absence (mixture A, symbols) and presence (mixture C, lines) of mesitylene. Pressures range from 1.4 to 2 atm argon.
(channel 6) from the methyl group. Other reactions are not included since they are much more endothermic or can readily be reversed. Addition of hydrogen at the methyl site of olefinic portion of 1PP (channel 4) followed by ejection of methyl (reaction d) must lead to styrene and methyl. Addition at the phenyl site (channel 3) will lead (reaction 2) to propene and phenyl. The possibility of a neophyl rearrangement [15] (channel b) can lead to the formation of 2-phenylpropyl-1 radical, which will decompose readily (channel c) to form methyl and additional styrene. Thus, the present data does not yield independent information on the specificity of H addition to the olefin. The sum of styrene and propene may be a measure of addition to both sites of the double bond (k3 + k4). Ambiguity arises from the possibility that the addition reaction (channel 3) may be reversed. Thus, k a + k4 so deduced must be a lower limit. However, the H + 1PP --* phenyl + propene reaction is approximately thermoneutral at 1000 K [16-18]. Rate constants for hydrogen and phenyl bond cleavage may be not be far apart. Propene, a measure of the phenyl released into the system, is only 7% of the styrene. Thus, little error is introduced into the addition rate constant by the neglect of the hydrogen ejection channel. At 1100 K, the sum of rate constants for styrene and propene formation is 1.4 • 1013 cm 3 mol- 1 s- 1 or very close to 0.5 times the value for butadiene given in Table 1. The results of Fahr and Stein can be used to arrive at an estimate of the specificity of hydrogen addition to the olefinic part of 1PP (channels 3 and 4). They found k(C6H5 + C2H4 ~ styrene + H) = 2.5 • 1012 exp(-3121/T) cm 3 tool -1 s -1. Assuming that this will also hold for terminal addition to propene and accounting for the reaction degeneracy of 2, one finds through detailed balance k(H + 1PP --* C6H 5 + C3H6) = 9.5 X 1013 exp(-3222/T) em 3 mol -I s -1. This is equal to 5 • 1012 em a mo1-1 s 1 at 1100 K. Combining this with the overall rate constant for
HYDROGEN-ATOM ATI'ACK ON 1-PHENYLPROPENE AT HIGH TEMPERATURES addition given earlier, the ratio of rate constants for hydrogen addition to the methyl and phenyl sites in 1PP is 1.8. This value is strongly dependent npon the thermodynamic properties of phenyl and its combination rate constant. For the former, the number used here, 328.5 kJ/mol, is from Burcat [18]. A slightly higher value of 340.5 kJ/mol [19] had been suggested and will lead to a rate constant of 1.5 • 1012 cm 3 mol i s- 1 for propene formation. Fallr and Stein assume a combination rate constant for phenyl of 3 • 1012 cm 3 mo1-1 s -1. A value of 1013 cm 3 mol 1 s-1 will raise the original value to 8.5 • 1012 cm a mol 1 s-1. The corresponding ratios are now 8 and 0.8. Although the results have wide scatter, these considerations demonstrate that a significant portion (although less than half) of the hydrogen atoms may add to the phenyl site of 1PP. Benzyl resonance energy is not a large influence on the position of hydrogen addition on the olefinic portion of 1PP. This sets a limit of 10-40% on the error introduced in assigning styrene and propylene yields to ka + k4. The acetylene yield can be a measure of propenyl formed (channel g) if isomerization to allyl is not important. Results on methyl attack on acetylene [8] and the thermodynamics leads to 4.5 • 1013 exp( - 15,900/T) s -1 as the rate expression for propenyl decomposition. The thermodynamic properties of propenyl are based on a vinyl C - - H bond energy of 459 kJ/mol [20] and molecular parameters estimated in the usual manner. Such isomerization processes can be treated as an internal abstraction with an additional contribution to the activation energy from the ring strain in the transition state [21]. For abstraction of allylic or benzylic hydrogens, the resonance energy has only a small effect on the rate constants [1]. The strain energy for a four-membered ring structure is 125 kJ/mol [22]. The transition state must be tight. A likely rate expression for isomerization is [20] 3 • 1011 e x p ( - 1 7 , 5 0 0 / T ) s -1 and should be unimportant compared to decomposition. Acetylene yields are not a measure of hydrogen addition to the propenyl site of the ring structure in 1PP (channel 5), since the reverse reaction is likely to be competitive with propenyl ejection (channel e). It represents instead a minimum value for ks. The activation energy for acetylene formation is higher than those for the displacement processes in Table 1. This suggests that the barrier for propenyl ejection is higher than the hydrogen addition barrier. Nevertheless, the rate constant is only a factor of 2.5 smaller than that for hydrogen addition to toluene, where displacement is a measure of hydrogen addition. Thermodynamic factors favor the displacement of methyl from the olefinic part of 1PP or the abstraction of a hydrogen by H atoms or a radical such as phenyl in comparison to displacement ofpropenyl. The ratio (styrene + propene)/acetylene is an upper limit to the ratio of rate constants, (k 3 + k4)/ks, which defines H-atom addition to the olefinie and ring
873
structures in 1PP. The data in Table 2 lead to a factor of 33 at 1100 K. The actual value of (k 3 + k4)/k 5 must be lower. If displacement of methyl from toluene is equivalent to addition at the propenyl site, one obtains a value of 12. This is the same value from the data on Table 1, using butadiene and benzene as a basis. Fahr and Stein [13] found the rate expression for vinyl addition to benzene to be k(C2H3 + C6H 6 styrene + H) = 1.5 X 10lz e x p ( - 3 2 2 2 / T ) c m a tool -1 s -1 or 8 • 101~ cm 3 mo1-1 s -1 at 1100 K. These are different (factor of 1.8) from the original estimate [13J since a newer value for vinyl combination from Fahr et al. [23] has been used. The thermodynamics and the rate expression for propene yield obtained here gives k(propenyl + C6H6 ~ 1PP + H) = 1.6 X 1013 exp(-5940/T) cm 3 tool -1 s -1 or 7.3 X 101~cm 3 mol 1 s-1 at 1100 K. Despite the differences in the rate expressions, agreement in the rate constants at 1100 K where both measurements were made is good. Fahr and Stein caution their readers on the use of their rate expression beyond the measurement range. The estimated uncertainty in the activation energy in this study is 13 kJ/mol. Thus, the disagreement may not be significant. Abstraction of a hydrogen from the methyl group (channel 6) will lead to the formation of 1-phenylallyl radical. The resonance energy of this radical is 66 or 18 kJ/mol higher than those for benzyl or allyl [24]. Internal displacement of a hydrogen will lead to the quantitative formation of indene [25] (channel f). Hydrogen, as well as methyl and phenyl, can be a abstracting radical. Thus, indene concentration sets an upper limit for the rate constant of hydrogen abstraction (k6). The rate expression given here can be compared with others leading to the formation of other resonance-stabilized radicals. From Table 1, it appears that the value from indene concentration is about a factor of 2 larger. A larger rate constant may be consistent with the 18 kJ/mol extra resonance energy. However, earlier studies indicate that resonance energy has very small effects on rate constants for abstraction [1]. The values given here may have additional contributions from methyl abstraction. This is supported by the rate expression, where the A factor is larger than that for other hydrogen-abstraction processes. These results demonstrate that the main role of hydrogen atoms is to strip alkyl groups from the olefinic portions of large unsaturated compounds containing olefinic and aromatic groups. Abstraction also makes a contribution. Next in order of reactivity is the removal of alkyl structures from the ring. The reversibility of hydrogen addition to these systems makes removal of phenylic and vinylic structures more difficult. Under combustion conditions, most of these effects can be almost quantitatively inferred from data on simpler molecules. There would appear to be a good basis for making estimates. Complicat-
874
REACTION KINETICS
ing factors are the isomerization pathways and uncertainties in the rate constants for decomposition of radicals that involve removal of phenyl and vinyl groups as a result of the reversibility of the initial addition process.
11. 12.
REFERENCES
13.
1. Robaugh, D., and Tsang, w.,J. Phys. Chem. 90:41594163 (1986). 2. Tsang, W., and Walker, J. A., Twenty-Second Symposium (International) on Combustion, The Combustion Institute, Pittsburg, 1989, pp. 1091-1097. 3. Tsang, W., Cui, J. P., and Walker, J. A., "Single Pulse Shock Tube Study of the Reactions of Hydrogen Atoms with Complex Aromatics," Proceedings of the 17th International Symposium on Shock Tubes and Waves, AIP Conference Proceedings 208, American Institute of Physics, New York, 1990, p. 63-74. 4. Tsang, W., He, Y. Z., Mallard, W. G. and Cui, J. P., "Single Pulse Shock Tube Study of the Reactions of Hydrocarbons with Aromatics, IV: Chlortoluenes," Proceedings of the 16th International Symposium on Shock Tubes and Waves, VCH, New York, 1988, pp. 467~74. 5. He, Y. Z., Mallard, G. W., and Tsang, W., J. Phys. Chem. 92:2196-2201 (1988). 6. Cui, J. P., He, Y. Z., and Tsang, W., J. Phys. Chem. 93:724-727 (1989). 7. Nicovich, J. M., and Ravishankara, A. R., J. Phys. Chem., 88:2534-2541 (1984). 8. Kerr, J. A., and Parsonage, M. A., Evaluated Kinetic Data on Gas Phase Addition Reactions: Reactions of Atoms and Radicals with Alkeues, Alkynes and Aromatic Radicals, Butterworths, London, 1972. 9. Harris, G. W., and Pitts, J. N.,J. Chem. Phys. 79:39943987 (1982). 10. Tsang, W., "Comparative Rate Single Pulse Shock
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18.
19. 20. 21. 22. 23. 24. 25.
Tube in the Thermal Stability of Polyatomic Molecules," in Shock Tubes in Chemistry (A. Lifshitz, Ed.), Marcel Dekker, New York, 1981, pp. 59-99. Falconer, W. E., and Sunder, W. A., Int. J. Chem. Kinet. 3:395-410 (1971). McMillen, D. F., and Golden, D. M., Ann. Rev. Phys. Chem. 33:493--532 (1982). Fahr, A., and Stein S. E., Twenty-Second Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1988, pp. 1023-1029. Tsang, W., and Lifshitz, A., Annu. Rev. Phys. Chem. 41:559-599 (1990). Wilt, J., in "Free Radical Rearrangements" Free Radicals (J. Kochi, Ed.) Wiley-Interscience, New York, 1973, Vol. 1, pp. 333-502. Stull, D. R., Westrnm, E. F., and Sinke, G. C., The Thermodynamics of Organic Compounds, John Wiley and Sons, Inc., New York, 1969. Stull, D. R., and Prophet, H., JANAF Thermochemical Tables, 2nd Edition, NSRDS-NBS37, U.S. Government Printing Office, Washington, DC 20402, 1971. Burcat, A., Zelenik, F. J., and McBride, B.~ "Ideal Gas Thermodynamic Properties of Phenyl, Phenoxy and o-Biphenyl Radicals" NASA Technical Memorandum 83880, National Aeronautics and Space Administration, Lewis Research Center, Cleveland OH, 44155, (1985). Robangh D., and Tsang, W., J. Phys. Chem. 90:53635367 (1986). Cui, J. P., He, Y. Z., and Tsang, W., Energy Fuels 2:614-618 (1988). Walsh, R., Int. J. Chem. Kinet. 2:71-74 (1971). Benson, S. W., Thermochemical Kinetics, John Wiley, New York, 1974. Fahr, A., Laufer, A. H., Klein, R., and Braun, W., J. Phys. Chem. 95:3238-3242 (1991). Robaugh, D. A., and Stein, S. E., J. Am. Chem. Soe. 108:3224-3229. Tsang, W., and Walker, J. A., in preparation.
COMMENTS John H. Kiefer, University of Illinois at Chicago, USA. Your agreement with the Fahr and Stein rate for C6H~ + C2H2 depends very much on the chosen 108.5 kcal/mol for the CH bond energy in ethylene. If you use tile "latest" value of 111.2 [Berkowitz, J., Ellison, G. B. and Gutman, D.,J. Phys. Chem. 98:2744 (1994)] you would not agree at all. Author's Reply. We are in agreement with Dr. Kiefer's conclusion. Use of the higher value would have led to a discrepancy of a factor of 4 in the equilibrium constant. We believe that the equilibrium constant derived from the experimental numbers is accurate to within a factor of 2. It suggests to us that the "higher" value for the heat of formation derived from nonkinetic methods is in error.
Note that the number derived from kinetic methods, as given in Dr. Kiefer's reference, is near 107 kcal/mol.
R. W. Walker, Hull University, England. Do you have any comment on the apparently large A factor for the H-abstraction reaction at the allylic position in I-phenylpropene? As a rotation is lost in the transition state, a value nearer 1013might be expected. Author's Reply. As noted in the text, the rate expression for the abstraction of the resonance stabilized hydrogen contains components from abstraction by methyl. Therefore it is a maximum value. This is probably the reason for the somewhat larger than expected A factor.
H Y D R O G E N - A T O M ATTACK O N 1-PHENYLPROPENE AT H I G H TEMPERATURES WING TSANG ANDJAMES A. WALKER Chemical Kinetics and Thermodynamics Division National Institute of Standards and Technology Gaithersburg, MD 20899, USA
The reactions of hydrogen atom with 1-phenylpropene (1PP) have been studied in single-pulse shocktube experiments. Contribution from a large number of the possible reactive channels has been observed. The prime channels are the displacement of the methyl group resulting from addition to the olefinic structure and abstraction of a hydrogen from the methyl group followed by internal displacement of the hydrogen, forming indene. Displacement from the ring is less favored. The following rate expressions at temperatures of 990-1100 K and pressures of 1.4-2.0 atm have been determined. k(H + 1-phenylpropene ~ styrene + methyl) = 10la.6-+~ exp(-1200 --- 1000/T) k(H + 1-phenylpropene-' propene + phenyl) = 1014.~176exp(-5300 + 1500/T) k(H + 1-phenylpropene ~ propenyl + benzene) = 1014.~§176exp(-6300 -+ 1500/T) k(H + 1-phenylpropene-* 3-phenylallyl + H2 + Ha)<~ 10t42-+~ exp(-5600 _+ 1000/T) in units of cma tool 1 s- 1, where the relative and absolute uncertainties in the rate constants are factors of 1.10 and 1.25, respectively. An attempt has been made to relate these rate expressions to basic thermal processes, and similarities to those for the appropriate smaller olefins have been noted. Displacement rate constants are made smaller by the reversibility of the hydrogen addition reaction, and together with possible isomerizations, they represent an additional complicating feature,
Introduction Hydrogen-atom reactions with unsaturated compounds are of great importance in combustion. This paper reports on single-pulse shock-tube experiments involving hydrogen-atom attack on 1-phenylpropene (1PP). A summary of earlier work on simpler compounds and pertinent lower-temperature data can be found in Table 1 [1-9]. Quantitative determination of mechanisms and rates for these reactions are difficult because there are numerous channels for reaction: addition to the sites of unsaturation, followed by decomposition of the radical and hydrogen-atom abstraction. The latter will lead to the formation of a large radical, which if not oxidized, can grow while the former may lead to the breakdown of the molecule to smaller fragments. The significance of such branching ratios to combustion chemistry is clear. 1-Phenylpropene is a more complicated molecule than those which have been studied. It contains both aromatic and olefinic structures. The large number of such species present during combustion creates the need for defining how information on simpler molecules can be extended. Figure 1 contains possible reaction pathways. The numbers are the chan-
nels of primary interest. The letters refer to subsequent fast reactions that may lead to the observed products. Although the mechanism is complex, the conditions in single-pulse shock-tube studies lead to simplifying features. Kinetics and thermodynamics drive reactions to eompletion, and many other possible pathways and products are eliminated. The experimental approach is to use the decomposition reactions of a very labile organic compound, hexamethylethane (HME), as a source of hydrogen atoms. The processes are [10] HME = 2 t-butyl = isobutene + H
(1)
with k 1 = 3 • 1016 exp(-34,500/T) s -1. The subsequent breakdown oft-butyl is rapid under the hightemperature conditions of single-pulse shock-tube studies. Other reaction possibilities are unimportant. The general instability of nearly all large organic radicals (certain resonance stabilized radicals are exceptions to this rule) under the conditions of these studies (high temperatures and high dilutions of molecule undergoing unimolecular decomposition) facilitates the interpretation of the data. Referring to Fig. 1, one sees the possibility of associating reaction pathways with particular unsaturated stable products.
867
868
REACTION KINETICS TABLE 1 Past work on hydrogen-atom attack on organics at high and low temperatures Rate expression Rate constant (1100 K) (cm mo1-1 s 1) Displacement [group]
Abstraction/H
1.7 x 1013exp(-- 1808/T)[CH3] 3.3 • 1012 6.7 • 1013exp(-3255/T)[CHa] 1.2 X 10WCH 3 3.5 X 1013exp(-3710/T)[CHa] 1.2 • 1012 4.8 • 10 la exp(-4795/T)[C1] 0.6 • 10lz 1.~ X 1013exp(-257S/T)[CHa] 1.2 • 1012 2.2 • 1013exp(-3990/T)[OH] 0.6 • 1012 ~9 • 10la exp(-4531/T)[Cl] 0.4 • 1012
1.7 X 1014exp(-4024/T) 0.7 X 1012 3.7 • 10TM exp(-4342/T) 0.8 • 10 ~2 9.6 • 10la exp(-4265/T) 0.6 • 10 lz
Compound attacked by H [Ref.] A. High Temperature Results Isobutene [2] Mesitylene [3] p-Chlorotoluene [4]
Toluene [1] Phenol [5] Chlorobenzene [6]
1.2 • 1014exp(-4138/T) 0.9 • 1012 1.1 • 1014exp(-6240/T) 0.4 • 1012
B. Lower-temperature results: rate constants are derived from rate expression and refer to addition without displacement. gutadiene [8] 4.1 X 1013exp(-655/T) (2 sites) 300 4.5 X 10~z 1100 2.3 • 10'z t-Butene-2 [9] 2.1 • 1013exp(- 1046/T) 300 6.3 • 10n 1100 0.8 • 1013 Isobutene [9] 3.7 • 1013exp(-849/T) (terminal addition) 300 2.2 X 1012 1100 1.7 • 1013 Benzene [7] 4 • 1013exp(-2170/T) ( 6 sites) 300 2.9 • 101~ 1100 5.6 X 10lz
Every isobutene that is generated implies a hydrogen released into the system. By carrying out the reactions in a vast excess of 1PP and mesitylene(1,3,5trimethylbenzene) (MES) in comparison to H M E (ratio of these two compounds to H M E is in excess of 100 to 1), with the latter present in concentrations near 100 ppm, practically all the hydrogen atoms will react with 1PP and MES. For MES [3], the result of hydrogen-atom attack are displacement of methyl leading to meta-xylene formation, H + MES = meta-xylene + CH3
(2)
and abstraction of benzylic hydrogens,
and MES are m u c h more stable than H M E (in the unimolecular sense), rate constants for products from 1PP and MES can be determined from the following relation: [product~/1PP]/[m-xylene/MES] =
Substitution of the rate expression for k2 will then yield rate constants for the process initiated by hydrogen-atom attack on 1PP. The yields of isobutene also serve as an internal t h e r m o m e t e r for the determination of the temperature of the system. T h e relations are
1/T
H + MES = 3,5dimethylbenzyl + H2 with the rate expressions given in Table 1. Since 1PP
k(product~)/kz.
with
= [38 - In(k,)]/34,500
HYDROGEN-ATOM ATTACKON 1-PHENYLPROPENE AT HIGH TEMPERATURES
H
.~
+ C3He
G
+H*
869
+ CH=CHCH 3
9
g. C2H2+CH3 *
+H*
FIG. 1. Mechanismfor the hydrogen atom-induced decompositionof 1PP.
k 1 = t 1.1n[1 - (1.03-isobutene/HMEi) where t is the heating time of 500/is and the factor of 1.03 refers to the propene formed during isobutene decomposition. The use of the internal temperature removes the chief uncertainty in such experiments. It depends on HME being present in small amounts relative to other reactants and short chain lengths for reactions involvingthe main components. Despite the large number of studies on hydrogenatom reactions with unsaturated organic compounds, there are no data on hydrogen-atom attack on 1PP or even related molecules. The most pertinent results are lower-temperature data on hydrogen-atom attack on the C4 olefins and benzene. These are summarized in Table 1. The values at 1100 K from the lowtemperature data are extrapolations. At room temperature, the main process is addition. Displacement does not occur, since the low temperatures prevent radical decomposition. There are large differences in rate constants. These can arise from small differences in activation energy. The data in Table 1 suggest that rate constants for addition to the aromatic ring may be an order of magnitude or more smaller than addition to an olefinic sti~lcture under the present reaction conditions. An important thrust of this work is to determine this branching ratio for 1PP. For olefins, relative rate constants for addition to a molecule are affected by the stability of the radicals [It] that are formed. Hence, the propensity for terminal addition. Terminal addition to butadiene will lead to a radical that is stabilized by allylic resonance, and one would expect that this will be the preferred mode. There are no data as to whether the full effect
of the allylic resonance energy, of the order of 50 kJ/ tool [12], is reflected in the rate constant. The data on butadiene and the butenes demonstrate that such effects do not occur between molecules. The same issue arises in the present system, except that benzylic resonance is substituted for allylic resonance. The values of the two resonance energies are similar. The differences in rate constants between trans butene-2 and the cis compound at room temperature are small. Fahr and Stein [13] have reported on the rate constants for phenyl and vinyl addition to benzene and ethylene at temperatures very close to those used here. Their data can be related to some of the present results through the thermodynamics, It will also be used to obtain information on the specificity of addition to the olefinic portion of 1PP,
Experimental Experiments are carried out in a heated singlepulse shock tube. This permits studying the reactions of high-boilingorganic compounds. Details of the experimental procedures have been given in earlier publications [1]. Analysis of reactants and products was by gas chromatography with flame ionization detection. A 6-ft Poropak N column was used to analyze all substances up to propene and was operated at 90 ~ All substances with boiling points higher than isobutene were analyzed with a 30-m dimethylsiloxane wide bore capillary column operated in the programmed temperature mode. Trans and c i s - l P P and
870
REACTION KINETICS 0.5
I::l
"'" *
0.0
=lib
,.o
" ;;';i " ; ;
o
"~.-0.5 o
OI o b-I
V v
-1.5
-2,0
~ 1020
= 1040
~ 1060
' 1080
TEMPERATURE (K) MES were purchased from Chemical Samples;* HME was from Aldrich Chemicals and, except for vigorous degassing, were used without further purification. Argon was ultrapure grade from Matheson. The only impurity was trace amounts of methane and was corrected for in the subsequent analysis. The major lower boiling point impurity in 1PP was allylbenzene. There are also some unidentified higher-boiling impurities in much smaller amounts than the allylbenzene in trans-lPP. The concentration of allylbenzene in the trans compound was about 0.2% in comparison to 1% in the cis mixture. A majority of the data and all the quantitative results on rate constants were derived from experiments with the trans compound. The results on the c/s compound served as a convenient check of the effect of impurities on the integrity of the results. Four sets of experiments were carried out. The compositions of the mixtures were as follows: 1. 100 ppm HME in 1% trans-lPP in argon; 2. 100 ppm HME in 1% cis-lPP in argon; 3. 136 ppm HME in 0.5% trans-lPP and 1.2% MES in argon; and 4. 100 ppm HME in 0.5% cis-lPP and 1.0% MES in argon. The invariance of the experimental results with changes of relative concentrations of HME, 1PP, and MES testifies to the absence of contributions of 1PP and MES decomposition in these studies and the validity of the postulated mechanism. This is a powerful test. The experiments were carried out at 1.4-2.0 atm pressure and in the temperature range 990-1100 K. *Certain commercial materials and equipment are identified in this paper in order to specify adequately the experimental procedure. In no case does such identification imply recommendation of endorsement by the national Bureau of Standards, nor does it imply that the material or equipment is necessarily the best availablefor the purpose.
11 O0
120
FIG. 2. Product distribution from the hydrogen atom-induced decomposition of 1PP. Large symbols are for reactions with trans-lPP. Results from mixture A. Small symbols are for reactions with cis-lPP. Results from mixture B: (#) styrene; ([-1) methane, ( i ) indene; (0) ethane; (C)) benzene; (0) propene; and (V) acetylene. Pressures range from 1.4-2 atm argon. Results
Data from studies with the first two mixtures can be found in Fig. 2. The yields have been normalized in terms of per hydrogen atom introduced into the system. There is very little difference in the data from the trans- and cis-lPP mixtures, except that at the lower temperatures, there are more propene and benzene from the cis results. This is probably due to the presence of impurities. Otherwise, it is not necessary to draw a distinction between the two sets of results. Trans ** cis isomerization is observed. This has no consequence in this work. Styrene is the main product. Indene is the next product of importance. All other products are in much smaller concentrations. Quantitative interpretation of the latter will have larger errors than that for the major products since there may be contributions from labile impurities or small side reactions that have been neglected in the analysis. They are a reflection of the "noise" in such studies. Uncertainties in the relative concentrations of the major products are expected to be in the range for standard gas chromatographic analysis or of the order of ---3%. For the minor products, this should be increased by a factor of 2. The presence of allylbenzene as an impurity prevented determination of contributions from this channel. The propene concentrations used here have been corrected for its formation from t-butyl decomposition. Large yields of methane and ethane are found. These are from fragments formed during styrene formation. The sum of methane plus twice the ethane is slightly larger than the styrene and other compounds, acetylene and propene from isobutene, whose presence imply simultaneous ejection of methyl. If the system is behaving as assumed, these should be exactly equal. This may be a reflection of small side reactions that have been neglected. From the propene yields, it is clear that only small amounts
HYDROGEN-ATOM ATTACK ON 1-PHENYLPROPENE AT HIGH TEMPERATURES
871
1.0
A
-~
0.5f 0
_ ~
. 00[ [0
~
n~
~
~O'v
1~
[]
g, f
o 9
_..
_
A A
O w
A
A
0
---
cP
o 9
o~
o
v
9
-1.0
-1.5
0.88
I
I
I
I
I
i
0.90
0.92
0.94
0.96
0.98
1.00
IO00/T(K)
FIG. 3. Ratio of rate constants for the production of styrene (i~), indene (11), propene (O), and acetylene (V), respectively, vs that for meta-xylene generation during the reaction of hydrogen atoms vs 1PP. Filled symbols are data for trans-lPP and mesitylene, mixture C. Open symbols are data for c/s-lPP and mesitylene, mixture D. Pressures range from 1.4-2 atm argon.
TABLE 2 Summary of experimental rate expressions k(Product channel) Product channel
Styrene Indene (from 3-phenylallyl) Acetylene (from propeny]) Propene Styrene + propene
k(Mesitylene)
k(Product channel) (cm3 mol -t s-t)
1100 K
0.65 x exp(2000 4-_ 190/T)
4.4 X 10~3exp(- 1200 _+ 1000/T)
1.4 • 10~*
12.6 • exp(-2400 + 300/T)
8.5 • 10~4exp(-5600 -2_ 1000/T)
5.2 X 10~~
2.2 x exp(-3100 _+ 510/T) 1.6 • exp(-2000 + 750/T) 0.85 X exp(1800 _+ 200/T)
1.5 x 10~4exp(-6300 _+ 1500/T) 1.1 X 10~4exp(-5300 -+ 1500/7") 5.7 X 10~3exp(- 1400 4- 1000/T)
4.7 • 10~ 9.1 • 10n 1.5 X 10~3
Uncertainties in column 2 are least-squares values, while those in column 3 are estimates and include 4.2 kJ/mol uncertainty in the rate expression for displacement of methyl from mesitylene. The uncertainties in the relative and absolute rate constants are factors of 1.1 and 1.25, respectively. Variations in the A factors are commensurate with these values. See text for further details. of phenyl radicals are formed. Together with its high reactivity, this is the reason for the absence of addition products such as toluene. The normalized yields of styrene are greater than 1. When all other products directly derivable from hydrogen reactions with 1PP are summed, the chain length is near 2.5. When MES is added, the chain length is reduced to 1.5. Figure 3 contains rate data pertaining to the ratio of rate constants for the formarion of the appropriate product from 1PP and the rate constants for the displacement of the methyl group from mesitylene-forming meta-xylene. Mso included are data fi'om the cis-compound. They track that of the trans mixture except for propene where, as noted earlier, there appears to be an extraneous sonrce.
Table 2 contains a summary of the rate expressions and constants derived from this study. The rate expressions in the second column in Table 2 are given in terms of the production of particular compound(s) compared to that of meta-xylene from MES and are a quantitative summary of the experimental results. The results in the third column are from multiplying those in the second column by k2. Also given in Table 2 are rate constants at 1100 K. This serves as a convenient reference point for comparisons. The value of the rate constants cover a wide range. For the less important channels, the rate constants, instead of rate expressions, are probably more meaningful. Estimated uncertainties for the relative rate expressions are 4.2 kJ/mol in the activation energy and a factor of 1.6 in the pre-exponential factor for styrene and
872
REACTION KINETICS 2.0
"d
w
1.5 0 k, n.
db
9
9
~ 1.0
~ 0
0.5 i 0.0 0.9
ill I
I
0.92
0.94
1000/T
(K)
indene and 8.4 kJ/mol in the activation energy and a factor of 2.5 in the A factor for acetylene and propene. In absolute terms, the uncertainty in the rate expression of the methyl displacement reaction from MES leads to an additional uncertainty of 4.2 kJ/mol in the activation energy and a factor of 1.6 in the A factor. These uncertainties are larger than that derived from the statistical scatter. They are based on experience from the consistency of earlier studies [10,14]. The relative and absolute uncertainties in the rate constants should be factors of 1.10 and 1.25, respectively. Figlwe 4 contains data on the ratio of products from hydrogen-atom attack on 1PP as compared to that producing styrene and demonstrate the effect of MES addition. For propene and acetylene, the results match. In the case of indene, the inhibiting effect of mesitylene leads to the observed lower yields. The additional mode for indene production in the absence of mesitylene is probably the abstraction by methyl of a hydrogen from 1PP, leading to the 3-phenylallyl radical, Only hydrogen-atom attack and the consequences were considered in Fig. 1. Methyl addition processes are reversed and will not make any contributions.
Discussion
The observations summarized in the third column of Table 2 will now be correlated with the thermal processes given in Fig. 1. Under the present conditions, the thermal instability of the larger radicals formed from hydrogen reactions simplify mechanistic possibilities. The only reaction they can undergo is unimolecular decomposition. Figure i lists four reaction channels. Two of these involve addition (channels 3 and 4) to the olefinic portion of 1PP. Another process is addition to the propenyl site (channel 5) on the ring and the last is hydrogen abstraction
0.96
FIG. 4. Ratio of rate constants for the production of indene (1), propene (O), and acetylene (y), respectively,vs styrene generation during the hydrogen atom-induced decomposition of 1PP in the absence (mixture A, symbols) and presence (mixture C, lines) of mesitylene. Pressures range from 1.4 to 2 atm argon.
(channel 6) from the methyl group. Other reactions are not included since they are much more endothermic or can readily be reversed. Addition of hydrogen at the methyl site of olefinic portion of 1PP (channel 4) followed by ejection of methyl (reaction d) must lead to styrene and methyl. Addition at the phenyl site (channel 3) will lead (reaction 2) to propene and phenyl. The possibility of a neophyl rearrangement [15] (channel b) can lead to the formation of 2-phenylpropyl-1 radical, which will decompose readily (channel c) to form methyl and additional styrene. Thus, the present data does not yield independent information on the specificity of H addition to the olefin. The sum of styrene and propene may be a measure of addition to both sites of the double bond (k3 + k4). Ambiguity arises from the possibility that the addition reaction (channel 3) may be reversed. Thus, k a + k4 so deduced must be a lower limit. However, the H + 1PP --* phenyl + propene reaction is approximately thermoneutral at 1000 K [16-18]. Rate constants for hydrogen and phenyl bond cleavage may be not be far apart. Propene, a measure of the phenyl released into the system, is only 7% of the styrene. Thus, little error is introduced into the addition rate constant by the neglect of the hydrogen ejection channel. At 1100 K, the sum of rate constants for styrene and propene formation is 1.4 • 1013 cm 3 mol- 1 s- 1 or very close to 0.5 times the value for butadiene given in Table 1. The results of Fahr and Stein can be used to arrive at an estimate of the specificity of hydrogen addition to the olefinic part of 1PP (channels 3 and 4). They found k(C6H5 + C2H4 ~ styrene + H) = 2.5 • 1012 exp(-3121/T) cm 3 tool -1 s -1. Assuming that this will also hold for terminal addition to propene and accounting for the reaction degeneracy of 2, one finds through detailed balance k(H + 1PP --* C6H 5 + C3H6) = 9.5 X 1013 exp(-3222/T) em 3 mol -I s -1. This is equal to 5 • 1012 em a mo1-1 s 1 at 1100 K. Combining this with the overall rate constant for
HYDROGEN-ATOM ATI'ACK ON 1-PHENYLPROPENE AT HIGH TEMPERATURES addition given earlier, the ratio of rate constants for hydrogen addition to the methyl and phenyl sites in 1PP is 1.8. This value is strongly dependent npon the thermodynamic properties of phenyl and its combination rate constant. For the former, the number used here, 328.5 kJ/mol, is from Burcat [18]. A slightly higher value of 340.5 kJ/mol [19] had been suggested and will lead to a rate constant of 1.5 • 1012 cm 3 mol i s- 1 for propene formation. Fallr and Stein assume a combination rate constant for phenyl of 3 • 1012 cm 3 mo1-1 s -1. A value of 1013 cm 3 mol 1 s-1 will raise the original value to 8.5 • 1012 cm a mol 1 s-1. The corresponding ratios are now 8 and 0.8. Although the results have wide scatter, these considerations demonstrate that a significant portion (although less than half) of the hydrogen atoms may add to the phenyl site of 1PP. Benzyl resonance energy is not a large influence on the position of hydrogen addition on the olefinic portion of 1PP. This sets a limit of 10-40% on the error introduced in assigning styrene and propylene yields to ka + k4. The acetylene yield can be a measure of propenyl formed (channel g) if isomerization to allyl is not important. Results on methyl attack on acetylene [8] and the thermodynamics leads to 4.5 • 1013 exp( - 15,900/T) s -1 as the rate expression for propenyl decomposition. The thermodynamic properties of propenyl are based on a vinyl C - - H bond energy of 459 kJ/mol [20] and molecular parameters estimated in the usual manner. Such isomerization processes can be treated as an internal abstraction with an additional contribution to the activation energy from the ring strain in the transition state [21]. For abstraction of allylic or benzylic hydrogens, the resonance energy has only a small effect on the rate constants [1]. The strain energy for a four-membered ring structure is 125 kJ/mol [22]. The transition state must be tight. A likely rate expression for isomerization is [20] 3 • 1011 e x p ( - 1 7 , 5 0 0 / T ) s -1 and should be unimportant compared to decomposition. Acetylene yields are not a measure of hydrogen addition to the propenyl site of the ring structure in 1PP (channel 5), since the reverse reaction is likely to be competitive with propenyl ejection (channel e). It represents instead a minimum value for ks. The activation energy for acetylene formation is higher than those for the displacement processes in Table 1. This suggests that the barrier for propenyl ejection is higher than the hydrogen addition barrier. Nevertheless, the rate constant is only a factor of 2.5 smaller than that for hydrogen addition to toluene, where displacement is a measure of hydrogen addition. Thermodynamic factors favor the displacement of methyl from the olefinic part of 1PP or the abstraction of a hydrogen by H atoms or a radical such as phenyl in comparison to displacement ofpropenyl. The ratio (styrene + propene)/acetylene is an upper limit to the ratio of rate constants, (k 3 + k4)/ks, which defines H-atom addition to the olefinie and ring
873
structures in 1PP. The data in Table 2 lead to a factor of 33 at 1100 K. The actual value of (k 3 + k4)/k 5 must be lower. If displacement of methyl from toluene is equivalent to addition at the propenyl site, one obtains a value of 12. This is the same value from the data on Table 1, using butadiene and benzene as a basis. Fahr and Stein [13] found the rate expression for vinyl addition to benzene to be k(C2H3 + C6H 6 styrene + H) = 1.5 X 10lz e x p ( - 3 2 2 2 / T ) c m a tool -1 s -1 or 8 • 101~ cm 3 mo1-1 s -1 at 1100 K. These are different (factor of 1.8) from the original estimate [13J since a newer value for vinyl combination from Fahr et al. [23] has been used. The thermodynamics and the rate expression for propene yield obtained here gives k(propenyl + C6H6 ~ 1PP + H) = 1.6 X 1013 exp(-5940/T) cm 3 tool -1 s -1 or 7.3 X 101~cm 3 mol 1 s-1 at 1100 K. Despite the differences in the rate expressions, agreement in the rate constants at 1100 K where both measurements were made is good. Fahr and Stein caution their readers on the use of their rate expression beyond the measurement range. The estimated uncertainty in the activation energy in this study is 13 kJ/mol. Thus, the disagreement may not be significant. Abstraction of a hydrogen from the methyl group (channel 6) will lead to the formation of 1-phenylallyl radical. The resonance energy of this radical is 66 or 18 kJ/mol higher than those for benzyl or allyl [24]. Internal displacement of a hydrogen will lead to the quantitative formation of indene [25] (channel f). Hydrogen, as well as methyl and phenyl, can be a abstracting radical. Thus, indene concentration sets an upper limit for the rate constant of hydrogen abstraction (k6). The rate expression given here can be compared with others leading to the formation of other resonance-stabilized radicals. From Table 1, it appears that the value from indene concentration is about a factor of 2 larger. A larger rate constant may be consistent with the 18 kJ/mol extra resonance energy. However, earlier studies indicate that resonance energy has very small effects on rate constants for abstraction [1]. The values given here may have additional contributions from methyl abstraction. This is supported by the rate expression, where the A factor is larger than that for other hydrogen-abstraction processes. These results demonstrate that the main role of hydrogen atoms is to strip alkyl groups from the olefinic portions of large unsaturated compounds containing olefinic and aromatic groups. Abstraction also makes a contribution. Next in order of reactivity is the removal of alkyl structures from the ring. The reversibility of hydrogen addition to these systems makes removal of phenylic and vinylic structures more difficult. Under combustion conditions, most of these effects can be almost quantitatively inferred from data on simpler molecules. There would appear to be a good basis for making estimates. Complicat-
874
REACTION KINETICS
ing factors are the isomerization pathways and uncertainties in the rate constants for decomposition of radicals that involve removal of phenyl and vinyl groups as a result of the reversibility of the initial addition process.
11. 12.
REFERENCES
13.
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COMMENTS John H. Kiefer, University of Illinois at Chicago, USA. Your agreement with the Fahr and Stein rate for C6H~ + C2H2 depends very much on the chosen 108.5 kcal/mol for the CH bond energy in ethylene. If you use tile "latest" value of 111.2 [Berkowitz, J., Ellison, G. B. and Gutman, D.,J. Phys. Chem. 98:2744 (1994)] you would not agree at all. Author's Reply. We are in agreement with Dr. Kiefer's conclusion. Use of the higher value would have led to a discrepancy of a factor of 4 in the equilibrium constant. We believe that the equilibrium constant derived from the experimental numbers is accurate to within a factor of 2. It suggests to us that the "higher" value for the heat of formation derived from nonkinetic methods is in error.
Note that the number derived from kinetic methods, as given in Dr. Kiefer's reference, is near 107 kcal/mol.
R. W. Walker, Hull University, England. Do you have any comment on the apparently large A factor for the H-abstraction reaction at the allylic position in I-phenylpropene? As a rotation is lost in the transition state, a value nearer 1013might be expected. Author's Reply. As noted in the text, the rate expression for the abstraction of the resonance stabilized hydrogen contains components from abstraction by methyl. Therefore it is a maximum value. This is probably the reason for the somewhat larger than expected A factor.