HOCK TUBE
Crs-
BANS
ISOMERIZATIONSTUDIES
282
pans Isomerization Studies.
IP
y Peter M. Jeffers Department of Chemistry, State Universitu College, Cortland, New York 130.46 (Received April 12, 1975)
Rates of isomerization of cis- and trans-1,2-dichloroethylene and cis- and trans-perfluoro-2-butene were measured relative to cis-2-butene isomerization. A a/4-in.single-pulse shock tube served as the reactor. Rate constants derived for the isomerizations agree well with available results of low-temperature studies. Rate constants found for the temperature range 1050-1350°K are log k,,t(C4Fs) = 13.21 - 54,700/4:.58T, log kt--cc(C1Fs) = 12.98 - 55,000/4.58T, log k,+t(CzHzCh) = 12.35 53,400/4.58T1log k+,+c(CzH&l,) = 12.26 52,700/4.58T. Combination of shock tube and low-temperature conventional results yields vnlues of log k6,t(C2H2C12) = 13.2 - 57,400/4.58T and log k,,+,(CSe) = 13.2 - 54,900/4.58T.
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Part I of this series reported results on cis-trans isomerization of cis-2-butene and cis- and trans-l12-difluoroethylene. It was evident from that work that the shock tube relative rate technique employed was e:rrcellently suited for finding the limiting high-pressure rate constants for homogeneous gas-phase isomerization reactions. A. recent summary* of thermal isomerizations lists only nine studies spread over a 35year period, and in most of these investigations some culties were experienced due to heterogeneous or radical catalysis. Neither of these problems arises with the shock t u method. Pn addition, the shock tube results for a en reaction usually cover a consXclerably higher temperature range than is found with conventional methods due to the inherent very short residence times. For these reasons, this series of studies continues. This paper includes results on cis- and trans-1,2-dichloroethylene which can be compared with the study reported in Part of difluoroethylene and on cis- and traazs-perfluoro-2- utene which is of interest to compare v7ith both 2-bute e and difluoroethylene. There have conventional studies of trans-C4Fs3 and cis1z4 both at a 300" lover temperature range, while of the reactions in each direction as was done here allows a check for consistency with the known equilibrium constants for these systems.
The 19-mm id. Pyrex shock tube has already been ckscribed.' The d y alteration was a new end block still housing a BaTiOS pressure-sensing crystal, but with a 1-nim hole leading to a rubber septum through which samples were rawn directly into a Precision syringe. Direct use of the E3ampBing Co. gas ~yiyringo,a completely grease-free system, gave much better results with dichloroethylene experiments than were obtained in preliminary work where gas samples were withdrawn into sample bulbs and subsequently oompy.essed in B mercury transfer system before being injected. into the gas chromatograph. Apparently,
some of the C ~ H Z Cabsorbed I~ on grease of tile shock tube valve, the sampling bulb stopcock or joint, or the stopcocks or joint of the transfer apparatus. Materials. Phillips research grade cis-%butene was used as supplied and contained O.OSyo of the trans isomer. A cis-trans mixture of Matheson perfluoro%butene was separated using a 6 mm X 15 in column c k80~~d~ of 20% Kel-F-oil No. 3 ( H e ~ ~ r l e t t - ~ ~on 100 mesh Chromosorb-1'. The column was kept a t 0". About 0.5 ml of the liquid G z s mixture (bp 0") was injected a t a time, and two repeat ~ h r o m a ~ o ~ r a ~ h ~ c separations of the crude products yielded Irans-CJ?s containing 0.3ojb of the cis isomer aiid eis-C4F8with 0.8% trans impurity. No other impurities were noted. Eastman practical grade 1,2-dichloroethylene (cistrans mixture) was separated with % 25 ITLM X 1.83 m 10% silicon gum rubber column a t room temperature. Two chromatographic separations gave 99.97% pure trans- and 99.3% pure cis-C2W2C1,, with the other isorner as the only impurity. Liiide high-purity Ar was used for sample preparation, and h ~ g ~ ~ - ~ ~ ~ r ~ t y fts the driver gas. Analysis. M1 analyses were performed with a Hewlett-Packard Model 5750 gas ~ ~ r o m ~ ~ ~ o using the flame ionization detector Samples containing perfluoro-Bbutene and 2-butene were passed ~ u r % ~ e ( through a 6 mm X 1.83 m ~ ~ ~ ~ ~ " s apolypropylene glycol column at room ~ ~ m ~ e r ~then t ~ r e , through a 6 mm X 3.05 m column 3f Porapslk and ~on, 150". Areas were determined by t ~ ~ a n ~ u ~ a L calibrations indicated the trans-C4Fg area liad t o be multiplied by a factor of 1.03. Mixtures containing dichloroethylene and %butene could be separated (1) Part I: P. M. Jeffers and W, Schaub, 1.Amer. Chemn.SQC.,91, 7706 (1969). (2) S. W. Benson, "Thermochemical Kinetics," Wiley, New York, N. Y., 1968, p 74. (3) E. W. Schlag and E. W.Kaiser, Jr., J . .!.mer. Chem. Soc., 87, 1171 (1965). (4) L. D. Hawton and G. P. Siemeluk, Can, J. Chem., 44, 2142 (1966).
The Journal of Physical Chemistry, V Q ~76, . N o . 20, 1975
5 1.9
f.
I 1.2
_____L.___p_II
I
. I
1.6
log k
2.8
7.4
2.0
CIS-2-PUTCNE
Figure 2. Plot of log k(t -+ c ) for C 9 8 vs. log k ( c -+t) for CaHa: 0 , 2% of each reactant; 0, 0.2% of each reactant; 0.02% of each reactant.
l.G
2.0
log k
2.4
2.8
3.2
+,
3.6
CIS-2-RUTBNr
Figure 3. Plot of log Icjc -+- t ) for zH2C12 08. log kjc --p t ) for 0.04% of each reactant. C4E8: 0, O2y0 of each reactant;
chosen as E, for cis-%butene. Results for cis-CsFg are shown in Figure 1. The least-squztres slope is 0.888 P 0.03 which implies an activation energy of 55.0 i 1.9 kcal/mol. The ~ r e e x ~ o n e ~ t i€actor 6 1 . ~ is then found to be 10”3.2210.2. can be determined from the e x ~ e r ~ ~ rate ~ n tcons ~ stants for %butene and the known Arrhenius pssameters, and is 1060-1250°K. ~ r ~ ~ was s studied ~ ~ ~ * ~ * from 1080 to 2240°K, th results as shown ius F i ~ u r e2. Thc slope of 0.893 -fi 015 gives an activation energy of 55.3 f 1.0 kcal/mol and 61. ~ r e e ~ ~factor o ~ of~ ~ ~ ~ a
[email protected]~
j
,/i
l-.---L.---& 41 8
1.2
1.6
log k
I
2.0
2.1
--I 2.8
CIS-?-BUTENE
Figure 1. Plot at log ~ I ’ C4 t) for C4Favs. log k(c t ) for C4X& 0, 2% oi each reactant; 3., Q.2%of each reactant; e, O.Q2% of each resetant.
~ i c ~ ~ o Isomertzation. ~ o e ~ ~ Three ~ ~ nsets~ of ~Biocki~ were made with cis-1,2-dichloroetbylene and cis-2butene mixtures with initial cntratioms of each reactant of 2,0.2, and 0.04Or,. rate plot, Figure 3, is 0.860 0.03 which gives .Ea=
*
-f
(5) R. E. Ruff and 8. H. Bauer, S. Chem. Phys., 36, 1754 (1962).
~ H O C XT . rm~ d;ns--'Tmm
ISOMERIZATION STUDIES
2831
3.5
f'
3.1
10
1.0
1.4
I
I
1.8
2.2
I
2.6
1
3.0
I
8.0
9.0
10.0
11.0
12.0
13.0
14.0
1 0 ~ 1 ~ log k
CIS-2-BUTENE
Figure 4. Plot of iog k ( t --t c ) for C2H2Clzus. log k ( c -+ t ) for C4Hs; 0 , 0.2% CzH&lz-O.l% C4Hs. C4R8: 0, 2% @&~C1~-1%
53.4 f 2.0 kcal/niol and log A = 12.36 f 0.3. The temperature range is 1120-1350°K. Two series of shocks with trans-CzW2C12 and cis-C4Hs had initial concentrations of 2-1% and 0.2-0.1%, respectively. Figure 4, whicla shows the results, has slope of 0.85 f 0.01 and implies a rate constant of 1012.26*o.1 PXP (-52,700 f 6OO/RT) sec-' over the temperature range 1080-1275°K. With the previously stated initial pressures in the shock tube, the shock compression yielded total reaction pressures of 3500-4000 Torr, so that the isomerizations should have all been in the unimolecular highpressure regions. The per cent conversion in the isomerizations ranged from 2 to 5% as a lower limit up to as much as 40y0. The lower bound was set by the detector sensitivity and by the di%culty of quantitatively measuring convemions j i i s t slightly larger than the background impurity level. The upper conversion limit was selected sufficiently below the temperature range where side products appeared to give reasonable assurance that the komerizatnon was not free-radical catalyzed. All shock experiments where the conversion was acceptable are represented on the relative rate plots.
Discussion The comparison standard for the CzHaClz and C4Fs isomerizations was cis-2-butene, for which a rate constanr, of i ' ~ -exp(-62,000/RT) was chosen. This value is a "rounded off " version of the previously determined' k, -= 10"3.38exp( - 6lJ600/RT), and is within the experimental uncertainty. Both the A factor and the &ctivationenergy agree exactly with the values calculated b y Benson.2 The value found by Schlag and Kaiser3for perfluoro%butene isomerization is kt = ioi352*0 exp( -.56,400 =t160/RT) and agrees v, ith the previously stated results )\ ithm the sum of experimental uncertainties. Using
Figure 5 . Arrhenius plot for cis-perfluoro-2-buterie showing low T results3 ( k = 101a.42-55,100'4,6*T), high T results of tjhis study ( k = 10'3.21-54,1"'4,68T), and the best line connecting the two sets ( k == 1013.21-54,900/4.58T ).
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their values of AH(t c) = 816.8 ltcal/mol and A S = -0.486 eu, one finds k(c t) = k(t c)/K = 1013.4ze-55~100/RT. Schlag and Kaiser's results cover the temperature range 703-750°K while the present study was a t 1060-1250°K. Plotting both sets of data on a log k us. 1/T graph (Figure 5 ) and connecting them yields avalue of k(cis trans) = 10'3.21exp( -54,900/ R T ) , in excellent agreement with each individually determined value and especially with the shock tube result. Benson's calculated value for log A is 13.1 which is also in excellent agreement. The ratio OS kJk-1 for perfluoro-%butene found here is i O o . 2 ~ + ( 3 0 0 / 4 . a * r ) = 1.9 at 12OO"K, in agreement, with Klz0oo = 1.8 deduced from Schlag and Kaiser's3 thermodynamic parameters. The difference of 300 ca1/ mol in forward and reverse activation energies agrees reasonably with Schlag and Kaiser's vzlue of 41-1" = 816.8 cai/mol. The activation energy for ezs-%butene now seems rather well established at 62 kcal/mol, so that the decreasc in E, in going t o the perfluoro molecule is about 7 kcal. For compwison, Part P I of this series reported E , €or cis-difluoroethylene 89 62.8 kcal/mol. The present study indicated k(c @)for dichloroethylene to be 1012.36 exp(-53,400/RT), compared to 10l2 exp( -56,00O/RT) found by Hawton and Semelul~.~The agreement in activation energies is probably within experimental error. However, if the shock tube results (1120-1350°K) are combined with the conventional data (807-845°K) on a log k us. 1 / T plot, (Figure 6), a value of k ( e i-) -- 1013 exp(- 57,4OO/RT) is obtained. For this reaction Benson2 calculates log IC = 13.2 - 57,0O0/4.589' which is nearly exact agreement. The differences between the relative rate value for eis-C&Cl2 and the value determined from combination of high- and low-temperature data appear surprisingly large in view of the OW relative
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The Journal of Physical Chemistry, Val. 76, No. 90, 1978
I _ _ _ r _ l -
Ea, koal/raol
GzHzD2C CzHzFzd
ezazclz
65 62 62 57
@aF*
dB
C4N8
LO^/^ Figure 6. Arrelinius plot for cis-1,2-dichloroethyleneshowing Iow '2' results4 ( k = 10'2 78--66,000'* b e T ) , high T' results of this study ( k = lox* 38-53 400'4 S8T), and the best line connecting the two (le I(p'3 ?--51,iiOO/J 5 7 7 1.
rate experimental scatter. However, the value of the log of k(relative)/lc(combined data) i s 0.06 at 1120°K and 0.19 at 1350"R, arid variations of this amount are not beyond the observed scatter. The experimental section mentions difficulties in preliminary studies of Ihe c ~ ~ c h ~ o r o e which ~ ~ ~ ~required ~ ~ n e sdevelopment of very careful c~ampkhandling procedures. Since the C2lla2C& analj eis did cause more trouble than usual, some small systernrztie error could have entered the Si;SUlh Xfwvl,on and Bemeluk9s suggested rate constant was ~ asurface/volume t~n~ raiio = obtained by r x t r a ~ ~ ~to c), indicating that l,hey experienced some heterogeneous ~ ~ o ~ t r ~ ~to~ ~their i t ~ reaction. ons In view of these difiicriities, the rate constant obtained by combining the low- and high -temperature studies is undoubtedly the best chojc? of th. three. r k h ~rela,tivc rate ,Pesults for C,H&12 give Ll/lc-l = loo O s - (7aCi'4 58'0 = 0.51 u t 1200"K, which is in reasonable agreemeal with ft d u e of K12000 = 0.75 derived fronx Pit eer u ~ i dI3 oBlenberg's6 thermodynamic data. The diffeaenec of 700 cal/mol in forward and reverse ~ c ~ energies j v is ~within ~ experimental ~ ~ ~ limits ~ ~of
wetices are found in the preexponen-
AHzu8(byCL1Ogunlatlonj ,5 kCal/Sd
-32 --28 -27 -52 --A6
'7 4
I 5 2
rc=cSb
A
i 337 Y. 331 I 346
I 354 I 346
a Eeats of hydrogenation were calculated from known heats of formation or from heats af forvriaiiori sstmiated from $enson's tables of group contributions.2 8. a.Bauer, private communication. c B. S. Rabinovitch and F.9.Looney, S , Chem. Phys., 23, 2439 (1955), 23, 315 (1955). Reference 1.
The entries in Table I are listed in decreasing order of activation energy, and the same tiend is observed in heat of hydrogenation, with the exception of diehloroet,hylene. Lin and Laidler? suggest the eorrelation E, -2AH (hydrogenation) for cis-trans isomerization, but there are significant deviations from that simple relation. The correlation between iuztivation energies and double bond lengths is not as good ~esthe heat of hydrogenation relation. Lin and Laidler? recognize tbe close and direct relation between double bond strength ( A N for RRC-CRR' C-C HR') and activation energy for isomerization. Howeverl few data are available on double bond strengths, and estimationsS require knowiedge of specific C-H bond strengths and r m m " e stabrlization energies which are n o t alw-ajc3 avnilablc. I n view of the activation energv changes in going from CzHzDzto C2H2F2to CzH2CL7and from C1 C4Fs, investigation of C&ls could prove quite interesting.
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Aclcnotdedgments. The author ~vouldlike to thank the State University of S e w York Research Foundation for financial support of this project. The following undergraduate students contributed t o the development of this work: Thomas I\Ic@arrick, David Tyminski, Ronald Yost, Water. Schsub, and Robert Utter.
t is1 factors of %-buiene, perflnoro-2-butenel difluoro-
ethylene, and ~ ~ ~ ~ ~ ~ ~ so ~ that o e tdirect h y compari~ ~ n e , (6) K. S. Pitzer and J. L. Hollenberg, J . Amer. Chem. Xoc., 76, 1493 (1954). son oi the a c ~ i v energies ~ ~ ~ omight ~ ~ be expected to be (7) M. C. Lin and K. J , Eaidler, Can. J . Chem., 46, 973 (1968). ~ ~ ~ e 'I['abk a ~I[ contains ~ ~ some ~ ~ relevant f ~ physical ~ ~ . (8) 8. W. Benson, D. M. Golden, and K~Egger, J . Amer. Chem. Xoc., quantities. 87, 468 (1965).
The Journal
of
Phgaieal Ciremistry, Vol. 76,N O . $0,19'7%
