1128
Kern et al.
(CH3)&SH should be approximately one-half that of H2S since tert-butyl mercaptan has only one “transferable” hydrogen atom.” Inspection of Table I shows that this is a t least qualitatively the case. Also, it would be anticipated that irZl for CH3SH,which has four transferable hydrogen atoms, would be significantly larger than that for (CH3)3CSH,an expectation which is also roughly borne out. These observations suggest that the parameters controling the rate of hydrogen transfer in amines are probably also operative in the analogous mercaptans. The low rate constants for hydrogen or proton transfer reactions in methyl and ethyl sulfides stand in sharp contrast to those observed for dimethyl- and diethylamines. The thermochemical data for (CH&S sug ests that reaction 6 is endothermic by ca. 8-15 kcal mol-‘?3 Despite an ion energy between 23 and 16 kcal mol-’ in excess of this endothermicity no hydrogen transfer occurs. It is not clear how this possible excess energy is dissipated and it must be assumed that either only a fraction of the excess ion energy is accessible during the lifetime of the collision complex or factors other than energy content, such as structure, are paramount in limiting the transfer rate for this reaction. The very low values of kl/lZmO (Table I) for sulfides may be a direct consequence of this situation. References and Notes (1) (a) Postdoctoral Fellow (1972-1974); (b) Postgraduate Fellow. (2) T. Su and M. T. Bowers, J. Am. Chem. Soc., 95, 7609 (1973). (3) T. Su and M. T. Bowers, Int. J. Mass Spectrom. Ion. Phys., 12, 347 (1973). (4) T. Su and M. T. Bowers. J . Chem. Phvs.. 58. 3207 (19731. (5) J. M. Brupbacher, C. J. Eagle, and E. ischuikow-Roux, J.’Phys. Chem., 79, 671 (1975). (6) B. H. Solka and A. G. Harrison, Int. J. Mass Spectrom. Ion. Phys., 14, 295 (1974).
(7) The term hydrogen transfer is used here synonymously to denote
(8) (9) (10) (11) (12) (13)
(14) (15) (16) (17) (18) (19) (20) (21) (22)
(23) (24) (25)
unit mass transfer since, in the case of reactions of type (1) or (2), it is not possible, in general, to distinguish between hydrogen and Droton transfer with the exoerimental setuo used. 6 . K. Gupta, E. G. Jones, A. G. Harrison, a i d J. J. Myher, Can. J . Chem., 45, 3107 (1967). A. G. Harrison, Int. J . Mass Soectrom. Ion Phvs.. 6. 297 (1971). W. E. W. Ruska and J. L. Franklin, Int. J. Mass Sbectrom. Ion Phys:, 3, 221 (1969). W. T. Huntress, Jr., and R. F. Pinizzotto, Jr., J. Chem. Phys., 59, 4742 (1973). B. H. Solka and A. G. Harrison, Int. J . Mass Specfrom. Ion Phys., 17, 379 (1975). The ionization potential of t-C4HQSHwas not available and that of n-C4H,SH was used in its place based upon the observation that the ionizatlon potentials of f-C4H9CI(10.61) and n-C4HQCI(10.67) are essentially equivalent. J. L. Franklin, J. G. Diliard, H. M. Rosenstock, J. T. Herron, K. Draxl, and F. W. Field, Natl. Stand. Ref. Data Ser., Natl. Bur. Stand., No. 26 (1969). J. L. Beauchamp, Annu. Rev. Phys. Chem., 22, 527 (1971). S.W. Benson, “Thermochemical Kinetics”, Wiley, New York, N.Y., 1968. S.W. Benson, J. Chem. Educ., 42, 502(1965). L. W. Sieck, L. Hellner, and R. Gordon, Jr., Chem. Phys. Lett., 10, 502 (1971). T. Su and M. T. Bowers, J . Am. Chem. Soc.. 95. 1370 (1973). G. P. Nagy, T. C. T. Thynne, and A. G. Harrison, Can. J. Chem:, 46. 3609 11968). R. S. Hemswok, J. D. Payzant, H. I. Schiff, and D. K. Bohme, Chem. Phys. Lett., 26, 417 (1974). As defined here for hydrogen transfer In amines, only hydrogen atoms on the nitrogen or on the carbon atom CY to the nitrogen are In a geometrically favorable position for hydrogen transfer in an aligned ion-polar molecule collision complex. Such hydrogen atoms are designated “transferable”. Estimatedvalue of CH,SCH,-H bond dissociation energy 94-99 kcal mol-’. R. J. W. Le FBvre, “Advances in Physical Organic Chemlstry 11”, Academic Press, New York, N.Y., 1965. R. D. Nelson, Jr., D. R. Lide, Jr., and A. A. Maryott, Net/. Stand. Ref. Data Set‘., Natl. Bur. Stand., No. 10 (1967).
The Reaction of Cyanogen Chloride and Hydrogen Behind Reflected Shock Wavedaib J. M. Brupbacher, C. P. Esneault, R. D. Kern,’ T. Nlkl, and D. E. Wilbanks Depatfment of Chemistry, University of New Orleans, New Orleans, Louisiana 70 122 (Received December 14, 1976) Publication costs assisted by the National Science Foundation
The rate law for the production of HCN over the temperature range 1850-2900 K was established by recording the time-dependent infrared emission from this species at 3.0 pm in the reflected shock zone. Four mixtures of ClCN and H2dilute in argon, differing in the ratio of initial reactant concentrations and initial shock pressures, were studied in order to determine the various order dependencies. The formation of the product was in all experiments observed to be nonlinear with respect to reaction time. The data were fit to the ,equation 1 ~ H c N / ~ H c N=, ~exp(-k[ClCN],,” ~ 5[Hz]?’[Ar]0.4t2), where h = 1021~s*0~06 exp(-70.3 f 0.6/RT) cm3 mol-'^-^. The units for the activation energy are kcal mol-’. Experiments in which the reflected shock zone was analyzed with a time-of-flight mass spectrometer revealed the products to be HCN, HC1, and C2NP Computer calculated profiles of HCN using a 14 step atomic mechanism with available literature rate constants failed to reproduce the experimental profiles.
Introduction Previous reports from this laboratory have concerned the nonlinear time dependence of product formation and the importance of this observation with regard to the existence of complex mechanisms in exchange s stems involving simple molecules: Hz + D2,’ HC1 + Dz, HCN + D2,4and HBr + D2.5 The nature of the multistep sequence for the first three reactions is not known in detail but it has been suggested that excitation to the higher
H
The Journal of Physical Chemistry, Vol. 81, No. 12, 1977
vibration and/or rotational levels of the ground electronic state is a prerequisite to product formation.6 Evidence has been presented for the exchange of HBr + Dz to proceed via atomic pathways.’ Two metathetical reactions of limited complexity have also been studied, C2Nz+ H2 2HCN7and Hz + C02 HzO + C0.8 In both of these systems, the time dependence for product formation was shown to be quadratic and arguments were made to demonstrate that their respective
-
-
Reaction of Cyanogen Chloride and Hydrogen
mechanisms were not atomic and that the rate laws could be explained in terms similar to the three exchange systems previously m e n t i ~ n e d . ~However, -~ both of the investigations were performed with limited objectives (testing of symmetry predictions for CzNz + Hz and formation of known amounts of water from a practically thermoneutral reaction) and as a consequence the individual reactant concentrations and total mixture pressures were not varied extensively. The reaction of cyanogen chloride and hydrogen is an appealing prospect for a more complete rate law determination for several reasons: it is a relatively clean reaction at high temperatures; the infrared emission from one of the major products HCN is free from interference of other emitting species; the products and possible intermediates appear in a conveniently observed portion of the timeof-flight mass spectrum; and lastly, most of the elementary rate constants necessary for computer calculation of the product formation profiles (assuming an atomic mechanism) are available in the literature. Although the metathesis has received surprisingly little attention, the pyrolysis of ClCN has been studied with the single pulse shock tube techniquegand the dissociation of hydrogen has been reported by many shock tube workers employing a variety of analytical methods."
Experimental Section The source of the kinetic data reported herein was taken from a shock tube which was equipped to record simultaneously the infrared emissions from HCN and ClCN in the reflected shock zone. Experimental details pertaining to the apparatus and procedure have been described.a An interference filter centered at 3.0 pm (0.10-pm bandpass at half-peak height) was used for HCN emission while a 4.45-pm narrow band filter served for ClCN. The following mixtures were tested for interference at the 3.0-pm filter observation port: 2% ClCN; 2% HCl; and 3% C2N2. The balance of the three mixtures was argon. Interference was observed only at the 4.45-pm station. Therefore, only the initial signal from that detector was used and solely for the purpose of establishing time zero for product formation. Four reacting mixtures, labeled A-D, respectively, were prepared by the method of partial pressures: 1.5% ClCN-1.5% Hz,P1= 5 Torr; 1.5% ClCN-1.5% Hz,PI = 10 Torr; 3% ClCN-3% H2,PI = 5 Torr; 3% ClCN-9% Ha, P1= 5 Torr. The diluent for all four mixtures was Linde argon (99.95%). Cyanogen chloride was purchased from Matheson and further purified by three consecutive liquid nitrogen bulb-to-bulb distillations in which only the middle fraction was retained in each step. Mass spectrometric analysis revealed impurity levels of 0.6% HCN and 1.1% HC1. No other foreign species were detected. Linde H2 (99.95%) was treated by trapping the gas onto a previously purged Linde type 4A molecular sieve at 77 K. After slight warming, the middle fraction of the desorbing vapor was employed for mixture blending. Three calibration mixtures were shocked to test the relation of detector signal amplitude and HCN concentration: 1.5% HCN-1.5% H,; 3% HCN-3% Hz; 3% HCN-9% Hz. The diluent was argon. Hydrogen was added to suppress the formation of cyanogen from the reaction 2HCN C2N2+ Hz and to simulate a collision partner environment more similar to reaction conditions. Hydrogen cyanide evolved from KCN upon addition of concentrated sulfuric acid was collected and purified by bulb-to-bulb distillation. Two Biomation 610 B transient recorders were connected in parallel to monitor the emission signal from the
-
1129
3.0-pm filter station. One recorder was set to digitize the signal at selected sampling intervals of 0.5-2.0 ps in order to maximize data collection during the initial stages of the reaction. These settings were chosen to minimize any heating effects due to reaction exothermicity (-25 kcal mol-'). The second recorder was set at a 5-ps sampling rate in order to determine the maximum signal intensity achieved by HCN production. Some 255 data points were generated by each Biomation. Polaroid pictures of oscilloscope traces of the emissions passing through the 3.0and 4.45-pm filters were taken at sweep speeds of 50 and 10 ps/cm, respectively. The faster speed picture was used to determine time zero for the reaction from an analysis of the initial ClCN emission at 4.45 pm. The other picture provided a check on the Biomation recorders. A time-of-flight (TOF) mass spectrometer was utilized to sample the reflected shock zone which contained mixture A in a diluent of neon-1% Ar. The primary purpose was to detect the presence of minor products and intermediates. Peaks corresponding to HCN, HC1, ClCN, and CzN2were measured at 20-ps intervals during a total observation time of 440 ps. A mixture of 1.5% HCN, 1.5% ClCN, 1.5% C2N2,balance Ne-1% Ar, was prepared for static analysis in order to measure the relative ionization sensitivities of those species containing CN. Hydrogen was used as the driver gas for all experiments performed in either the TOF or IR emission shock tube.
Results Dynamic sampling of reaction mixture A by the TOF at 20-ps intervals revealed that the major products were indeed HCN and HC1 and that, at high temperatures, minor amounts of C2N2were formed. C12was not observed nor was any other readily identifiable intermediate. The temperature range covered was 1784-2455 K. The peak heights corresponding to m / e 27 (HCN), 52 (CzN2),and 61 (CNC1) were measured as a function of reaction time. An expression for the mole fraction of HCN for an equimolar mixture of reactants was used: ~HCN =
[HCNl/([C1CNlo + [H21u)
= [ HCN]/2[ ClCN],
(1) (2)
Mass balance for CN is given by the following equation: [ClCNIo = [HCN] + 2[C,N,]
+
[ClCN]
(3)
Substitution of eq 3 into eq 1 along with the relative ionization sensitivities as determined from static analysis of a mixture of known concentrations of the various species yielded the working equation
(4) where P represents the respective peak heights. Other mole fraction equations were likewise constructed and plots l displayed in Figure 1. of fHCN, fclCN,and f ~ c are In addition to identifying the species present during the course of the reaction, the TOF data showed clearly the near disappearance of ClCN and the nonlinear time dependence of product formation. The long time mole fraction levels recorded by the TOF runs were in accord with equilibrium calculations on this particular mixture. The calculated mole fractions are listed in Table I. The chlorine amounts predicted were below the detectability threshold in our experiments. The instrumentation which processed the infrared emission signals produced a 10- to 40-fold increase in the number of data points during the initial stages of HCN The Journal of Physical Chemlstry, Vol. 81,
No. 12, 1977
1130
Kern et al.
Figure 2. Infrared emission record from mixture B reacting at 2104 K. HCN emission is lowest trace on left-hand side. The sweep speed is 50 pslcm.
1,
experiments conducted with the 1.5% HCN and 3% HCN in argon mixtures demonstrated that, within experimental error, the signal intensity doubled with a twofold increase of HCN concentration over the temperature range spanned for the reaction mixtures. These facts, coupled with the circumstancethat time zero is more accurately determined in the IR shock tube than in the TOF shock tube, indicated that analysis of the HCN emission signal was the best source of kinetic data for this system. A typical oscilloscope record for a reacting mixture is displayed in Figure 2. The abruptly increasing signal on the left-hand side of the figure is due to ClCN emission and was used primarily to determine time zero for the reaction. It is interesting to note that the ClCN emission signal does not diminish to a level consistent with that shown in Figure 1or Table I. This observation is easily explained by the emission at 4.45 ym of other species formed during the reaction, thus rendering the signal unsuitable for kinetic analysis. The signal at 3.0 pm was due entirely to HCN emission and was treated according to the following equation:
x
?!
* * *
J
x
1
11
4
X
I
Q=I / L x= ~
H C N / ~ H C N , ~ ~ ~
(5)
where I is the signal intensity at known reaction intervals and ,I is the maximum signal intensity plateau observed during a given run. The further equation of intensities to mole fractions is derived from the results of the calibration experiments. The data were fit successfully to the following equation
1 - Q = exp(-lz'tf) 1BB
200
50a
400
300
Figure 1. Mole fraction profiles of HCN, CICN, and HCI for TOF experiment at 2339 K.
TABLE I: Mole Fractions in ClCN-H, Reaction at Equilibrium (Initial Mixture Equimolar) T,K HCN HCl ClCN H, C,N, 1700 2000 2300 2500 2800
0.441 0.498 0.427 0.496 0.414 0.493 0.406 0.491 0.396 0.486
0.002 0.004 0.007
0.030 0.038 0.046 0.009 0.052 0.013 0.059
0.028 0.035 0.040 0.042 0.046
C1, 3 X
1.7 X 6.2 X 1.2 X 2.8 X
formation. Thus, the prgduct profile could be determined at early reaction times thereby reducing the effects of heat release. Shocked mixtures of 2% ClCN, 2% HC1, and 3% CzNz (each diluted with argon) revealed an absence of signal at the 3.0-ym filter detector station. Calibration The Journal of Physical Chemistry, Val. 81, No. 12, 1977
(6)
where z is the time dependence for product formation. The value of z was determined by a plot of the In In of the left-hand side of eq 6 vs. In t. The slopes in all cases were linear and fell into the range 1.7-2.1 with no apparent pattern emerging with regard to temperature or mixture composition. A typical plot is shown in Figure 3. Since a constant time power is helpful in comparing rates, the value of 2.0 was chosen for z in all rate calculations. After selection of the quadratic time dependence, the value of k'for each experiment was calculated. In order to minimize the effect of temperature change in the reflected shock zone due to reaction exothermicity, only those points with Q less than 0.5 were utilized. The best fit profile along with the experimental data were plotted for all runs. A typical example is displayed in Figure 4. An Arrhenius plot of the four mixtures studied (A-D) is shown in Figure 5.
1131
Reaction of Cyanogen Chloride and Hydrogen TABLE 111: Arrhenius Parameters for Computer Simulation Programa
TABLE 11: Rate Constants for ClCN-H, Reaction 106p, k 1 0 6 ~ ~ 10-14 k mol cm3 mol-' mol cm3 mol-' T, K cm-' s-' ~ r n - ~ s-~ 1877 1899 1942 2003 2005 2032 2045 2130 2198 2330 2331
A. 1.5% ClCN-1.5% H,, P, = 5 Torr 18.8 1.82 0.526 2368 2.01 1.83 0.558 2423 2.02 29.3 1.85 0.689 2538 2.06 50.3 1.88 1.29 2617 2.08 80.7 1.88 1.13 2659 2.09 90.1 1.89 1.45 2761 2.12 184 1.89 2.03 2819 2.13 193 l.93 2.78 2829 2.14 260 1.95 6.07 2843 2.14 163 2.00 13.6 2897 2.15 273 2.00 17.5
1968 1989 2068 2104 2229 2257 2263
B. 1.5% ClCN-1.5% H,, PI = 1 0 Torr 3.73 0.848 2298 3.96 3.73 1.48 2319 3.98 3.80 2.25 2363 4.01 3.83 2.91 2375 4.01 3.91 7.91 2401 4.03 3.94 11.2 2620 4.16 3.94 13.5
Reaction a b C
d
e f g
h i j
k 1 m n
15.0 11.3 22.9 18.6 24.8 90.1
1972 2018 2040 2092 2109 21 29 2178 2183 2236 2258 2276 2289
C. 3% ClCN-3% 2.00 0.785 2.02 1.26 2.03 1.71 2.05 2.61 2.06 2.55 2.06 3.95 2.08 3.99 2.09 5.53 2.11 8.10 2.12 8.56 2.12 10.8 2.13 11.3
H,, PI = 2333 2373 2390 2526 2544 2564 2566 2648 2654 2656 2691
5 Torr 2.14 2.16 2.16 2.22 2.22 2.22 2.22 2.25 2.25 2.25 2.26
18.9 19.7 22.3 51.2 64.5 78.3 56.5 88.3 102 98.2 133
1855 1867 1869 1946 1950 1952 1992 1998 201 1 2016 2075 2091 2150
D. 3% ClCN-9% 2.08 0.373 2.09 0.319 2.09 0.393 2.13 0.705 2.13 0.905 2.13 0.821 2.15 1.21 2.15 1.09 2.16 1.52 2.16 1.43 2.19 2.65 2.20 2.12 2.23 4.03
H,, P,= 2228 2236 2313 2331 2377 2410 2441 2445 2474 2533 2547 2561 2668
5 Torr 2.26 2.26 2.27 2.30 2.32 2.33 2.35 2.35 2.36 2.38 2.38 2.39 2.43
7.30 7.29 13.4 15.9 23.3 29.5 27.5 29.8 35.0 53.6 48.8 62.5 106
a
=
k (CICN):(H,)ob(Ar)c
1021.78+,0.06
exp(-70.3
cm3 mol-' s-'
16.53 13.97 14.00 13.05 13.78 13.78 13.92 13.53 13.78 14.52 16.79 9.82 16.81 10.94
9 16 9 9 Est Est 17 '18 Est Est 19 20 21 22
Units: A = cm3 mol-' s-I; E* = kcal mol-'.
(a)
H2tAr=2H+Ar C1 t ClCN + C1, t CN CN t ClCN + C,N, t C1 H t ClCN= HCN t C1 H t ClCN + HCl C1+ H, CN
+
HCl
t
H,
H t C,N,
--L
+ CN
+H +H
HCN
F'
HCN t CN
H t C1, + HCl t C1 HCNtAr+H+CNtAr HC1t A r * H t C l + A r C,N2 + Ar
= 2CN t
Ar
C1, t Ar + 2C1 t Ar
(7)
?
Ref
ClCN t Ar --L CN t C1+ Ar
where a, b, and c are the respective individual orders. A computer program, into which the reflected shock zone gas density for each experiment was input, employed an iterative routine to calculate the set of orders which gave the least statistical variance. The best fit values were a = 0.5, b = 0.1, and c = 0.4. The concentration independent rate constant k was calculated for all IR experiments and is represented by the following Arrhenius equation:
h=
E* 91.5 88.9 34.0 6.0 5.0 5.0 5.5 7.0 10.0 2.9 117.1 70.0 98.6 48.0
Discussion The inert gas dependence and the nonlinear production of HCN argue persuasively against a direct bimolecular mechanism involving the reactants and suggest that the reaction is proceeding via a series of elementary steps. Of prime consideration is an atomic mechanism consisting of the following steps:
The order of the reaction with respect to ClCN, Hz, and Ar was found from the following equation
h'
log A
0.6/RT)
(8)
The units for the activation energy are kcal mol-'. The rate constants and the best fit line are presented in Figure 6 and listed in Table 11. The reflected shock pressures varied from 0.28 to 0.89 atm.
(n)
All but four of the forward direction rate constants have values in the literature. Reasonable estimates for those not measured were and the values used in a computer calculated profile of HCN are listed in Table 111. The appropriate thermodynamic data necessary for the reverse reaction rate constants were taken from the JANAF tables.14 Calculations were performed at 1900,2300, 2600, and 2800 K for each of the four mixtures studied, A-D, at their respective initial shock pressures.15 The resulting profiles were observed to have nonlinear shapes similar to those recorded experimentally. In fact, the resulting profiles could be fit for each of the mixtures by eq 5 and 6. Values of the time dependence z were found to be in the range 2.08-2.11. The respective values of k' for each mixture were used to calculate the individual Arrhenius lines. Lastly, the reaction orders a , b, and c were determined, which yielded the following result:
k' = h (ClCN)2*50(H2),,0b2 (Ar)0.77
(9)
A grand Arrhenius line for the computer generated profides was obtained
k=
lozg5exp(-94.5/RT)
(10)
The Journal of Physical Chemistry, Vol. 8 1, No. 12, 1977
1132
Kern et al.
Figure 3. Time power plot for an IR experiment at 2178 K, z = 1.93.
$
a
40
am
120
TIME
1
20a
160
(
240
I
280
3%.
3x4
I_JSEC)
Flgure 4. Reaction profile for HCN from an IR experiment at 2178 K.
The units for the preexponential factor are (cm3m ~ l - ~ ) - ’ ~ ’ ~the apparent activation energy for the metathesis reflects this fact. One possible reconciliation involves the proposal of a Comparison between an experimental HCN profile and reactant catalyzed dissociation occurring with an activation a computed profile is shown in Figure 7. The Arrhenius energy some 19-21 kcal less than that measured with an line for eq 10 appears in Figure 8. The fractional orders, inert gas dependence, and noninert gas. linear time dependence are all predicted by an atomic ClCN + H, C1 t CN + H, (0) mechanism. The most serious discrepancy with regard to H, t ClCN H t H t ClCN (PI experimental results is the magnitude of the activation There exists no evidence to support this proposal and there energy: 70.3 vs. 94.5 kcal mol-’. Variation of the rate is indirect evidence that decompositions occur with an constants of the three center reactions did not produce activation energy not much different from that recorded “agreement”. The most significant steps are the dissowith an inert gas. The dissociation of H2 have been ciations of the reactants and the computed magnitude of --t
-+
The Journal of Physical Chemistry, Vol. 81,
No. 12, 1977
1133
Reaction of Cyanogen Chloride and Hydrogen
Ei
/
I
'A/
8; 0
,
54
109
1L3
21 7
27 1
326
380
T I M E (USEC)
Figure 7. Comparlson of experimental (dashed line) and atomic mechanism (solid line) profiles for HCN at 2178 K. ..
3 8
3 5
* e
4 5
10'IT
5 8
5 5
( OK1
Figure 5. Arrhenius plot of k'rate constants for mixtures A-D: (0) A; (0)B; ( X ) C; (V)D.
i
\
\
II I I .
vi ?/.
9 3 6
,e
3 5
< a
~B'/T
' 5
5 Q
3 ,
(OK)
Figure 6. Arrhenlus plot for data in Table 11. Symbols identlcal wlth those used In Figure 5.
measured with Ar, H2,and H as collision partners.16 The respective rate constants differ but the difference is not of the magnitude required to explain the results herein.
1@
/T
