5352
J . Phys. Chem. 1986,90, 5352-5357
EDTA, oxalate ions have a much wider pH range of utility, and that oxalate does not reduce R ~ ( b p y ) , ~and + cannot function as the chemistry of their radicals seems to be more straightforward. a substitute for EDTA in systems containing R ~ ( b p y ) , ~and + If the species resulting from the reaction of OH with oxalate are MV2+. Experiments directed toward this point are under way in our laboratories. the same as those generated by the oxidation of oxalate in the photochemical system, a fact that remains to be demonstrated, Acknowledgment. This research was supported in part by then the appreciable half-lives of C204*-and C204H' (-0.3 and Consiglio Nazionale delle Ricerche of Italy and in part by grants -0.2 ps, respectively) and their oxidizing abilities will cause a from the Office of Basic Energy Sciences, Division of Chemical diminution in the efficiency of formation of electron carriers, such Sciences, US Department of Energy (M.Z.H.), and the National as MV". Institutes of Health (M.A.J.R.). The collaboration between Finally, note should be made of a disagreement in the literature Q.G.M. and M.Z.H. is part of the US-Italy Cooperative Research regarding the use of oxalate as a scavenger for R ~ ( b p y ) ~ ~ + . Program. CFKR is supported jointly by the Biomedical Research Rubinstein and Bard3 claim that this reaction occurs in their Technology Program of the Division of Research Resources of electrogenerated chemiluminescence system where the oxidation NIH (RR 00886) and by the University of Texas at Austin. The of R ~ ( b p y ) , ~in+ the presence of oxalate results in emission from technical assistance of Dr. A. Martelli, G. Gubellini, A. Monti, the excited complex. On the other hand, Furlong et aL3' state and V. Raffaelli is appreciated. R d t r v NO.MV2+.4685-14-7: MV'*. 25239-55-8:. N,O. 10024-97-2: HCOi-, 71-47-6; c@:-, 338-70-5; COz'-, 85540-96-1; (CH,),CHOH: 1
(31) Furlong, D. N.; Wells, D.; Sasse, W. H. F. J . Phys. Chem. 1985, 89, 1922-1928.
,
67-63-0; H20, 7732-18-5.
Hydrochlorlnation of Alkenes. 2.' Reaction of Gases Hydrogen Chloride and 2-Methylpropene Francis Costello,2 David R. Dalton,* and John A. Poole Department of Chemistry, Temple University, Philadelphia, Pennsylvania 19122 (Received: January 27, 1986)
Mixtures of gaseous hydrogen chloride and gaseous 2-methylpropene at total pressures less than 1 atm yield, exclusively, gaseous 2-chloro-2-methylpropane. Kinetic measurements have been made by proton magnetic resonance and infrared spectroscopyand by observing the pressure change accompanying the reaction. It is concluded that surface catalysis is required for product formation and that the reaction, which occurs at the walls, is most probably between strongly adsorbed hydrogen chloride and weakly adsorbed 2-methylpropene; Le., a Rideal-Eley mechanism obtains.
More than 5 decades ago, Maass and Coffin3 reported their attempts to understand the process by which gaseous 2-chloro2-methylpropane formed when gaseous hydrogen chloride was mixed with gaseous 2-methylpropene. They found that although product formed in what was probably a "heterogeneous" process, reproducible results which might allow them to begin to define a pathway could not be obtained. A few years later, Kistiakowsky and co-workers4q5examined in detail the catalyzed thermal unimolecular vapor-phase decomposition of 2-chloro-2-methylpropane to hydrogen chloride (gas) and 2-methylpropene (gas) and the catalyzed (glass wool) equilibrium between these three gaseous species. They were able to obtain the equilibrium constant attending the reaction and thus were able to calculate the rate constant for the formation of 2-chloro-2-methylpropane(eq 1). On the basis of their studies kfo, = 101~*~e(-ZS.~O*Z400)/R~ cm3 s-l (1) they commented that "...the association of halogen hydrides and isobutene in the gas phase proceeds according to entirely normal kinetics ..." and, in contrast to the findings of Maass and C ~ f f i n , ~ that the reaction "...cannot be observed directly merely because of cooperation of a high energy of activation and an unfavorable equilibrium." (1) Haugh, M. J.; Dalton, D. R. J. Am. Chem. SOC.1975, 97, 5674. (2) Taken, in part, from: Costello, F. the Ph.D. Dissertation Temple University, 1985. (3) Maass, 0.;Coffin, C. C. Can. J . Res. 1930, 3, 526, 533. (4) Brearley, D.; Kistiakowsky, G. B.; Stauffer, H. C. J. Am. Chem. SOC. 1936, 58, 43. ( 5 ) Kistiakowsky, G. B.; Stauffer, H. C. J . Am. Chem. SOC.1937, 59, 165.
0022-3654/86/2090-5352$01.50/0
Noller et aL6 studied the reaction between the gases hydrogen chloride and 2-methylpropene over calcium chloride, magnesium sulfate, and aluminum sulfate catalysts in a static reactor. By measuring the pressure change accompanying the reaction, over a limited range of compositions and pressures, they found their data could be fitted to eq 2, an integrated form of the Rideal-Eley equation where P = P,,,,, at any time t , C = the sum of the initial
-Aka,jdt = In
2P-C+K-A 2P- C + K f A
+ In CC ++ KK +- AA
(2)
While it was clear that their primary interest was to study the effects of various catalysts on this reaction, they did propose specifically that chemisorbed hydrogen chloride reacted at the catalyst surface with 2-methylpropene to form product which was, simultaneously with its formation, desorbed. That the reaction yielding gaseous 2-chloro-2-methylpropane nevertheless appears to proceed in the absence of added catalysts3 indicates either that there is another pathway (aside from what may be the bimolecular reverse of the unimolecular decomposition4v5)or that the surface of the vessels used to contain the reaction mixture can fulfill the catalytic function. Since we had already shown' that the gases hydrogen chloride and propene yielded gaseous 2-chloropropane via a high-order process where minimal involvement of wall catalysis was evidenced, we undertook examination of the reaction of 2-methylpropene and hydrogen chloride in the absence of added catalysts with the preconceived (6) Andreu, P.; Noller, H.: Paiz, M. An. Quim. 1969, 65, 130.
0 1986 American Chemical Society
Hydrochlorination of Alkenes TABLE I: Summary of the Experimental Data Obtained by Following the Decrease in Pressure Attending the Reaction of Gaseous 2-Methylpropene and Hydrogen Chloride To Form Gaseous
2-Ch~oro-2-methv~~ro~ane~ -U/AT, expt Pi,", Torr P H ~Torr , Plotalo, Torr Torr-min-I 25 104 4.2 X 1 79 5.0 X 2 25 93 118 93 143 8.2 X lo-* 3 50 55 145 9.7 x 10-2 4 90 94 168 1.03 X lo-' 5 74 79 169 1.26 X lo-' 6 90 9.11 X 7 88 101 189 8 154 90 244 2.1 x 10-1 9 90 208 298 2.23 X lo-' 10 254 89 343 4.68 X lo-' 11 200 208 408 9.00 X lo-' 12 351 90 44 1 8.95 t lo-' 13 243 199 442 6.97 X lo-' 356 445 1.11 14 89 90 538 2.1 1 15 448 16 90 456 546 1.24 17 202 385 587 1.85 18 413 192 606 1.47 19 300 300 600 1.98 20 200 500 700 3.38 'PiM" = initial pressure of 2-methylpropene. PHclo= initial pressure of hydrogen chloride. P,,,,I" = Pis," + PHCIo.A P / A T is the change in total pressure in unit time.
idea that this reaction might fall into the same category. In the event, we have been proven wrong. Experimental Section
Reagents. 2-Methylpropene (C. P. grade) and hydrogen chloride (Electronic grade) were obtained from the Matheson Gas Company, East Rutherford, NJ. The gases were handled after extensive purification as described earlier.' Apparatus and Equipment. Vacuum (generated with a Precision rough pump and a Fisher oil diffusion pump) was monitored by a Pirani gauge, and pressure measurements were made by using a Baratron (1-1000-Torr) capacitance bridge manometer with a stainless steel diaphragm (linked to a digital VOM; 1 V = 100 Torr). All experiments were carried out at total pressures of less than 1 atm. Commercial, polished high-resolution, thin-walled, 12-mm N M R tubes with vertical twist valves were used for the *HN M R measurements. Infrared cells with KBr windows (IR measurements) and the vacuum rack (pressure measurements) were constructed of Pyrex glass. The windows (IR measurements) were cemented to the glass with Glyptal. Kinetics obtained by 'H N M R utilized a Varian XL-100-15 spectrometer operating at 100.0 M H z in the CW mode. An external I9F lock was used. Kinetics obtained by infrared spectrophotometry utilized a Perkin-Elmer Model 225 grating spectrophotometer. Kinetics obtained by direct pressure measurements utilized a 34.0-cm3 portion of the vacuum rack bounded by two stopcocks and the Baratron capacitance manometer. Experimental Technique. General. Hydrogen chloride and 2-methylpropene gases cannot be cofrozen and allowed to revaporize without significant formation of 2-chloro-2-methylpropane o c c ~ r r i n g .It~ is not clear whether reaction occurs during condensation, liquifaction of the condensate, or vaporization of the liquid. This requires, in a static system, that mixtures of the gases be prepared by utilization of an overpressure in one part of the system. Calculations of needed pressures were made assuming ideality, but in the event, pressures were measured before and after mixing. ( a ) Pressure Measurements. The gas which was to be the one at lowest pressure was introduced into the triangular portion of the vacuum rack and its pressure measured directly by the Baratron gauge. The gas which was to be the one used at higher pressure was then introduced until the appropriate reading was obtained on the gauge. The second gas could usually be introduced
The Journal of Physical Chemistry, Vol. 90, No. 21, 1986 5353 TABLE 11: Summary of the Experimental Data Obtained by Following the Increase in the Area of the Carbon-Chlorine Stretching Frequency at 590 cm-I Attending the Reaction of Gaseous 2-Methylpropene and Hydrogen Chloride in Cell W expt 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Pi,,",Torr
Torr 55 65 61 20 61 45 12 53 67 18 54 54 102 101 173 179
PHC]",
56 51 103 55 116 54 105 165 53 106 171 171 160 161 158 160
Ptotalo, Torr 111 116 164 175 177 199 217 218 226 224 225 225 262 262 331 339
AP/A T , Torr-min-I 5.74 x 10-4 2.83 X lo4 3.55 x 10-3 1.78 x 10-3 3.64 x 10-3 4.26 x 10-3 1.75 X 7.5 x 10-3 7.85 x 10-3 1.81 x 10-2 5.02 x 10-3 5.7 x 10-3 1.73 X 1.86 X 4.00 X 4.69 X
"Areas have been converted to pressure through use of a Beer's law plot. Pi,," = initial pressure of 2-methylpropene. PHClo= initial = Pi,," + PHclo.A P l A T is the pressure of hydrogen chloride. PlOh~" change in total pressure in unit time.
TABLE III: Summary of the Experimental Data Obtained by Following the Increase in the Area of the Carbon-Chlorine Stretching Frequency at 590 cm-' Attending the Reaction of Gaseous 2-MethvI~rooene - . . and Hvdrogen Chloride in Cell K' -
I
expt
P,-",Torr
PHC,",Torr
1 2 3 4 5 6 7 8 9 10 11
50 57 125 57 60 161 165 106 162 155 157
61 55 52 123 125 52 53 115 102 152 171
Ptotalo, Torr 111 112 177 180 185 213 218 221 264 307 328
AP/ AT, T0rr:min-l 4.1 x 10-4 4.73 x 10-4 1.33 x 10-3 1.56 x 10-3 1.73 x 10-3 2.57 x 10-3 4.81 x 10-3 2.85 x 10-3 4.0 x 10-3 7.8 x 10-3 6.8 X lo-'
"Areas have been converted to pressure through use of a Beer's law plot. Pi,," = initial pressure of 2-methylpropene. PHClo= initial pressure of hydrogen chloride. PtOu~" = Piso" + P H C ~ "A. P I A T is the change in total pressure in unit time.
in a matter of seconds, and time t = 0 was taken as that time when the appropriate pressure was reached. Table I contains a summary of the experimental data obtained in this fashion. ( b ) Infrared Measurements. Two different (nominally 10-cm path length) cells were used. The cells had volumes of 29.7 (cell D) and 29.9 cm3 (cell K) as measured by expansion of argon from a portion of the vacuum rack of known volume. A Beer's law plot of the product (2-chloro-2-methylpropane) was prepared so that absorbance (or percent transmittance) values could be converted directly to Torr. As with the pressure measurements (vide supra), the gas at lower pressure was introduced into the cell, which had been thoroughly evacuated at a pressure of less than 1 Torr for 14 h prior to use. The second gas was then introduced rapidly, having calculated what overpressure was required to produce the desired final pressure in the cell, and the cell closed. The first infrared spectrum was taken immediately (monitoring the C-Cl stretching band at 590 cm-I), and this measurement was taken as time t = 0. Tables I1 and I11 contain summaries of the experimental data obtained in this fashion. ( c ) ' H N M R Measurements. A number of 12-mm, highly polished, thin-walled N M R tubes (of volumes 18 i 1 cm3) with vertical twist-type valves were utilized, and all experiments were run in duplicate. A small, but noticeable, variation might be observed from tube to tube, but in general, duplicate experiments were run without difficulty. All tubes were cycled through an
5354 The Journal of Physical Chemistry, Vol. 90, No. 21, 1986 TABLE IV: Summary of the Experimental Data Obtained by Following the Increase in the Area at Which the Methyl Groups of Gaseous 2-Chloro-2-methylpropane Come into Resonance and the Decrease in the Area at Which the Methyl Groups of 2-Methvl~ro~ene Come into Resonance" APlAT, expt Pi,", Torr PHC'",Torr Ptotalo, Torr Torrmin-' 1 117 100 21 7 2.12 2 119 100 219 1.93 200 300 4.70 3 110 4 100 202 302 4.62 5 190 204 394 8.72 X 10-1 8.58 X IO-' 6 218 204 422 7 302 100 402 5.97 x 10-1 8 303 100 403 6.90 X IO-' 9 328 100 428 5.66 X IO-I IO 392 100 492 4.62 X 10'' 11 394 100 494 4.26 X IO-' 200 510 2.26 12 310 2.27 13 316 200 516 14 489 100 589 2.13 X 10-1 15 49 1 100 59 1 2.88 X IO-' 1.05 16 400 200 600 17 283 300 583 5.72 18 284 300 584 4.83
Costello et al.
PRESSURE
(torr)
25
1 p' c
~
-
I
4-
.. 1
I
1
I C
2 0
3 r
TIME
lilr)
Figure 2. Plot of the change in the total pressure for the reaction of hydrogen chloride with 2-methylpropene as followed by 'H NMR. The initial pressure of hydrogen chloride is held constant at 100 Torr. The initial pressure of 2-methylpropene is varied from 491 (curve 1, 0)to 302 (curve 2, V) to 117 Torr (curve 3, 0).
DAreas have been converted to pressure by comparison to known samples. Pi,,"= initial pressure of 2-methylpropene. PHclo= initial A P / A T is the pressure of hydrogen chloride. P,,,,,"= Piso" + PHCIo. change in total pressure in unit time. 3c
f
7
30
20
E0
TIYE lbrl
T'Ht
,mill
Figure 1. Increase in pressure of 2-chloro-2-methylpropane (gas) with time. For curve 1 (0),the initial pressure of 2-methylpropene was 79 Torr, and the initial pressure of hydrogen chloride was 25 Torr. For curve 2 ( 0 ) , the initial pressure of 2-methylpropene was 351 Torr, and the initial pressure of hydrogen chloride was 90 Torr.
annealing oven before each run and were thoroughly evacuated, with heating, at a pressure of less than 1 Torr for 24 h prior to use. Tubes were filled as described for the infrared cells in section b above. The first 'H NMR spectrum was taken immediately. Since no reference standard was incorporated, chemical shift values are not reported. The reaction was followed by observing the growth in the singlet due to the nine identical protons of 2chloro-2-methylpropane and the simultaneous diminution in the signal due to the six identical methyl protons of 2-methylpropene. Machine integration was supplemented by use of a planimeter to obtain a r e a s corresponding to the two types of protons. Since liquid formation was not observed, ideality was assumed and the moles of product formed were converted to pressures (Torr). Numerous samples of known pressures of mixtures of 2methylpropene and 2-chloro-2-methylpropane were run for comparison purposes. Table IV contains a summary of the experimental data obtained in this fashion. Results and Discussion The results summarized in Tables I-IV were obtained by observing the increase in concentration of 2-chloro-2-methylpropane (gas) with time. Typical plots of that increase (expressed as Torr of product) vs. time are shown in Figures 1-3. As seen, while the rate of product formation is a function of the initial concentrations of reactants, and perhaps other things, there is no evidence,
Figure 3. Plot of the change in pressure vs. time for the reaction of 2-methylpropene with hydrogen chloride as followed by infrared spectroscopy in cell K. The initial pressure of 2-methylpropene was 157 Torr, and the initial pressure of hydrogen chloride was 171 Torr.
RATE
' '1
( t o r r l m in )
1 P
2 0
3 3
4 0
i n
ppIcl'pelo
Figure 4. Plot of experimentally derived pressure data (Table I) for the reaction between hydrogen chloride and 2-methylpropene. The rate of pressure change (Torr/min) is plotted against the reduced initial pressure of hydrogen chloride. The initial pressure of 2-methylpropene is held constant at ca. 90 torr.
over the course of at least 90% of the reaction, no matter how measured, of significant diminution of the rate as the starting material is consumed. This is among the criteria often utilized to establish the presence of a catalyst.' Other criteria may include (7) Moore, W. J. Physical Chemistry, 3rd ed.;Prentice Hall: Englewocd Cliffs, NJ, 1962; p 306.
The Journal of Physical Chemistry, Vol. 90, No. 21, 1986 5355
Hydrochlorination of Alkenes
(torrlminl RATE1 .o
the surface reaction and BHCl and O h are, respectively, the fractions of active surface sites covered by hydrogen chloride and 2methylpropene. Classically,I2 the corresponding expansion for the isotherm for the surface coverages for the two gases is
i
OHCI = WHCIPHCI)/(~ + KHCIPHCI + KisoPiso)
(8)
= ( K i s Z i s o ) / ( l + KHCIPHCI + Kisopiso) (9) where K H c I and Kisoare, respectively, the adsorption coefficients (Le., the ratios of the effective rate constants for adsorption and desorption) for hydrogen chloride and 2-methylpropene, PHcland Pi,, are their respective pressures. Thus, the rate is given by eq 10 or, in terms of initial pressures and initial rates, by eq 11. Biso
0- 0--0 I
1 .[I
I
2.0
3,o
,
4.0
5.0
Figure 5. Plot of experimentally derived pressure data (Table I) for the reaction between hydrogen chloride and 2-methylpropene. The initial rate of pressure change (Torr/rnin) is plotted against the reduced initial pressure of 2-methylpropene. The initial pressure of hydrogen chloride is held constant at ca. 90 Torr.
(a) retardation of the rate of the reaction upon increasing the concentration of one (or both) of the reactants once the rate has been maximized for the available surface,* seen for this reaction in Figure 2; (b) demonstration of zeroth-order dependence on one of more of the reactants' (an approach to zeroth order at the higher pressures of hydrogen chloride is seen for this reaction in Figure 4); and (c) the appearance of fractional orders, as well as nonintegral multiples of the reaction rate, when the concentration of one of the reactants is increased while the other is held constant? For the reaction of 2-methylpropene with hydrogen chloride (both in the gaseous state) the exponential increase in rate with increasing 2-methylpropene pressure while the hydrogen chloride pressure is held constant is seen in Figure 5. That all of these may be variously observed under the various conditions employed to examine the reaction between 2methylpropene and hydrogen chloride gases, coupled with a variation in rate, consistent for each method, but different in the different vessels used in the methods, directed our attention to the possibility that the surfaces of the various reaction vessels might ' be serving the role of catalyst for the formation of 2-chloro-2methylpropane. We consider the case where both hydrogen chloride gas and 2-methylpropene gas are competitively adsorbed on the surface (eq 3 and 4). We assume here, as in the most general case, that the adsorption need not necessarily be to the same extent or in the same way for both gases. For example, the hydrogen chloride may be chemisorbed (held more tightly) and the 2-methylpropene may be physically adsorbed (held less tightly) on the various surfaces. We assume further that the surface reaction (eq 5) is rate controlling and that the product, 2-chloro-2-methylpropane, physically adsorbed, is desorbed in a separate step (eq 6). The last step is, however, indistinguishable from desorption occurring simultaneously with formation. The process described in eq 3-6 is that which is particular to the Rideal-Eley mechanism.I0
ksKisoKHCIPisd)HC1
rate =
(10)
(1 + KisoPiso + K H C I P H C I ) ~
- ( l + Kis$isoo
PHCloPisoo
+ KHClpHClo)2
(11) rateo ksKis$rHCl Then, using the methods of linear least squares and the notation of ref 12, wherey = (PHcloPisoo/rate0)1/2, c1= (ksKi&Hcl)-1/2, c2= (Kiso/ksKHCI)1/2, and c3= (KHcl/k,Kiso)1/2for the various conditions of Piso'and PHCloexpressed, for example, in Tables I-IV, we may write
+ C3PHCIo
( P H c l o ~ i s o o / r a t e 0 )= 1 ~C1 2 + C2Pi,,"
(12)
and Y I = CI + G(Pis0')i YZ =
. . 9
cl
. .
+ G(PHCIO)I + C2(Piso0)2 + c 3 ( p H C I o ) 2
.
Yn =
Cl
+ CZ(f'iso')n + ~ ~ ( P H c I ' ) ~
The values of C1, C2,and C3,minimizing the sums of the squares of the residuals vl, v2, ...,v,, given by
. = CI. + ~ 2 ( p i s 0 0 ) l . .
+ c3(PHClo)l
-Yl
. . un = C,
+ C 2 ( P i s o o ) n + ~ ~ ( P H C I-' ~n) ,
are determined by solution of eq 13-15.
n
C(pisoo)gj i= 1 n
clC(pHClo)i i= 1
n
= 0 (14)
n
+ C2C(PHCIo)i(Pisoo)i + C3Z(pHC1°)? i= 1
-
I=]
n
E(pHcIo)rYi i= I
= 0 (15)
Evaluation of C,, C,, and C, for the 20 pressure measurements given in Table I provides the following values: C, = 239.1; C, = 0.0512; C, = -0.1281.13 Clearly, C1>> C2 and C,, the latter two approaching zero.
The rate for the surface reaction may be written in terms of the Langmuir isotherm" (eq 7) where k, is the rate constant for rate = ksOHCIOiso
(7)
(8) Bond, G. C.; Newham, J. Trans. Faraday SOC.1960, 56, 150. (9) See ref 7, p 305 and: (a) Pease, E. N. J. Am. Chem. SOC.1923, 45, 1196. (b) Bond, G. C.; Wells, P. B. J . Catal. 1965, 4 , 211. (10) Rideal, E. K. Proc. Camb. Phil. SOC.1939, 35, 130.
(11) Langmuir, I. J . Am. Chem. SOC.1918, 40, 1361. (12) Butt, J. B. Reaction Kinetics and Reactor Design; Prentice-Hall: Englewood Cliffs, NJ, 1980; p 143 ff. (13) Presumably, negative values for any of the coefficients indicate that the expressions for which they stand are likely to be further from equilibrium than the coefficients with positive values. Additionally, since the adsorption constant for 2-methylpropene must be incorporated in the denominator of the rate equations (eq 12 and 16) on the right-hand side, and this material is (presumably) only weakly adsorbed, small variations due to scatter will be magnified by the closeness of the constant to zero. Finally, as it is the square of the coefficientsrequired for evaluation in eq 1 1 , it is their magnitude which is of primary importance.
5356 The Journal of Physical Chemistry, Vol. 90, No. 21, 1986
Costello et al.
TABLE V Comparison of the Observed and Calculated Results Derived from the Assumption of a Rideal-Eley Mechanism for the Reaction between Hydrogen Chloride and 2-Methylpropene in the Gas Phase
IR measurementsu
pressure measurements" calcd obsd
exptl
239.9 228.5 229.8 236.7 230.9 233.6 230.7 235.5 217.4 240.7 222.7 245.6 226.0 198.1 250.5 185.3 200.1 235.6 216.1 185.2
1
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
IH NMR
cell K
216.8 215.6 238.1 225.9 259.9 237.6 312.4 256.9 289.7 2 19.8 215.0 188.0 263.4 169.0 138.2 182.0 205.0 232.3 213.2 172.0
calcd 103 103
2.46 x 2.43 x 2.04 x 2.37 x 2.35 x 1.83 x 1.80 x 2.09 x 1.78 X 1.78 X 1.75 X
cell D obsd x 103
103
2.73 2.57 2.21 2.11 2.08
103
1.80 x 103
103 103
1.35 x 2.07 x 2.03 x 1.74 X 1.99 X
103 103
lo3 IO3 lo3
x 103 x 103 x 103 x 103 103
103 103 lo3 10'
calcd 4.56 x 4.11 X 3.61 x 1.21 x 3.43 x -7.03 X 9.40 X 3.17 x 6.15 X 3.04 x 3.04 x 7.04 X 1.42 X -2.94 x -3.28 x
measurements" calcd obsd
obsd 2.32 x 3.42 x 1.33 X 1.93 x 1.39 X 1.36 x 8.19 X 1.06 x 8.31 X 1.36 x 1.27 x 9.71 X 9.35 x 8.26 X 7.86 X
103 IO3 103 103 103 10' lo2 103 IO2 103 103 lo2 IO2 103 103
103 103 IO3 103 lo3 103 lo2 103 lo2 103 103 lo2 102
IO2 lo2
99.7 102.0 37.1 23.7 117.7 142.3 264.5 265.2 282.9 325.3 215.9 220.2 383.5 384.0 276.0 141.8 142.6
74.3 78.5 68.4 66.1 210.8 227.7 224.9 209.6 240.7 291.3 165.7 166.9 479.1 412.9 276.0 121.8 137.8
" Measurement of (PisooPHClo/rate)1/2.
I
I I
0 0 . c
A A
iO
122
150
I
I
213
250
I
IO0
150
I
Figure 6. Plotted comparison of the data from the pressure measurements as listed in Table V. Circles (0)represent the results listed under
the heading "obsd" and the dashed line (-- -) the least-squares fit to those data. Triangles (A) represent the results listed under the heading "calcd" and the solid line (-) the least-squares fit to those data.
Table V provides a comparison of (PisooPHCIo/rate0)1~2 calculated with (PisooPHclO/rateO)l/Z observed for each of the experiments listed in Table I and by utilizing eq 12. These are further compared in Figure 6. It will be. noted that the correspondence between observed and calculated values of (PboPHclo/rate)1/2is generally quite good, with only four (experiments 7 , 9 , 12, and 15) providing significant scatter.13 The mechanism associated with such a correlation is that expressed in eq 3-6. Thus, as shown in Figure 4, when the ratio (PHcIo/Pi,")is less than about 2.0, i.e., Pi," is constant at ca. 90 Torr and PHa0is less than about 180 Torr, not all available sites are occupied and 1 >> KHcIPHcIO (eq 11). When the ratio (PHcIo/Pisoo) is greater than about 4.0, Le., Pisoois constant at ca. 90 Torr and PHclois greater than about 360 Torr, the surface sites available for coverage by hydrogen chloride have been saturated, the surface reaction between hydrogen chloride and 2-methylpropene is rate controlling, and KHcjPHclO >> 1. Values for the coefficients C1, C,, and C, utilizing the 16 infrared measurements carried out in cell D are 8.18 X IO3,-13.70, and -15.90, respectively, and for the 11 experiments in cell K are 2.80 X lo3, -5.71, and 0.870, re~pectively.'~The observed and calculated values of ( P ~ s o o P H ~ I o / r a t eutilizing o ) l ~ Z the data obtained from the experiments in cell D (Table 11) and cell K (Table 111) and eq 12 are shown in Table V and compared graphically in Figure 7. In cell D, but not in cell K, and in contrast to the pressure measurements, the differences between experimentally observed
io
15
,
ldl
15"
I "5
rP;~lp;,,i'/2
Figure 7. Plotted comparison of the data from the infrared measurements in cells D and K as listed in Table V. The observed and calculated results for cell D are presented as open circles (0) and open triangles ( A ) , respectively, while the observed and calculated results for cell K are presented as closed circles ( 0 )and closed triangles (A),respectively. The lines are the least-squares fits to the observed data.
and calculated values for (PisooPHC10/rate0)1/2 are larger at the higher pressures of hydrogen chloride. Further, most reactions in cell K, where scatter is considerable, occur at rates either slightly slower or the same as those in cell D. That C, and C3 remain negative (although the latter for cell K is no longer so) even where the data have less scatter, implies that both the magnitude of the experimental scatter and the weak adsorption of 2-methylpropene may help account for the sign of these coefficient^.'^ Further still, however, again C1 >> C, and C3, and again it follows that the surface reaction to form product is rate controlling. Finally, in the 'H NMR experiments, with highly polished and annealed surfaces and with a glass different from that used in the infrared and pressure measurements, the expression correlating the data (eq 12) generate a large random fluctuation, and a new expression (eq 16), assuming a more reactive surface, where hydrogen chloride is much more strongly adsorbed and/or the 2-methylpropene is held on two sites, appears to obtain. (PisooPHCIo/rate0)l/2 = C1+ C2(P,soo)1/2 + C3PHCIo
(16)
Table V and Figure 8 provide a comparison of (PisooPHCIo/ rate0)'/' calculated with (PisooPHclO/rateO)l/Z observed for the ' H NMR experiments and by utilizing eq 16 in place of eq 12 (with Cl = -1.18 X 10,; C, = -25.12; C, = -0.54).'2 Here, in contrast to the pressure and IR measurements (vide supra), the
J . Phys. Chem. 1986, 90, 5357-5360
50
130
150
200
250
100
lP;clP;J1/2
Figure 8. Plotted comparison of the data from the NMR measurements as listed in Table V. Circles (0)represent the results listed under the heading "obsd" and the dashed line (- - -) the least-squares fit to those data. Triangles (A) represent the results listed under the heading "calcd" and the solid line (-) the least-squares fit to those data.
relative magnitude of C3suggests that the adsorption of hydrogen chloride rather than the product forming step is rate controlling. While it is clear from Table V and Figures 6-8 that the correlation between the observed rates and those calculated from eq 12 and 16 is not perfect, general trends are successfully reproduced and we conclude that the product is doubtlessly formed in a surface-catalyzed process. Thus, although a gas-phase complex between hydrogen chloride and 2-methylpropene is not precluded,14 our results indicate that not much, if any, product arises directly from such a complex. In this vein, there is ample precedent for the involvement of glass in addition reactions of olefins both in the gas15 and in the solution phase in nonpolar solvents at low (14) Such complexes, normally observed only at low temperatures in the solid (e.g., argon matrix) phase, may be detected utilizing thousands of accumulated infrared spectra (FTIR) in the gas phase at room temperature Kasloff, C., Hui, Y . W.; Poole, J.; Dalton, D. R., unpublished observations).
5357
concentrations.16 Additionally, isomerization and polymerization have been observed in an infrared study of the adsorption of C, alkenes in the vapor phase on Vycor glass.I7 Indeed, in our own work, surface involvement was also demonstrated for the reaction between gaseous propene and hydrogen chloride.' However, it was clear there, a t higher pressures with a lower boiling alkene, that overall fourth-order kinetics, presumably involving hydrogen chloride dimer and a hydrogen chloridealkene complex, governed product formation. Here, closer to the boiling point of the alkene, where physical adsorption dominates, and at lower pressures, where the concentrations of gas-phase alkene-hydrogen chloride complex14 and hydrogen chloride dimersI8 will be dramatically lower, the surface involvement is more pronounced. This, coupled to the greater reactivity of 2-methylpropene,Ig doubtlessly accounts for the different behavior.
Conclusion The reaction between gaseous hydrogen chloride and gaseous 2-methylpropene to yield gaseous 2-chloro-2-methylpropane at total pressures of less than 1 atm is surface catalyzed and follows a pathway consonant with the Rideal-Eley mechanism. Hydrogen chloride (gas) and 2-methylpropane (gas) are both adsorbed and, while adsorbed, react to form 2-chloro-2-methylpropane. The kinetics are entirely normal for this process. Registry No. 2-Methylpropene, 115-11-7. (15) De La Mare, P. B. D.; Scott, R. A.; Robertson, P. W. J . Chem. SOC. 1945. 509. (16) Robertson, P. W.; Clark, N. T.; McNaught, J. K.; Paul, G. W. J . Chem. SOC.1937, 335. (17) Little, L. H.; Klauser, H. E.; Amberg, C. H. Can. J . Chem. 1961, 39, 42. (18) Rank, D. H.; Sitaram, P.; Glickman, W. A,; Wiggins, T. A. J . Chem. Phys. 1963, 30, 2673. (19) Poutsma, M. L. J . Am. Chem. SOC.1965, 87, 4285.
Kinetic Nitrogen Isotope Effects on Methyl Transfer to Amines Joseph L. Kurz,* Michael W. Daniels, Karen S. Cook, Department of Chemistry, Washington University, St. Louis, Missouri 63130
and Moheb M. Nasr Department of Chemistry, Lindenwood Colleges, St. Charles, Missouri 63301 (Received: February 5, 1986)
Values of l4N/I5Nisotope effects on rates of methyl transfer to pyridines in aqueous solution are only weakly dependent on the identity of the leaving group or of the pyridine; all measured values fall in the range 0.995-0.998, which is consistent with a moderately early transition state in which the N-CH3 bond order is near 0.2 or 0.3. In aprotic solvents (acetonitrile or 1,2-dichloroethane), values are smaller by a factor near 1.003. Values for methyl transfer to quinuclidine and N,Ndimethyl-4-toluidine are larger (by factors up to ca. 1.008) than for methyl transfer to pyridines, which qualitatively reflects the difference between the I4N/l5Nequilibrium isotope effects on protonations of pyridines (0.978-0.98 1) and of quinuclidine (0.988). (0.987) or N,N-dimethyl-4-toluidine
Introduction Kinetic isotope effects (KIEs) provide the most precise available probes of the bonding present in transition states (TSs). Since isotopic substitution does not appreciably perturb the electronic potential surface, KIEs are vibrational in origin and their values depend to a good approximation only on the temperature and on the fundamental frequencies of the reactants and of the TSs. An observed KIE thus gives direct evidence concerning the force field in the TS and consequently concerning the extent to which bonds have been made and broken. For methyl transfer to water (eq 1) there exists a range of evidence1v2that a change in solvation is rate-determining and that 0022-3654/86/2090-5357$01.50/0
both H20-CH3 bond making and CH3-X bond breaking take place after the rate-determining TS has been passed. An obvious
H 2 0 + CH3X
-
C H 3 0 H 2 ++ X-
(1)
(1) (a) Kurz, J. L.; Lee, J.; Love, M. E.; Rhodes, S . J . Am. Chem. SOC. 1986,108,2960. (b) Kurz, J. L.; Kurz, L. C. Isr. J. Chem. 1985,26,339-348. (c) Kurz, J. L.; Lee, J.; Rhodes, S . J . Am. Chem. SOC.1981,103, 7651-7653. (d) Kurz, J. L.; Lee, J. J . Am. Chem. SOC.1980, 102, 5427-5429. (e) Kurz, J. L.; Lee, Y.-N. J . Am. Chem. SOC.1975, 97, 3841-3842. (0 Kurz, J. L. Acc. Chem. Res. 1972, 5, 1-9. (9) Kurz, J. L.; Harris, J. C. J . Am. Chem. SOC.1970, 92,4117-4119.
0 1986 American Chemical Society
