KINETICSOF METHYLIODIDEREACTION WITH TOLUENE
Dec. 5, 1960 [COXTRIBUTION FROM
THE
5987
CHEMISTRY DEPARTMENT, UNIVERSITY OF MICHIGAN, ANI; ARBOR,MICHIGAN]
Kinetics of the Reaction of Methyl Iodide with Toluene BY ROGERF. K L E M MAND ~ RICHARD B. BERNSTEIN RECEIVEDAPRIL25, 1960 The kinetics of the reaction of methyl iodide with toluene have been studied a t 326, 354 and 374" in a static system at total pressures from 180-780 mm. The ratio of methyl iodide ( M ) to toluene ( T ) was varied from 0.021-0.63. I t was found that methane was the sole end-product of the methyl iodide carbon, so that the methane production could be used as a measure of the extent of reaction. Molecular iodine accounted for only a fraction (0.25-0.70) of the iodine from the reacted M, due to the formation of H I and benzyl iodide. Analysis of the kinetic data (with emphasis on the measurements a t low k~(M)'/2(T)l/$,with activation energies E1 = extent of reaction of M ) yielded the expression: -d(M)/dt = kl(M)(T) 51.6 and Ez = 45.2 kcal./mole. Auxiliary experiments showed that the reaction was accelerated by added iodine. KO evidence for an inert gas pressure effect or a surface reaction was found. The C12/C1akinetic isotope effect was 1.019 i 0.002 over the range studied, compared to 1.026 zk 0.001 for the reaction of M with HI. Experiments on the pyrolysis of M alone were carried out using a high speed flow system with He carrier at 475-575". Product analyses indicated the relation0.072 CzHs 0.22 C 0.50 12. ship: CHaI -+ 0.64 CH,
+
+
+
+
(Mallinckrodt A.R.) was further purified6 and then treated Introduction by the procedure of Szwarc.7 Blank runs of the toluene The kinetics of the thermal decomposition of alone a t the highest reaction temperature yielded no methmethyl iodide are rather complex and not well ane. By allowing the reaction of methyl iodide with excess understood. The early work has been sumtoluene to proceed longer than the time for complete demarized by Steacie.2a Szwarc,2b employing the composition ( > 99.9%), it was found that the number of toluene carrier technique, investigated the re- moles of methane formed was equal to the initial number of action a t temperatures near 500"; more recent moles of methyl iodide taken, within the experimental unsimilar work has been reported by H ~ r r e x . ~ Un- certainty of f 2%. Moreover, the C12/Cla ratio of the Con from the above methane corresponded (within f 1 fortunately, in the case of iodides a t least, the ap- derived part per mil) with that for COz from quantitative combustion plicability of the toluene carrier technique is of the methyl iodide. Since the Cla/C12ratio in the CHJ dubious, due to various complications. Here, for happened to be smaller than the ratio in tank COS (and in example, (a) iodine reacts with toluene readily to most hydrocarbons) by the factor 0.926, the above result a tracer determination implying that < 2% of the produce HI, which in turn reacts with methyl constitutes CHd is derived from the toluene. iodide yielding methane, and (b) the possibility Consideration of these important observations indicates of a direct bimolecular reaction (CH3I 4-C G H ~ C H ~that ) the methane production may be used as the measure may not be excluded. The present paper reports of the extent of reaction of the methyl iodide, Le., -d(CHaI)/ = d( CHd)/dt. an investigation of the kinetics of the reaction of dt The standard kinetic experiment consisted of evaporating methyl iodide with toluene using a conventional premixed amounts of CHaI and CBHSCH~ (the latter usually static method a t lower temperatures (326-374') , in excess) into the thermostated reactor. Total pressures where complexities would presumably be less ranged from 180-780 mm. After times ranging from 5 min. to 6 hr. the contents of the reactor were passed through important. It was found that the kinetic data were traps a t -196" and the volatile product (essentially pure quite reproducible and that the observations could CHI, as determined by gas chromatography and by quantibe represented by the simple empirical rate expres- tative combustion t o COZ)was transferred to a measuring volume. When isotopic assays were also desired, the CHr sion -d(M)/dt
= Kl(M)(T)
+ kz(M)'/z(T)'/2
(1)
where M and T refer to methyl iodide and toluene, respectively. Experimental4 Kinetic experiments weie carried out in a conventional vacuum apparatus. The Vycor reaction vessel was identical with the one used in a previous study6 and had been conditioned by the deposition of a carbonaceous coating. Good reproducibility of results for given initial conditions over the duration of the investigation indicated the negligible influence of the condition of the surface. Methyl iodide (Eastman Org. Chem. No. 164) was further purified,6 distilled zn vacuo and stored in the dark a t -78". Vapor pressure, boiling point measurements and the infrared spectrum of the vapor agreed with the literature.4 Toluene (1) Chemistry Department, University of Alberta, Edmonton, Alberta, Canada. (2) (a) E. W. R. Steacie, "Atomic and Free Radical Reactions," Reinhold Publishing Corporation, New York, N. Y.,1954. (b) M. Szwarc, Thesis, Manchester Univ., 1947. (3) C. Horrex and R. La Page, Discussions Faraday SOL, 10, 233 (1951). (4) For further details see Ph.D. dissertation of R. F. Klemm, University of Michigan (1959), available from University Microfilms, Ann Arbor, Michigan. (5) M. E. Russell and R. B. Bemstein, J . Chcm. Phys., S O , 607 (1959). (6) A. Weissberger, r l d., "Techniques of Organic Chemistry," Vol. VII. 2nd Ed., Interscience Publishers, Inc., New York, N. Y . , 1957.
was oxidized to COz by cycling through CuO (850", then 300°), water being removed at -78". The iodine (originally condensed a t 196") was titrated with thiosulfate; to 5 X typical analyses were in the range 7 X g. at. of iodine. Gaschromatographic analysis showed that no ethane (< 1.5%) was formed under the present experimental conditions. Auxiliary experiments showed the presence of small quantities of H I and benzyl iodide in the products. Separate studies on the reaction of Iz,with toluene a t 326" indicated extensive formation of HI. A number of high temperature experiments were also carried out with a flow system using He carrier gas; the CHaI was present a t a level of 0.02-0.2 of the total pressure. A cyclone "stirred flow" reactor8 was used, followed by a conventional vacuum system to separate CH3I and Ipfrom more volatile products. A silica gel adsorption trap a t - 196" isolated the product CHI. Alternatively, the total mixture issuing from the reactor was sampled by gas chrcmatography. Results are presented in Appendix I. For both static and flow experiments the carbon in the CHaI was converted t o COz by a modified Wilzbach-SykesO combustion. The mass spectrometric technique for C12/C13 assay was standard.'O Isotopic fractionation results are presented in Appendix 11.
-
(7) M. Szwarc, J . Chem. Phys., 16, 128 (1948). (8) T. J. Houser and R. B. Bernstein, THISJOURNAL, 80, 4439 (1958) (9) K. E. Wilzbach and W. Y.Sykes, Sciertce, 120, 494 (1954). (10) M. E. Russell and R. B. Bernstein, J . Chcm. Phys., S O , GI3 (1959).
5988
ROGERF. KLEMM AND RICHARD B. BERNSTEIN
Vol. 82
0.800D W
t-
2 W a
5 0.700'C M
n
I 0
Fig. 1.-Typical dependence of x / a ~upon t.
2
ResuWJ
60 0.600-
l2
Fig, 1 is a typical graph of the time dependence of the fraction of methyl iodide reacted, based on CH4 formation. For comparison the data for IZ production is also shown. It is clear that molecular IS does not constitute the sole end-product of I atoms from the reacted CHJ. For a number of reasons the kinetic data for 1 2 formation are less reliable (and less extensive) than the comparable data for CH4. However, in every case the ratio of the IZ recovered (in g. at. of I)/CHJ reacted (inmoles) was < 1. This ratio varied rather systematicallylover a range from 0.25 to 0.70, generally increasing with increasing extent of reaction. The distribution of the remainder of the iodine between the H I and G H I C H ~ products I was not measured, however. Figs. 2 and 3 show semilogarithmic plots of the methane data for typical sets of experiments. Ranges of initial concentrations used in the present study were as follows: 1o4MOfrom 3.80-27.2; 10To from 0.42-1.78. Initial slopes, designated k e l p (min.-l), of such lines were helpful in deducing the empirical rate expression. Additional hitherto unreported observations are briefly summarized. A. I n the reaction of M with T in the range 326374" : (a) Addition of 1 2 (in concentration ratios relative t o MO ranging from 0.25 to 3.0) caused an increase in the CHI production. Quantitative evaluation of the 1 2 dependence was not feasible using the present experimental technique. (b) The reaction is unaffected by inert gas (SFJ pressure in the range 180-760 mm. and is increased by only 507* upon 100-fold increase in the S/ Vratio. (c) Pyrolysis of M alone (in clean Pyrex reaction vessels) was accompanied by extensive carbon deposition, whereas in the presence of T (11) A compilation of original data has been deposited as Document number 6336 with the AD1 Auxiliary Publications Project, Photoduplication Service, Library of Congress, Washington 25, D. C. A copy may be secured by citing the Document number and by remitting $1 25 for photoprints or $1.25 for 35". microfilm, in advance by check or money order payable to: Chief, Photoduplication Service, Library of Congress. (12) The abbreviated notations employed are: Ma, initial concentration of CHII (mole/l.); TI, initial concentration of CaHrCHa (mole/l.); Qa. initial molar ratio of methyl iodide to toluene; X/M, molar concentration of product/M,; 1,fraction of methyl iodide reacted, assumed equal to x/aa for methane.
a
a LL
0500-f
t
20
40
TIME, (MIN.) 60 80 100
\ c)
120
144
I20
I?
Fig. 2.-Dependence of rate on Mp.
\
0 (3 -I
+
. . I
20
40
60
TIME, (MIN.) SO IO0
Fig. 3.-Dependence of rate on TO.
the reaction was clean, yielding no visible surface film or denosit.
KINETICS OF METHYLIODIDE REACTION WITH TOLUENE
Dec. 5 , 1960
5989
(pseudo-first order behavior) ; thus kexp is identified with klT,-,(l b), consistent with eq. 2, of course. The fit of the time-dependent solution (eq. 4) to the experimental points is shown in Fig. 5, where tc (the time calculated using eq. 4-6 with rate constants of Table I) is plotted vs. tobsd. for each experiment for which f _< 0.7 and t 5 180 min. More than SO% of the points lie within the designated band corresponding to an error of 10%. At each temperature, however, there appears to be a systematic deviation for the points a t relatively large t (and/or high extent of reaction). Thus eq. 1 must be regarded as an approximation best describing the observations a t low extent of reaction.13 Over the limited temperature range studied, linear Arrhenius plots of kl and ko were obtained. Leastsquare treatments yielded
+
1 4
I
I
6
8
I l l
326' 1 374' 3540
i O z x IM,T,)~"'
Fig. 4.-Plot
kl(l. mole-' min.-l) = 1.9 X IO1' exp( - 51,60O/RT) (7) and kz(min.-') = 5.2 X 10l2exp( 45,20O/RT) (8)
-
For evaluation of rate constants.
A detailed discussion of various plausible steps (d) The average C1z/C13kinetic isotope effect in the reaction mechanism and the question of the isotope effects is presented elsewhere. However, over the temperature range studied is 1.019 0.002 compared to 1.026 f 0.001 for the reaction no mechanism has been found which satisfactorily encompasses all the present experimental results, of M with H I (see Appendix 11). B. I n the high temperature pyrolysis of M in yielding the empirical rate equation (eq. 1). Speculation of mechanism tilerefore does not apthe range 475-575" (Appendixes I and 11): (a) CzHe is identified as a significant product. pear warranted a t the present time. (b) The rate of C H 1 production increases upon Acknowledgments.-The authors appreciate the addition of toluene. financial support received from the US.A.E.C., (c) The CI2/Ci3 kinetic isotope effect is 1.010 Div. of Research, Contract AT(ll-1)-321 and the (at 486"). Michigan Memorial-Phoenix Project. Thanks are Empirical Rate Expression.-Inspection of the due to Mr. J. Doshi for assistance with the mass dependence of kexp upon the initial concentrations spectrometric measurements. of 111 and T in the absence of added IZsuggested the Appendix I High Temperature Experiments.-The thermal decomposition of methyl iodide alone and in the presence of toluene leading to the form of the rate expression, eq. 1. was investigated4 using a high speed flow technique in the I n Fig. 4 are shown least-square lines of k e x p l range 475-575' and 2-5% decomposition. From product To vs. (iVIoTo)-l/~ a t the three temperatures of the analyses for experiments made over a fair range of initial conditions the stoichiometry for the pyrolysis of methyl iomeasurements, from which the rate constants kl dide alone could be approximated by1: CHaI + 0.64CH' + and k2 were evaluated; the results are listed in 0.072 CZHB+ 0:22C + 0.501z. This may be compared to Horrex's's stoichiometry in the lower pressure region over Table I.
+
TABLE I SUMMARY OF RATECONSTANTS T ("C.) 326.0 354.4 374.4
kl (1. mole-' min.-l)
10' X k z ( m h - 1 )
0.0287 ,197 .736
0.175 0.938 3.00
Since for the present experiments ( T ) s TO, eq. 1becomes -d(M)/dt = ki(M)To
+ kz(M)l/pTo'/*
(3)
+
the same temperature range: CHa! + 0.75CH1 0.25C 0 5012. From the present results it is seen that ethane production accounts for an appreciable fraction of the decomposed methyl iodide. Addition of toluene in excess gave rise to a significant increase in the over-all rate of methane formation, as would be expected from the results of the static experiments a t lower temperatures.
Appendix I1 C W 1 3 Isotope Effect.-The is defined in the usual way:
isotope fractionation factor S
which is integrated to yield the equation
where b = k~'kl(MoTo)i/2,a = 1 - (1 - f)'Izand = 1 - ( M ) / M 0 , the fraction reacted. For cr/(l+ b)
