ON THE MECHANISM OF GASEOUS REACTIONS. I1 HOMOGENEOUS CATALYSIS IN THE DECOMPOSITION OF METHYL ETHYLETHER WILLIAM URE
AND
JOHN T. YOUNG
Department of Chemistry, The University of British Columbia, Vancouver, Canada Received June 10, 19%
During the course of a series of experiments on the thermal decomposition of methyl ethyl ether, some anomalies were observed in the measured rates where different preparations of the ether were used (1). Under the same conditions of temperature and pressure different samples of the gas showed widely different rates. It was suspected that the presence of some impurity in varying amounts was producing a catalytic effect. Since the ether was prepared from sodium methoxide and ethyl iodide, a likely impurity in the product would be the latter subst,ance, and in some of the experiments a brownish deposit, probably iodine, was observed in the capillary tubing above the furnace enclosing the reaction flask. Clusius and Hinshelwood have shown that the alkyl iodides will produce homogeneous catalysis in the case of the decomposition of several ethers, the catalytic effect being largely due to iodine vapor formed by the decomposition of the halide (2). Since it was desired to study the decomposition of methyl ethyl ether in absence of catalysts, the method of preparation was modified in such a way as to ensure the virtual absence of any iodide in the product. The results of this work have been given in the previous communication of this series. Afew experiments have been carried out for the purpose of substantiating the conclusion that the observed discrepancies in rates were due to catalysis by the alkyl halide, and these are reported here. The results of some calculations as to the mechanism of the catalytic process are also given, since, as in the case of the uncatalyzed decomposition, consecutive reactions appear to be involved. EXPERIMENTAL
The apparatus employed was the same as that used in the previous work except for the addition of a small bulb containing ethyl iodide. I n carrying out a rate measurement a small quantity of ethyl iodide vapor was first admitted to the hot reaction chamber, and heated for a few minutes before the sample of ether was introduced. This should ensure complete decom1183
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WILLIAM URE AND JOHN T. YOUNG
position of the iodide a t these temperatures (2). Since in previous experiments where catalysis was suspected, the residual gas after decomposition showed approximately the same composition as that obtained in the uncatalyzed process, the gas mixture from the experiments in which ethyl iodide was added was not analyzed, and it was assumed that the reactions taking place were substantially the same as those in the previous cases. RESULTS
The effect of the addition of ethyl iodide is to produce a marked increase in rate, as may be seen from the values given in table 1. I n fact, in run 33 where the highest concentration of iodide was used, the pressure increase was so rapid that it was impossible to obtain pressure readings until the reaction was substantially complete. I n the table t 5 0 represents the time in minutes for the pressure increase to reach one-half of its final value, and TABLE 1 The e$ect of ethyl iodide upon the rate of decomposition of methyl ethyl ether
I
NO,
-
25 33 34 35
INITIAL PRESSURE
TEMPERATURE
~ .
I
PRESSURE DUE TO IODIDE
tro
degrees C.
cm.
cm.
minutes
487 487 487 480
24.96 11.4 24.1 26.95
0 9.1 2.5 0.06
66.7 (0.5)
I
I
1.14 I
9.17
in the case of a single unimolecular reaction these values would be inversely proportional to the reaction rate constant. The reactions here are more complex, nevertheless the time values give an approximate idea of relative rates. Run No. 25 shows the time for the uncatalyzed reaction. The same preparation of ether (sample 5) was used in all of these experiments. The fourth column gives the pressure in the reaction chamber justbefore the ether was admitted, this being due to the decomposition products of ethyl iodide. Since the connecting tubes outside the furnace were not heated some iodine would condense in these, as was observed in run 33. For this reason the concentrations of catalysts cannot be accurately compared. However it is evident from run 35 that even a small quantity of iodide produces a considerable acceleration. I n ta.ble 2 are given the time values for experiments carried out with several preparations of ether a t various temperatures, showing the great differences obtained. The effect of the variation of the initial pressure (and consequent variation in the concentration of the catalyst) is well shown in the second and third lines where a decrease in pressure to onefifth of its value decreases the rate approximately three times. The time
1185
ON THE MECHANISM OF GASEOUS REACTIONS. I1
values for sample 5 represent the rates for the uncatalyzed process, since it is believed that this preparation of ether was virtually free from iodide. The last line refers to one of the experiments (run 35) in which iodide was added. The effect of increasing the surface exposed in the reaction tube was rather unexpected. With the tube packed with glass wool, a decrease in rate was observed, and the course of the reaction was no longer even approximately unimolecular, as shown by the values of tZ5,t5,,, and t76 (table 3, previous paper). I n the presence of Pyrex rods a similar decrease was noted, but in a lesser degree. The explanation of these results is possibly that of adsorption of iodide or iodine on the surface with reduction in its catalytic activity. TABLE 2 T i m e values for experiments with different samples of methyl ethyl ether at various temperatures SAMPLE OF ETHER
INITIAL PRESSURE
t60
at 456-7°C.
cm.
2 3 3 3 3 4 5 5
8 and 12 14 and 17 3.2 15 15 and 20 10.3 25 to 30 27
minutes
29.5 377.6
a t 486-7°C.
I
I
I
a t 509-10°C.
a t 561-5'C.
minutes
minutes
18.9 2.1
2.1
minutes
5.6 15.9 13.5t
30.6* 7.4t
66.7
34.3
2.2
* In presence of glass wool.
t
In presence of Pyrex rods. $ Ethyl iodide introduced to a pressure of about 0.06 cm.
The various samples of ether were prepared from the same reagents. The values in table 2 indicate that in ether sample 2 the concentration of iodide was lower than in sample 3. This was to be expected, since in the preparation of sample 2 a smaller proportion of iodide was used in comparison with the sodium methoxide. The chief difference in the method used for sample 5 was in connection with the time allowed. The reaction mixture of methoxide and iodide consists of two layers with the heavy iodide forming the lower. The reaction a t the interface is rather slow unless the temperature is raised to such a value that there is danger of the reagent distilling over. I n the preparation of sample 5 , the mixture was allowed to stand a t room temperature until the lower layer had completely disappeared, a matter of six days, after which treatments with sodium and
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WILLIAM URE AND JOHN T. YOUNG
several careful fractionations were carried out. The results seem to point unmistakably to the presence of iodide in the other preparations, THE KINETICS O F THE CATALYTIC PROCESS
Clusius and Hinshelwood (3) have shown that the decomposition of acetaldehyde catalyzed by iodine follows a unimolecular course with respect to the aldehyde itself, and that the catalytic decomposition of ethyl ether consists of consecutive unimolecular reactions. The same state of affairs is to be expected in the present case, in which a large proportion of the methyl ethyl ether decomposes into acetaldehyde and methane. The treatment of the experimental results is somewhat simpler than that given for the uncatalyzed process (1). Here a solution of the differential equations is possible, but it is easier to retain the differential form in treating the data. The value of kl, the rate constant for the ether, may be obtained as in the previous case or may be estimated from the slope of the pressure-time curve a t the origin. The equation for ICz’, the unimolecular constant for acetaldehyde, becomes dP -= dt
k~ ( P j - 1.65 Pi) e--L1*
+ kr‘b
dP where as before - is the slope a t time t of the curve obtained by plotting dt the total pressure throughout decomposition against the time. PI and Pi are the final and the initial pressures respectively, and b is the partial pressure of acetaldehyde a t time t . I n table 3 is shown the results of applying this method to an experim.ent a t 480”C., in which 26.95 cm. of ether from sample 5 were used in the presence of ethyl iodide, the pressure due to the catalyst being about 0.06 cm. There is some doubt as to the correct value of kl,since k,’ is relatively large and the decomposition of the acetaldehyde is taking place at an appreciable rate even a t the start. The values of b show the expected rise and fall and values of kz’ for a unimolecular process given in the fourth column show fair agreement up to a time of 20 minutes, after which both b and the pressure of the ether become small. In the last column are shown values calculated for a bimolecular reaction on the part of the acetaldehyde and the variation is much larger than for the unimolecular case. In table 4 the same treatment is applied to an experiment at 466°C. in which 10.25 em. of ether from preparation 6 was used without further addition of catalyst. Here the agreement in the values of kz’ in column four is quite satisfactory. As in the uncatalyzed decomposition, the analyses indicate a part of the ether forms formaldehyde and ethane, followed by the rapid decomposition
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ON THE MECHANISM OF GASEOUS REACTIONS. 11
TABLE 3 Experiment on ether sample 5 and ethyl iodide at dS0"C. IC, = 0.146 min.-l t
P
b
minutes
cm.
cm.
0 1 2 3 4 5 6 7 8 9 10 20 30 40
26.95 30.15 33.15 36.05 38.85 41.45 43.85 46.15 48.05 49.9 51.5 62.3 66.7 68.5
0 2.75 4.75 6.25 7.15 7.95 8.25 8.45 8.65 8.60 8.60 5.47 2.84 1.47
k d (uni.)
kz (bi.)
0.131 0.135 0.126 0.130 0.116 0.121 0.113 0.110 0.102 0.091 0.111 0.086 0.068
0.048 0.028 0.020 0.018 0.015 0.015 0.013 0.013 0.012 0.011 0.020 0.030 0.040
TABLE 4 Experiment an ether sample 4 at 456°C. kl = 0.030 min.-1 ~
t
P
b
minutes
cm.
on.
0 10 20 30 40 50 60 70 80 90 100 110 120 150
10.25 13.45 15.98 18.10 19.85 21.25 22.37 23.30 24.20 24.98 25.60 26.03 26.35 27.00
0 1.34 2.16 2.52 2.60 2.56 2.45 2.27 1.92 1.55 1.23 1.03 0.87 0.51
A d (uni.)
ka (bi.)
L_
0.034 0.029 0.025 0.025 0.023 0.022 0.024 0.027 0.031 0.033 0.025 0.026 0.030
-
0.025 0.013 0,0099 0.0096 0.0090 0.0090 0.011 0.014 0.020 0.027 0.024 0.030 0.059
i v . -0.027
of the formaldehyde, and the slow decomposition of a part of the ethane to an equilibrium with ethylene and hydrogen. I n the former case the ethane decomposition was found to be relat'ively slow and heating for a day or two was usually required to reach a constant pressure. Thus in an experiment
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WILLIAM URE AND JOHN T. YOUNG
RUN NO.
TEMPERATURE
degrees C .
4
5 7 8 13 14 35
503 509 510 510 487 487 480
INITIAL PRESSURE
ETHER SAMPLE
cm.
11.9 7.7 21.0 17.3 14.1 13.4 26.95
2
2 3 3 3 3 5 + ethyl iodide
ON THE MECHANISM OF GASEOUS REACTIONS. I1
1189
a t 565'C. a n approximate calculation based on the pressure rise after 50 minutes gave for a unimolecular constant 3.4 X 10-3 min.-l, while from the work of Marek and McCluer (4)the constant for ethane decomposition min.-' is calculated to be 8.9 X I n the catalytic experiments in most cases there was noted a slow rise in pressure following the first relatively rapid increase. Moreover if a logarithmic plot is made, the lines appear to show two main slopes., This is illustrated in figure 1 in which are plotted the values of log ( P f - P ) against the time for a number of the catalytic runs. P is the total pressure at any time, and P, the final total pressure. If the process was a simple unimolecular one such a plot would produce a straight line. I n the figure it is shown that in most cases the lines are approximately straight in the first portion of any run; then there is a rapid change of slope to a much lower value. The first part of the plot evidently refers to the decomposition of ether followed by that of acetaldehyde, and corresponds approximately to a single unimolecular process, since kl and ICz' are close together in value, as shown in tables 3 and 4. That the second slope corresponds to the decomposition of ethane seems a reasonable assumption, since the rate is too slow for that of acetaldehyde, and moreover the amount of pressure change during the second stage is of the right order of magnitude as calculated from the equilibrium data for the reaction
I n the case of runs 4 and 5 (and some others not shown here), the initial rates are slow and the experiments were not carried on long enough to reach the equilibrium pressure. The experimental data for this final stage in the decomposition process is not sufficiently accurate to provide reliable values of the unimolecular constant for ethane, but approximate calculations indicate considerably higher values than would be expected a t the temperatures used. This points to catalysis also in the decomposition of the ethane. It is to be noted that a t the temperatures a t which the catalytic reactions were carried out, the uncatalyzed reactions were also taking placeatmeasurable rates, as shown in the previous paper, although in most cases the effect of the latter should be negligible. SUMMARY
1. The,decomposition of methyl ethyl ether is catalyzed in the presence of small quantities of ethyl iodide such as may remain in the ether as a result of its preparation. 2. The catalytic process consists mainly of the unimolecular decompo-
1190
WILLIAM URE AND J O H N T. YOUNG
sition of the ether followed by the unimolecular decomposition of the acetaldehyde formed. 3. Ethane seems also to be a primary product and the decomposition to an equilibrium mixture with ethylene and hydrogen is catalyzed as a result of the iodide. REFERENCES (1) UREAND YOUNG:J. Phys. Chem. 37, 1169 (1933). (2) CLUSIUSAND HINSHELWOOD: Proc. Roy. SOC. London l28A, 75 (1930). (3) Reference 2, pp. 82 and 88. (4) MAREK AND MCCLUER: Ind. Eng. Chem. 23,878 (1931). (5) PEASE:J. Am. Chem. SOC.64,1876 (1932).
