SOME OBSERVATIONS ON THE TRANSFORMATION OF CYCLOPENTADIENE INTO ITS DIMERIDE E. G. V. BARRETT AND L. J. BURRAGE K i n g ' s College, L o n d o n , E n g l a n d Receiced M a y 10, 19%
Cyclopentadiene is obtained in the first runnings of the crude benzene from coal tar, and readily polymerizes, a t temperatures up to lOO"C., yielding chiefly dimeride. Above this temperature polycyclopentadienes are also formed. The present experiments, which arose out of work in another connection, consisted essentially of an investigation of the rate of dimerization, utilizing for this purpose the accompanying fall in vapor pressure. EXPERIMENTAL
The crude dimeride was distilled and the fraction collected which boiled at 172.5"C. This was t8henredistilled and kept in a stoppered bottle in the dark. Some of the pure dimeride was boiled with iron filings in a flask fitted with a fractionating column. Ice-cold water was run through the condenser and the distillate collected in a receiver surrounded with ice, the fraction being employed which passed over a t 41°C. This monomeride changed over very rapidly into the dimeride and was therefore used immediately after it had been prepared. The apparatus employed was extremely simple and is shown in figure 1. The container for the liquid, A, which was similar in design to that described by Chaplin (l),was attached to the apparatus by a waxed groundglass joint B. A T-piece, C, made connection with the manometer, D, a long tube dipping into a mercury reservoir, which was open to the air, while the other arm of the T-piece connected through the tap E with a Kraus mercury condensation pump, backed by a Hyvac oil pump. Before each reading the zero was taken, and since the individual experiments only took a short time, there was no change in the zero over this period. This was checked by remeasuring after each pressure reading had been taken. In order to avoid the adjustment of a thermostat to the rather troublesome temperature of 12"C., which had been fixed as suitable for the comparison of vapor pressures, which would fall, during the course of the experiment, from the high value given by the monomeride to the much lower figure of the dimeride, measurements were always made a t a series of neighboring temperatures, the results being plotted in the form of log 1029
1030
E. G. V. BARRETT AND L. J. BURRAGE
p :1/T. The container was kept at room temperature between the readings, and that chosen (12°C.) represented the average room temperature during the year in which the experiments were made.
::
Air Ilercury Pump Trap Condensation Pump
FIG.1
TABLE 1 Zero reading = 27.98 cm.
I
PRESSURE
TEMPERATURE
TIME AFTER OPENINQ CONTAINER
degrees C.
minutea
em.H g
14.5
4 14
31.18 31.22 31.22
2 6 11
30.11 30.25 30.25
7 6.9 I
I I
5.0
0.0
-22.9
I
8 17
I
30.01 30.01
12 15 46
29.51 29.54 29.54
3 8 23 34 43
28.03 28.18 28.31 28.40 28.40
Two series of experiments have been carried out, in the first of which pure monomeride was introduced into the apparatus and the pressure read at various intervals over a period of approximately twelve months. In
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TRANSFORMATION O F CYCLOPENTADIENE
order to define the compositions giving rise to the above pressure a second series was carried out, in which definite mixtures were made and the pressures measured immediately. This gave a pressure-composition curve,
~
1/T
0.0040
0.0035
FIQ.2 Xme in days 150
200
250
0.P. I
20
$0
60 80 Time in days
100
120
FIQ.3
and hence the pressure-time series could be converted into compositiontime readings. I n table 1 the complete readings for the vapor pressure curve on the twenty-seventh day are given, and typical data expressed in figure 2, the pressures being in centimeters of mercury and the time in minutes.
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E. G. V. BARRETT A N D L. J. BURRAGE
The pressure figures at 12°C. were obtained from the log p : 1/T curves and are expressed in figure 3 which shows the change of pressure with time.
_.-
I
0.0035
0.0040 1/T
FIU.4 100
80
60
40
20
%
Wonomeride
A selection of curves from the second series is given in figure 4, the values at 12'C. being expressed in figure 5. It will be seen that the vapor pressure curve passes t.hrougha minimum at
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TRANSFORMATION OF CYCLOPENTADIENE
a composition of approximately 0.36 per cent monomeride. This curve made it possible to calculate the composition a t any point on the vapor pressure-time curve and hence the relation between per cent of monomeride and time has been obtained, the data being expressed in figure 6 . Time in days 150 200
100
250
.2
io
20
40
SO
Time in days FIQ.6 DISCUSSION
One may regard the polymerization as proceeding for the one stage only (monomeride-dimeride) under the conditions of experiment, since the pressure finally remained constant over a considerable period. The chief point of interest, however, lies in the fact that the dimeride forms liquid crystals. It is now well established that liquid crystals are phases intermediate between the crystalline (solid) and amorphous (liquid) states. G. Friedel (2) considers that there may be two or possibly more such phases between the true crystalline and the true liquid forms. H e then states that the order of succession must always be crystalline, smectic (soap-like), nematic (thread-like), liquid, and the reaction goes in this direction with increasing temperature or on dilution. The present work showed that the dimeride exhibited these properties, for, after 150 days there was an appearance of thread-like bodies in the liquid, the latter becoming considerably more viscous, while after 181 days the solution set to a stiff opalescent jelly. On raising the temperature the reverse process took place. On the 203rd day the container was surrounded with ice for the lowest point, but the pressure obtained was abnormally high. On removing the ice bath it was found that practically the whole of the solution had changed to a crystal mass (dimeride), leaving
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E. G. V. BARRETT AND L. J. BURRAGE
only a small amount of liquor which was richer in monomeride than should have been the case if the liquid crystals had been present, thus causing the pressure to correspond to a composition of some weeks previously. After this point the 0°C. figure was omitted, the tendency for crystal formation being thereby largely removed. After 229 days the solution became more fluid as it approached the pure dimeride state. When the solution was fluid equilibrium was rapid, but as soon as liquid crystals had formed it was very slow. The points in figure 2 which were in contact with the smectic mesophase have been marked with solid circles. A survey of the curves showed that the form was the same, whether the solution with which it was in contact was liquid or contained liquid crystals, a t any rate to within experimental error over the range measured. After one year the monomeride had completely changed into the dimeride and there was no evidence from the vapor pressure measurements that the change was proceeding further. At the close of the experiment the liquid had a slight opalescence, but there was no trace of resin formation. However, on exposure to air the liquid turned yellowish in color with distinct rapidity. From this it would appear that only the reaction, monomeride ---f dimeride, takes place in a vacuum, but that it proceeds to a resin in the presence of air. It would appear quite probable that the dotted lines in figures 3 and 5 represent the correct curves, since the pressure is about the same amount below the smooth curve in each case, and as one is obtained from the monomeride-+dimeride series and the other is the vapor pressure of a definite mixture, this cannot be a chance agreement. On the basis of the phase rule, the two forms of liquid crystal are considered to bedistinct phases and will therefore give rise to invariant points on the phase diagram. There are not enough points to show the position of each section of the curve, but the presence of a point so far removed from the smooth curve shows that the latter is discontinuous. The mesophases would appear to exert a considerable influence on the reaction-time curve, For example, reference to figure 6 shows that the amount of monomeride steadily decreases up to a period of 93 days, a t which point approximately 0.73 per cent remains. From this point up to a period of 235 days the curve is almost linear, and represents an extremely slow rate. From 235 days onwards, however, the change is again much more rapid. Between the two periods mentioned the liquid phase contained liquid crystals, and hence, the reason for the slow rate of change over this period may be due to hindrance to diffusion of the molecules in the solution, owing to the resistance occasioned by the presence of these liquid crystals.
TRANSFORMATION O F CYCLOPENTADIENE
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SUMMARY
1. The vapor pressure of a cyclopentadiene has been measured a t various temperatures during the change from 100 per cent monomeride to 100 per cent dimeride. 2. The vapor pressures of known mixtures of monomeride and dimeride have been determined a t various temperatures. 3. The formation of liquid crystals has been observed and the conditions noted. The authors with to express their thanks to Professor A. J. Allmand for the interest that he has taken in this work. REFERENCES (1) CHAPLIN: Proc. Roy. SOC.London A121,344(1928). (2) FRIEDEL: Ann. phys. 191 18,273 (1922).
