Preparation of Pure Cyclohexane d
W. F. SEYER, MAURICE M. WRIGHT, AND RAYMOND C. BELL University of British Columbia, Vancouver, Canada
HE best cyclohexane available through most of the leading chemical dealers has a freezing point between 3.5" and 4.5" C. Timmermans (23) and others have shown that the freezing point of the pure substance lies close to 6.4' C. Since the physicochemical properties of the hydrocarbon make it a useful solvent for many purposes, a simple method of purification seemed highly desirable. The impurities may arise from two sources: (a) side reactions during hydrogenation and (b) the nature of the benaene utilized. Although Sabatier (9) claimed that no side reactions took place when nickel was used as a catalyst, others found that this was not invariably true. Thus Burrows and Lucarini ( l ) ,studying the equilibrium of benzene, hydrogen, and cyclohexane, had to discontinue the use of nickel because of side reactions; they substituted platinum instead. The side reaction which is most likely to occur is the scission of the benzene ring with the formation of methylcyclopentane. Recently, Flynn (4), working in these laboratories, obtained information as to the extent of these side reactions by hydrogenating benzene over a nickel catalyst a t 170" C. By recycling the condensate over the nickel in an atmosphere of hydrogen, he obtained a completely saturated liquid whose freezing point was approximately -40" C. after the benzene had been removed by treatment with sulfuric and nitric acids. This c. P. benzene had previously been purified by three recrystallizations in order to remove traces of the xylenes and toluenes which are ordinarily present. Calcu-
T
lations based on the latent heats of fusion of cyclohexane and methylcyclopentane would indicate that a mixture of the two, with a freezing point of -40°, should have a composition of about 30 per cent methylcyclopentane. Thermodynamic calculations indicate that methylcyclopentane will be even more likely to form than cyclohexane. Thus from the data and method given by Thomas, Egloff, and Morrell ( l a ) , the free energy change for the reaction, CsHe 3H2 +C6Hn.CHs (1)
+
is AF04rl = -119,762; and for the reaction, C ~ H4B 3Hz+ CsHiz (2) the free energy change is AF"441 = -4534. It is possible that the method of making the catalyst may influence the course of reaction, for Ipatiev (7) claimed to have produced only cyclohexane when nickel sesquioxide was used. ~~
~~
Evidence is presented to show that methylcyclopentane is likely to be produced in the hydrogenation of benzene with a nickel catalyst. Pure cyclohexane can be produced from the commercial material by rectification in a Fenske, Quiggle, and Tongberg column.
Extensive recrystallizations of the best cyclohexane obtainable from Eastman Kodak Company and the British Drug House, as carried out both by Selwood (IO) and Flynn (4), showed little promise that cyclohexane could be purified this way. Hence rectification in an efficient column seemed to be the only alternative method of preparing this substance in a pure state.
Separation of Fractions
A rectification column was built according to the plans given by Fenske, Quiggle, and Tongberg ( 3 ) . The dimensions, however, were altered to suit the space available. The column proper was made of a copper pipe 2.54 cm. 0. d. and 4 meters long. It was filled with a No. 18 jack chain. The still was a copper-plated iron pot, insulated and bound with Nichrome wire so that it could be heated both from the sides and bottom. A reflux ratio of about 35 to 1 was maintained by keeping the temperature difference between the still and condenser head constant a t 11.5" C. The product was withdrawn in 100-cc. fractions every 2 hours. I n this manner over 3000 cc. of the Eastman Kodak Company's commercial cyclohexane were separated into thirty fractions. They were then examined for purity by density, refractive index, and freezing point measurements. DENBITY.The densities were measured with a hydrometer, graduated to four decimal places. It was calibrated by immersing it in two samples of cyclohexane whose densities had been carefully determined previously by means of a standard 50-cc. specific gravity bottle. All measurements
FIGUREI 1. COOLING CURVES 759
.
INDUSTRIAL AND ENGINEERING CHEMISTRY
760 FREEZING POINT
VOL. 31, NO. 6
TABLE I. PHYSICAL PROPERTIES OF FRACTIONS
CURVE
REFRACTIVE. INDEX CURVE
4270
~20W
e60
No. of Fraction d:' 1 0.7771 2 , 3 0.7775 4 5 6 0.7780 7 8 9 10 11 12 13 14 15
.. . .. .... .. .... ... .
0.7784 0.7784
... . 0.7784
... ..... 0.7784
ny 1.42562 1.42606 1.42627 1.42627 1.42627 1:42627 1.42627 1.42627 1.42632 1.42632 1.42637 1.42632 1.42627 1.42632
Freezing No. of zoint, FracC. tion 16 1:30 17 18 5:79 19 20 21 6:23 22 6.32 23 24 6140 25 26 6147 27 28 6148 29 30
..
..
d:'
.... .. . .
0.7784
.. . .
0.7784 0.7784 0.7784 0.7784
....
0.7784
....
0.7784 0:+784
....
Freezing Point, C. 1.42632 6 . 4 7 1.42637 1.42632 6:46 1.42642 1.42642 6:47 1.42632 1.42637 6:48 1.42637 1.42632 6:48 1.42632 1.42632 6:46 1.42632 6 . 4 4 1,42632
np
.....
1.42632
6146
..
~~
l l
1.78c
I*=
=~
775
1170
0
10
15
0
NUHBER OF FRACflON
E3
30
PROPERTIES OF FRACTIONS FIGURE 2. PHYSICAL
were carried out a t 20" C. in a constant-temperature bath whose variation was not greater than 0.03" C. The mercury thermometer had been calibrated against a platinum resistance thermometer recently checked by the Bureau of Standards for accuracy. REFRACTIVE INDEX. The refractive indices were measured by a Pulfrich refractometer constructed by Adam Hilger. Powdered sodium chloride sprayed into a Bunsen burner flame served as a source of monochromatic light. Water at 20" C. from the constant-temperature bath was led through the apparatus at such a rate that no difference in temperature reading could be detected between the bath and the refractometer cup.
Purity of the Fractions !
FREEZING POINT. The apparatus used was described by Seyer and Walker (11). The bath fluid was water and ice, 'and was kept stirred by a small motor. The cyclohexane was stirred by hand once every 5 seconds to obtain constant results, because the heat of fusion of this hydrocarbon is so low. For freezing point determinations two of the 100-cc. fractions were combined in order to have sufficient liquid in the freezing point chamber. To show the difference in behavior of the various fractions, the cooling curves of fractions 1-2 and 19-20 are given in Figure 1. The densities (corrected to vacuum), refractive indices, and freezing points of the fractions are listed in Table I. The results are shown graphically in Figure 2. Since the freezing point is the most sensitive to impurities of the physical properties measured, it is obvious that fractions 11 to 26, inclusive, constitute pure samples of cyclohexane. I n other words, a yield of 50 per cent of pure cyclohexane can be obtained from the commercial variety by using a column such
as has been described. A rerun of most of the fractions would certainly increase the yield. The average values of the densities, refractive indices, and freezing points of the pure samples are, respectively, 0.7784, 1.42635, and 6.47 * 0.02" C. These results agree well with those given by other investigators and summarized in Table 11. TABLE11. COMPARISON OF RESULTS WITH THOSEOF OTHER INVESTIGATORS Timmermans and Martin (13) Zelinsky (14) Cifford and Lowry (6) Intern. Critiaal Tables (6) Burrows and Lucarini ( I ) Nagornow and Rotinganz (8) Eisenlohr (8) Present values
Density
Refractive Index
0 7785 2 0 ° C ) 017788 t19.5'C.)
1.42886 (15'C.) 1.4289 (19.5'C.) 1.42900 (15" C.)
... ...
...
0.7786 (20" C.) 0.7783 (20' C.) 0.7784 (20' C.)
1.42640'(20° C.)
Freezing Point, O C. 6.40 6.40 6.28 6.50
..
...
6.54
1.42635'(20° C.)
6:47
The tables show that the present values fall well within those given by others. Considerable weight must be given the values of Timmermans and Martin (13), who were very careful to prepare a pure compound. The difference in their density values and those found here differ by only 0.0001. The refractive indices cannot be directly compared because of the lack of knowledge of an accurate temperature coefficient. The difference in the freezing point, 0.07" C., is worthy of note. The value given in this paper is an average of seven determinations, and the cooling curve of fraction 19-20 (Figure 1) shows a high degree of purity. The characteristics of the platinum thermometer used were described elsewhere (11). It was frequently checked against the freezing point of conductivity of water both during and a t the end of the freezing point experiments.
Literature Cited (1) Burrows and Lucarini, J . Am. Chem. SOC.,49,1157(1927). (2) Eisenlohr, Fortschritts der Chemie, Physik, physikal. Chena., 18, 521-66 (1925). (3) Fenske, Quiggle, and Tongberg, IND.ENQ. CHmM., 24, 408 (1932). (4) Flynn, J. B., unpublished master's thesis, 1933. (5) Gifford and Lowry, Proc. Roy. SOC.(London), A104,434 (1923). (6) International Critical Tables, New York, McGraw-Hill Book Co., 1926. (7) Ipatiev, J. Russ. Phys. Chem. SOC.,39,681-93 (1907). (8) Nagornow and Rotinganz, Ann. inst. anal. phys. chim., 2, 371 (1924). (9) Sabatier, IND.ENQ.CREM.,18,1006 (1926). (10) Selwood, P., unpublished research. (11) Seyer and Walker, J. Am. Chem. SOC.,60,2125 (1938). (12) Thomas, Egloff, and Morrell, IND.ENQ.C H ~ M29, . , 1260 (1937). (13) Timmermans and Martin, J. chim. phys., 23,733-87 (1926). (14) Zelinsky, Ber., 39, 2799 (1901).
