3134
LAWRENCE SILVER AND REUBMN RUDMAN
be quite sensitive to the approximations used, and the good agreement between the second- and third-law values gives further credence to the validity of the approximations employed in this investigation.
Acknowledgment. The partial support of the U. S. Atomic Energy Commission (AT( 11-1)-716) and a National Science Foundation Fellowship to D. E. W. are gratefully acknowledged.
Polymorphism of the Crystalline Methylchloromethane Compounds. 111. A Differential Scanning Calorimetric Study by Lawrence Silver and Reuben Rudman Department of Chemistry, Adelphi University,’Garden City, N w York 11630 (Received March 80, 1070)
The methylchloromethane compounds and carbon tetrabromide have been studied from above their melting points to 115°K using differential scanning calorimetry. Carbon tetrachloride, methyl chloroform ( l , l , l trichloroethane), and 2,%dichloropropane each form two successive high-temperature, disordered crystalline phases (Ia and Ib) when cooled from the melt, but when warmed from the low-temperature, ordered modification (phase 11), phase Ib forms and persists until the melt. &Butyl chloride (2-chloro-%methylpropane), neopentane (2,2-dimethylpropane), and carbon tetrabromide form only one crystallographic modification in the high-temperature region. Enthalpies of transition and fusion have been obtained for all but the two weakest phase transitions involved. These include AHIa+L,CCId = 419 cal/mol, AHIa+L,CHaCCla = 217 cal/ mol, AHIb-..L,(CHs)2CCla = 537 cal/mol, and AHII-.Ib,(CHI)8CC12 = 1415 cal/mol. The other data, with one exception, are in agreement with previously published values. It was not possible to obtain accurate data for AHII+Ib,CHaCCis inasmuch as the heat capacity is rapidly changing in the high-temperature phase. Similar difficulties were encountered by previous investigators; the two sets of results are compared.
Introduction The methylchloromethane (MCM) compounds, (CHa),CClr-,, where n varies from 0 to 4, consist of nearly spherical molecules and exhibit a number of solid-solid phase transitions. The high-temperature phases of such materials are generally termed “plastic crystals,” inasmuch as they are soft, waxy, and easily extruded. An initial low-temperature survey112 of these compounds revealed that carbon tetrachloride ( n = 0) exhibits unusual crystallographic behavior. It was found that, on cooling from the melt, a face-centered cubic (fcc) phase is formed which transforms to a rhombohedral phase, followed by a transformation to a monoclinic phase. On warming the monoclinic phase, the rhombohedra1 phase is formed; this phase persists until the melting point. Subsequent study by differential thermal analysis* and differential scanning calorimetry (DSC) verified the results of the X-ray investigation. The X-ray investigation2 of methyl chloroform (n = 1) and 2,2-dichloropropane (n = 2) revealed the existence of high-temperature structures similar to that of the rhombohedral phase of carbon tetrachloride. However, no evidence for the existence of any fcc phase was The Journal of Physical Chemiatry, Vol. 74, LVo. 16, 1070
discovered for these compounds, even though t-butyl chloride (n = 3) and neopentane (n = 4), as well as carbon tetrachloride, form fcc phases with similar unit cells and X-ray intensity distributions. The present study was undertaken in an attempt to investigate systematically the solid-solid phase transitions of all five MCM compounds, from their melting points to - 160”. I n addition, the possibility of similar phase transitions in carbon tetrabromide, which is isomorphous with carbon tetrachloride in the fcc and monoclinic phases, was investigated. The existence of fcc phases in methyl chloroform and %,&dichloropropane was uncovered during the course of the DSC investigation and verified by low-temperature singlecrystal X-ray diffraction techniques. The results of the X-ray diffraction studies have been described elsewhere.6 The present paper is limited to a description (1) R. Rudman and B. Post, Science, 154, 1009 (1966). (2) R. Rudman and B. Post, Mol. Cryst., 5 , 95 (1968). (3) K. Kotake, N. Nakamura, and H. Chihara, Bull. Chern. Soc. Jap., 40, 1018 (1967). (4) A. P.Gray, personal communication; Instr. News, 17, 3 (1967);
and Perkin-Elmer DSC literature. (5) R. Rudman, Mol. Cryst. L k u i d Cryst., 6 , 427 (1970).
POLYMORPHISM OF CRYSTALLINE METHYLCHLOROMETHANE COMPOUNDS of the various phase transitions and the thermodynamic data derived from the DSC thermograms of the MCM Compounds and of carbon tetrabromide. These data are compared, where possible, with previously reported data.
Experimental Section
3135
line and the peak slope (on the ascending side of the peak). A difficult problem arose in determining the accurate melting point of the MCM compounds with n = 0, 1, and 2. These compounds melted over considerable ranges of temperatures. It is known that the considerable premelting phenomena which have been observed for these compoundsg prevent the determination of a sharp melting point. I n fact, it might not even be proper to speak of a sharp melting point when discussing these compounds. The soft, plastic nature of these phases and the large degree of self-diffusion that is known to occurlosuggest that it might be most practical to speak of a melting point range. In the present investigation it was found that the end of the peak always occurred a t the same temperature. It was decided to report the (reproducible) temperature at which melting is completed rather than the (variable) temperature a t which premelting phenomena begin to occur. It was later found that similar difficulties had induced Rubin, et al.,ll to report the melting point of methyl chloroform for the completely melted system. Sample Preparation. The mass was determined to hO.01 mg for samples ranging in size from 2 to 10 mg. All samples used were quite volatile and the sample holders had to be sealed before weighing. A few sets of sample holders were weighed, sealed without any sample, and reweighed. No change in mass was determined. Aluminum sample holders were initially used. However, for MCM compounds where n = 1, 2, and 3 they exploded while they were being sealed or corroded very soon after sealing. A search of the literature revealed that a, strongly exothermic reaction occurs between heated aluminum metal and alkyl halides resulting in the formation of air-sensitive liquid alkylaluminum sesquihalides.12v1a Evidently the crimping process generates sufficient heat to initiate the reaction.
Materials. The MCM samples used were the highest quality commercially available and were treated as follows. Carbon tetrachloride, methyl chloroform, 2,2-dichloropropane, and t-butyl chloride were redistilled and examined by vapor-phase chromatography using both polar and nonpolar columns. Analysis of the chromatograms showed that methyl chloroform was 99.9% pure and tbutyl chloride was 99.7% pure, while both carbon tetrachloride and 2,2-dichloropropane showed no indication of any impurities even a t the highest resolution of the instrument and were assumed to be >99.95% pure. J. T. Baker C P grade neopentane, rated at 99.0% pure, was obtained in a pressurized cylinder and was used without further purification or analysis. The carbon tetrabromide was recrystallized by sublimation. Equipment. A Perkin-Elmer DSC-1B differential scanning calorimeter equipped with a low-temperature adapter was used.6 The DSC was calibrated with 99.999901, pure indium, using ranges of 8 and 16 with a chart speed of 1.5 in./min. An instrument constant of 10.92 f 0.06 mcal/cm2 was obtained, using AHr of indium equal to 0.775 kcal/mol.’ With this constant, AHr for a 99.99+% pure sample of lead was found to be 1.113 kcal/mol, compared to the reported value of 1.141 kcal/mol.’ Appropriate corrections were made for other ranges and chart speeds. A further indication of the accurate calibration of the energy input and temperature readings of the DSC was provided by the good agreement between the present and previously reported values for a number of the MCM transitions. Possible explanations for the dis(6) The temperature calibration of the instrument only reached crepancies which were observed in a few cases are found -looo, so it was necessary to offset the controls in the rear of the instrument and to recalibrate the dial settings with a series of known in the Discussion. materials. Instructions for this are included with the instrument, Each of the transitions was investigated at two or but the suggested standards, a8 well as a number of other materials we tested, did not melt sharply. A straight-line calibration curve more scan rates, ranging from‘O.625 to lO”/min. All of dial reading us. temperature was obtained by using the following readings for a given transition were in agreement, ~~-~ (n, sharp, reproducible transitions of the ( C H S ) ~ C Ccompounds transition, transition temperature in degrees Kelvin): 3, I -c L, within the limits of error. The thermogram peaks were 248.2; 2, I b -+ L, 239.4; 0, I1 ---c Ib, 225.5; 3, I1 + I, 219.7; 2, each measured several times with a compensating polar I1 -+ Ib, 188.2; 3,I11 ---c 11, 183.2; 4,I1 -+ I, 140.0. In this not& tion, I1 4 I indicates a transition from phase I1 to phase I ; L planimeter accurate to 0.1 em2. Areas ranging from represents the liquid state. 0.4to 21 ern2were measured. (7) “Handbook of Chemistry and Physics,” 49th ed, The Chemical The manufacturer’s specifications state that the temRubber Publishing Co., Cleveland, Ohio, 1968-1969, p D-33. (8) A. Dunlop, J. Amer. Chem. Soc., 77, 2016 (1955). perature of the DSC-1B is accurate to better than 2’. (9) A. R. Ubbelohde, “Melting and Crystal Structure,” Oxford The observed phase changes were reproducible to within University Press, London, 1965,p 230. 1”for sharp changes and agreed as well with previously (10) E. 0. Stejskal, D. E. Woessner, T. C. Farrar, and H. 8.Gutowreported values. For example, the monoclinic to sky, J. Chem. Phys., 31, 55 (1959). rhombohedral phase transition in carbon tetrachloride (11) T.R.Rubin, B. H. Levedahl, and D. M. Yost, J. Amer. Chem. Soc., 66,279 (1944). was observed a t 226.6OKl which compared quite favorably with the value of 225.6”K reported by D u n l ~ p . ~ (12) A. V. Grosse and J. M. Mavity, J. Org. Chem., 5 , 106 (1940). (13) G. E. Coates, M. L. H. Green, and K. Wade, “Organometallic Temperatures were read at the intersection of the base Compounds,” Vol. I, 3rd ed, Methuen and Co., London, 1967,p 297. The Journul of Physical Chemistry, Vol. 74,No. 18, 1970
3136
LAWRENCE SILVER AND REUBENRUDMAN
Table I: Carbon Tetrachloride
Temp, OK This work Dunlops Hicks, et al.a Kotake, et a1.8 Gray4 A H , cal/mol This work Hicks, et a1.a Gray4 AB, eu This work Hicks, et a1.a Gray4 a
---
Ib
Ib -+ I1
Phase transition I1 -+ I b
245 2
234.0
217.3
226.6
225.28
225.48
244.8
Obsdb 234.3
220.9
225.1
250.3
- 407
- 164
- 1063
-421
- 163
- 1050
1070 1095 1087
596 601 601
-1.66
-0.70
-4.89
4.72 4 859 4.83
2.41 2.401 2.40
L + 16.
I
Ia
-f
I&-+ L
247.8 250.41
244.8
Xb
I
-1.72
-0.70
-4.75
J. Hicks, J. Hooley, and C. Stephens, J. Amer. Chem. Xoc., 66, 1064 (1944).
Gold pans were used for these compounds and for carbon tetrabromide. All samples were sealed with the Perkin-Elmer volatile-sample sealer in special volatilesample pans and covers. As suggested by the manufacturer, small disks were placed inside the pans to prevent the vapors from depositing on the covers. The samples were weighed after investigation and remained constant within 0.01 mg, indicating no sample loss during the course of the DSC study. Errors. Enthalpy. Calculations of AH are based on the measured peak areas, the experimentally obtained instrumental constant, weight of the sample, and the C-12 system for molecular weights. The errors involved in each of these measurements, combined with the agreement between the experimental and literature values of AHr for Pb, indicate that AH is accurate to within 4%, with a precision of 2%, except for the weak, broad melting point transitions where an error of 8% is estimated. Temperature. The error in temperature accuracy is between 1 and 2". For temperatures in the range of 140-250°K the error is no larger than 1%. Entropy. The error in the calculated entropy is the square root of the sum of the squares of the errors in enthalpy and temperature. This is approximately 4% for the sharp transitions and 8% for the broad melting points.
Results and Discussion The five methylchloromethane compounds are now known to form a total of fifteen distinct crystallographic phases between their melting points and 113"K, at atmospheric pressure.14 Enthalpies of fusion and transition have been obtained from the DSC study for all but two of these. The results of this investigation are presented in Tables I-VI and, where possible, are compared with similar data obtained by other investigat ors. The Journal of Physical Chemistry, Vol. 74, No. 16, 1970
--r
Ib -+ L
x
-
3.86 245 9 I
419 42 1 1.71 1.71
Observed but not reported.
Table 11: Carbon Tetrabromide Phase transition---I-CII II+I
I+L
310.4
320.0 320.0 319.4
367.4
1581 1594.5
854
7--
L+I
Temp, OK This work Marshall, et a1.a Astonb A H , cal/mol This work Marshall, et al.a Astonb AS, eu This work Marshall, et a1.a Astonb
367.8
-846
-1563
366.7
880 -2.33
-5.03
4.94 4.98
2.33 2.4
a J. G. Marshall, L. A. K. Staveley, and K. R. Hart, Trans. Faraday Xoc., 52, 19 (1956). 6 J. G. Aston, "Physics and Chemistry of the Organic Solid State," Vol. I, D. FOX,M. Labes, and A. Weissberger, Ed., Interscience, New York, N. Y.,
1963, p 545.
I n an attempt to prevent confusion between this and other ~ a p e r s , ~ ,the S , l phases ~ have been labeled as in the previous studies. In order to accommodate the newly discovered phases, the high-temperature phases for n = 0, 1, and 2 have been labeled Ia and Ib, where the I a phase forms directly from the melt and, upon cooling, transforms to Ib. When the low-temperature modification (11) is warmed, it always forms I b which persists until the melting point; phase I a is never formed by warming phase Ib. When the sample is cooled, the formation of phase I b is extremely rapid. This is shown by the nearly perpendicular rise of the chart pen as the transition to phase I b occurs. This sharp rise is characteristic of the formation of phase I b and was (14) C. E. Weir, G. J. Piermarini, and 8. Block, J. Chem. Phys., 50, 2089 (1969), have detected the presence of yet another phase for carbon tetrachloride using high-pressure X-ray techniques.
3137
POLYMORPHISM O F CRYSTALLINE n/lETHYLCHLOROMETHANE COMPOUNDS
Table 111: Methyl Chloroform
-Temp, OK This work Rubin, et al.11 Crowe and Smythb A H , cd/mol This work Rubin, et al." Crowe and Smythb AS, eu This work Rubin, et al.ll Crowe and Smythb a
L -.L Ia
l a -+ I b
I b -+ I1
227
214.3
178
- 226
- 162
- 1433
...
a
a
... 50
-0.99
-0.76
-8.09
..*
... ... 0.24
1
I1 -+ I b
Ib-L
Ia-L
220.5 224.20 223.6
239.2 240.2 240.1
231.9
1227 1786 1780
451 450 450
217
5.56 7.967 7.92
1.87
0.93
...
1.85
R. W. Crowe and C. P. Smyth, J . Amer. Chem. Soc., 72, 4909 (1950).
Table IV : 2,2-Dichloropropane
a
...
206 205
Too small to be measured accurately.
Temp, "K This work Turkevich and Smythb A H , cal/mol This work van de Vloedc AS, eu This work van de Vloedc
Phase transitionI1 e I11 I11 -+ I1
Phase transition I b +I1 I I - I b
L+Ia
L +Ib
a
226.7 232.9
184.8 187.2
187.0 188.2
240.2 239.4
-544
-1373
1415 >750
537 790
-2.40
-7.43
7.57 >4
2.24 3.3
Ib-
L
Observed during X-ray diffraction investigation; see text.
A. Turkevich and C. P. Smyth, J . Amer. Chem. SOC.,62, 2468 (1940). A. van de Vloed, Bull. SOC.Chim. Belg., 48, 229 (1939).
noted on all the thermograms that were obtained in the present investigation and by Gray14even when the rate of cooling was as low as 0.62So/min. A comparison of the present data with previously reported values shows that in nearly all cases the DSCbased enthalpies are lower. This is due to the (partly) subjective choice of base line and is pronounced in those cases where the thermograms show broad, relatively small peaks. The difficulty in choosing a proper base line has been recognized as a potential source of errors4 I n spite of this, the only major discrepancy appears in the transition between phases Ib and I1 of methyl chloroform. However, in this particular instance both sets of previous i n v e s t i g a t ~ r s ~reported ~ * l ~ difficulties in the determination of the heat capacities. Carbon Tetrachloride. The data for carbon tetrachloride are found in Table I. The agreement between the present data and those of previous investigators is well within the experimental error. It should be noted that Gray4 used AHII,I~ and T I b + L as internal heat capacity and temperature standards, respectively.
The peaks corresponding to the melting of phases Ia and Ib were always broad and poorly defined. This was true regardless of the scan rate and sample size. Surprisingly, the thermograms obtained by Gray showed clearly defined, nearly identical peaks for the formation and melting of these phases. We could not duplicate this. However, it is noted that the areas under the two sets of peaks were equivalent and corresponded to equal enthalpy changes, within the experimental limits of error. Similar broad curves were obtained by us for the corresponding transitions of methylchloroform and 2,2-dichloropropane. In the original X-ray study,2 it was thought that the metastable phase Ia transformed spontaneously to the stable phase Ib, after standing for several hours. However, the present study shows that this transition becomes kinetically favorable a t a definite temperature, resulting in a sharp peak appearing on the thermogram. This was reproducible over several runs and at different cooling rates and agrees with the observations of Gray.4 Holding the sample temperature above 234°K for a long period of time did not result in the formation of phase Ib; the moment the temperature was lowered, phase Ib was formed. (A similar effect was noted for methyl chloroform.) It is most likely that during the X-ray investigation, momentary cooling of the sample resulted in the formation of phase Ib. The X-ray cooling apparatus was accurate to f3', the nominal temperature was 238"K, and occasional fluctuations in the air pressure over a period of several hours could very easily have lowered the temperature. An interesting phenomenon is that the melting point of phase Ia is approximately 4" lower than that of phase Ib. This fact has already been reported by several investigators.3)4 Carbon Tetrabromide. Carbon tetrabromide is known to crystallize in a manner similar t o that of car(15) R.W. Crowe and C. P. Smyth, J . Amer. Chem. SOC.,72, 4009 (1950). The Journal of Physical Chemistry, Vol. 74, N o . 16, 1970
3138
LAWRENCE SILVER AND REUBEN RUDMAN
Table V : &Butyl Chloride
Temp, OK This work Kushner, et aZ.a Dworkin and Guillaminb A H , cal/mol This work Kushner, et aZ.a Dworkin and Guillaminb AS, eu This work Kushner, et aZ.a Dworkin and Guillaminb
-
Phase transition I11 I11 -b I1
I1 -c I
I-L
181* 0 183.2 182.9
220.3 219.7 219.2
250.3 248-2 247.5
- 401
394 410 446.3
1281 1390 1405.1
410 480 495.0
-2.25
2.18 2.3 2.44
5.81 6.4 6.41
1.64 1.9 2.00
L-I
I 4 I1
245.2
209.8
178.5
- 414
- 1298
-1.69
-6.19
I1
a L. M. Kushner, R. W. Crowe, and C. P. Smyth, J . Amer. Chem. Soc., 7 2 , 1091 (1950). Phys., 63, 53 (1966).
Table VI: Neopentane
____L-CI
Temp, OK This work0 Aston and Messerlyb AH, cal/mol This work Aston and Messerlyb AS, eu This work Aston and Messerlyb
Phase transition-1-11 11-1
I-CL
255
134.4
139.0 140.02
253.8 256.5
-724
-623
612 615.9
774 778.2
-2.84
-4.64
4.40 4.39
3.05 3.03
a Based on calibration curve described in ref 6. b J. G. Aston and G. H. Messerly, J . Amer. Chem. Soc., 58,2354 (1936).
bon tetrachloride. The possibility that CBr4also forms a phase analogous to phase I b was investigated. The data in Table I1 show that this is not the case; only two crystalline phases are formed between 115 and 370°K. Methyl Chloroform. The present investigation revealed the existence of a previously unknown crystalline phase. I n the initial X-ray diffraction survey,2 phase I a was not discovered. The DSC thermogram clearly showed that it formed as the sample cooled and that its behavior is similar to that of carbon tetrachloride, Subsequent X-ray diffraction studies5 showe! that this phase is face-centered cubic with a = 8.39 A with 4 molecules/unit cell. This phase was not found originally due to the extensive supercooling that methyl chloroform exhibits. It will be seen from the data in Table I11 that the L + I a transition does not form until 227°K (the temperature, though not reproducible, did fall within a 3" range). This is nearly the same temperature as the I1 + I b transition observed by us on warming and reported by o t h e r ~ . ~During ~ ~ l ~ the first X-ray study, The Journal of Physical Chernietry, Vol. 74, No. 18, 1070
b
A. Dworkin and M. G,uillamin, J Chim.
when it was discovered that the temperature was approaching that of the major solid-solid transition and the liquid had not yet solidified, the tube containing the sample was touched with the tip of a cotton swab dipped in a Dry Ice-acetone slurry. This immediately initiated crystallization, but, as is now known, of phase Ib. However, on the basis of the DSC data, the X-ray sample was allowed to cool without disturbance until the material crystallized and phase I a was then identified. Although the temperature of the I a 3 I b transition is sharply reproducible, the supercooled I b -t I1 transition occurs within a 2" range. Phase I11 has been identified by heat capacity,16 dielectric constant,16 and nmr measurementslO as well as by X-ray diff raction.2 I n the present case, no indication of the formation of this phase was found during the cooling cycle. A small peak, of approximately the correct area, was found a t 205°K on warming. However, this peak was not significantly above the base line variation and was too small to be measured accurately. The melting point of phase I a is more than 7" below that of phase Ib. It is clear from the data in Table I11 that previous investigators studied phase Ib. This is consistent with the fact that they were heating the sample from below 125°K. The enthalpy of transition between phases I b and I1 determined in the present investigation does not agree with that determined on the basis of heat capacity measurements.llr16 I n the latter case, the enthalpies of transition and fusion were estimated on the basis of the running time in excess of that required by the normal heat capacity of the material in the transition or melting range.ls This in turn requires a reasonably accurate (16) L. M. Kushner, R. W. Crowe, and C. P. Smyth, J . Amer. Chem. Soc., 7 2 , 1091 (1950).
3139
POLYMORPHISM O F CRYSTALLINE AfETHYLCHLOROMETHANE COMPOUNDS
determination of the heat capacity on either side of the transition. Both sets of investigator^'^^'^ obtained similar heat capacity vs. temperature curves. However, in both cases the heat capacity between the temperature of transition and the melting point is seen to be sharply rising and not well defined. As these authors indicate, it was extremely difficult to determine C , in this region. As a result their values for AHt and AHm are only approximations. I n fact, AHm is reported as 450 f 300 cal/mol." Methyl chloroform is quite different from the other compounds in this series. A comparison of available datal' shows that in all cases except methyl chloroform the heat capacities are well defined on either side of the transition and the use of the approximation formula (eq 2, ref 16) is valid. However, in the case of methyl chloroform it does not appear to be valid. Crowe and Smyth16 emphasized this point by stating that the heat capacity values between the transition point and melting point are extremely uncertain and that it is difficult to estimate the energies involved in the two processes with accuracy. Rubin, et al.," made similar statements; e.g., "premelting effects occur to obscure the course of the true heat capacity curve." On the other hand, a difficulty in the interpretation of the thermogram lowers the accuracy of the data obtained by the DSC method. I n order to measure accurately the area under the peak a suitable base line must be drawn. I t is generally very easy to choose this parameter, since the base line on either side of the peak is clearly defined and lies along a straight line of nearly constant slope. I n the case of methyl chloroform, because of the rapidly changing heat capacity, the base line above the I1 -.+ I b transition is quite different in slope from that below the transition. A large error enters because of the ambiguity in choice of base line. However, it is important to point out that, regardless of the choice of base line, it was not possible to obtain as large a value for AHt as was previously reported.l17l6 The present value was determined on the basis of a straight line drawn from the beginning to the end of the peak. By judicious choosing of a base line we were able to obtain nearly equivalerit enthalpies of transition for I b + I1 and I1 -c Ib, which fell between the two values reported in Table 111. However, there was no justification for this choice and so these data are not reported. I n addition, there is the strong possibility that not all of the liquid froze during the cooling process until the I b + I1 transition occurred. This would account for a larger enthalpy of transition for I b -c I1 than for I1 -.+ Ib. Similar effects have been reported elsewhere for cases of severe supercooling. 18
It is of interest to note that during one cooling run a small shoulder was noted on the low-temperature side of the L 3 I a peak. I n this run the three enthalpies of transition were 245, 164, and 1369 cal/mol, respectively. It would seem that, in this case, supercooling effects were negligible. However, it was not repeated during any other scan. The conclusion drawn by the present authors is that neither the DSC nor the heat capacity value is accurate. The former method underestimates the enthalpy, while the latter overestimates it. 8,2--Dichloropropane (Table I V ) . Yon de Vloed19 determined the enthalpy values on the basis of freezing curves of binary mixtures. This method gives only approximate values and is not reliable. (For example, a value of AS, of 4.5 eu was determined for methyl chloroform.) No other reports of heat capacity or enthalpy data were found for 2,2-dichloropropanea The thermogram showing the L(?) Ib transition was identical in appearance with the formation of phases I b of carbon tetrachloride and methyl chloroform. However, a slight increase in the base line just prior to the onset of this transition was noted. A sample was set up on an X-ray diffraction camera and allowed to cool until crystallization was initiated, a t which time it was quickly warmed. X-Ray diffraction photographs showed the presence of a face-centered cubic unit cell with a = 8.45 8 and 4 molecules/unit cell.5 Evidently the formation of phase I a occurs nearly simultaneously with that of I b and involves a very small enthalpy change. t-Butyl Chloride and Neopentane. Each of these compounds exhibits only one phase (fcc) prior to the major crystallographic transition. The observed transitions are in agreement with those reported by previous investigators, as shown in Tables V and VI. Acknowledgments. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, and to the Research Corp. for partial support of this research. The authors thank R. Fyans for helpful discussions concerning the operation of the DSC-1B.
(17) Figure 1, J. Hicks, J. Hooley, and C. Stephens, J . Amer. Chem. Sot., 66, 1064 (1944) (n = 0) ; Figure 1, ref 11, and Figure 1, ref 15 (n = 1); Figure 3, ref 16 (n = 3); Figure 1, J. G. Aston and G. H. Messerly, ibid., (58, 2354 (1936) (n = 4); 2,2-dichloropropane n = 2 ) was determined by another method.
(18) E. M. Barrall, 11, R. S. Porter, and J. F. Johnson, J . Phys. Chem., 71, 1224 (1967). (19) A. van de Vloed, BUZZ. SOC.Chirn. Belg., 48,
229 (1939).
The Journal of Physical Chemistry, Vol, 74, No. 16, 1970
