748
DONALD €1. PAYNE AND E D G a R F.WESTRUM, JR.
limited wettability as those produced from fully fluorinated acids of the same total fluorocarbon content. The use of such partially fluorinated acids does, however, provide these advantages: (i) the presence of the long aliphatic chain can be expected to modify the solubility of the acid with respect to some of the more conventional hydrocarbon materials used as solvents in technical applications; (ii) the lowering of the melting point (for the more heavily fluorinated acids) also may facilitate certain types of applications; (iii) the decrease in volatility resulting from the presence of the hydrocarbon chain increases film durability and also lessens the possibility of toxicity hazards; and (iv) finally, and most important of all, the presence of the aliphatic hydrocarbon chain between the carboxyl group and the perfluorocarbon moiety decreases the acid strength of the perfluoroalkanoic acid20s21
Vol. 66
compound to that of a weak acid like the fatty acids. The presence of the intermediate aliphatic hydrocarbon chain may alter greatly the lubricating properties of a condensed monolayer adsorbed on a metal. An adsorbed monolayer offers resistance to abrasion and wear to an extent which is sensitive to intermolecular cohesion between molecules of the filn1.Ig Increasing the total chain length may or may not improve the durability of a lubricating film depending on whether the hydrocarbon portions of the chains adlineate to a certain extent or assume a quasi-liquid configuration. Film durability measurements and electron diffraction examinations are being made to elucidate further the properties of these unique monolayers. (20) T. J. Brice, E. G. Bryce, and H. M. Scholberg, Cham. Eng. I\rews, 81,510 (1953). (21) W. Sheppard and E. L. Muetterties, J. Om. Chem., Z6, 180
(1960).
HEAT CAPACITIES AND THERMODYNAMIC PROPERTIES OF GLOBULAR MOLECULES. IV. PENTAERYTHRITYL CHLORIDE, BROMIDE, AND IODIDE FROM 6 TO 300°K.1 BY DONALD H. PAYNE AND EDGAR F. WESTRUM, JR. Department of Chemistry, University of Michigan, Ann Arbor, Michigan Received November 16, 1981
The heat capacities of pentaerythrityl chloride, bromide, and iodide were determined by adiabatic calorimetry from 6 to 3OOOK. and the entropies, enthalpies, and free energy functions evaluated. The entropies at 298.15”K. are 61.54, 69.58, and 75.70 cal. mole-’ OK.-’ for the chloride, bromide, and iodide, respectively. Although the general temperature dependences of the heat capacities are of the usual sigmoid character, the chloride and the bromide both show a region of anomalously slow absorption of energy.
Introduction increments of the ammonium halides also probably Since the original suggestion by Pauling2 that are consistent with the order-disorder mechani~m.~ certain types of solid-solid transitions might be Thesp compounds are examples of relatively simple explained by the onset of rotation in the crystalline transitions because of the symmetry features of state, many investigations have been pursued in the ammonium ion. In solid-solid transitions in molecular and an attempt to determine the nature of the mech- general, these structures-both anism of such transformations. Although origin- crystallographic-are sufficiently complicated that ally rotational freedom was attributed only to rela- no adequate general correlation has been demontively simple molecules, Smyth,a through an ex- strated between the isothermal entropy increments tensive investigation of the dielectric constants of and the mechanism of transition, However, certain compounds involving transitions, extended Paul- stochastic correlations are possible.6 In transitions ing’s view to include many larger and more complex involving more complex molecules the factors to be considered apparently are more complicated, molecules. Perhaps the most extensive investigations have but the transitions are actually somewhat more concerned the transitions in the ammonium halides. susceptible to study because of the larger number Neutron diffraction investigations on the deute- of possible comparison compounds which may be rated ammonium halides4 indicate that these synthesized. At the initiation of this research a family of transitions are explained better by an orientational order-disorder type of mechanism than by a ro- compounds of the general type C(CH2X)4 (where tational one. Moreover, the transition entropy X may represent halogen or hydrogen atoms, or hydroxyl groups) was considered to possess in(1) Based upon a dissertation submitted to the H. H. R. School of herent interest. A solid-solid transition has been Graduate Studies of the University of Michigan (by D. H. P.)in partial fulfillment of the requirements for the degree ob Doctor of Philosostudied in neopentane, C(CH3)4, by Aston and phy. Messerly’ and in pentaerythritol, C(CH2OH)4 (2) L. Pauling, Phys. Rev., 36, 430 (1930). (3) C. P. Smyth, “Dielectric Behavior and Structure,” McGrawHill Book Co., New York, N. Y., 1955; J. Phgs. and Chem. Solids, 18, 40 (1961). (4) G. H. Goldsohmidt and D. G. Hurst, Phus. Rev.. 86,797 (1952): €1. A. Levy and 8. W. Peterson, abid., 86, 766 (1952).
( 5 ) C.C. Stephenson, R. W. Blue, and J. W. Stout, J . Chem. Phys., 20, 1046 (1952). (6) G. B. Guthrie and J. P. McCullough, J . Phys. and Chem. Solids, 18, 53 (1961).
749
THERMODYNAMIC PROPERTIES OF PENTAERYTHRITYL HALIDES
April, 1962
by Nitta and his eo-workers8 with the discovery of a remarkably high transition entropy increment in the latter compound. Comparison of the symmetry features of the high temperature cubic modifications of pentaerythritol with the symmetry features of neopentane suggested the halogen homologs of pentaerythrityl would be interesting compounds for investigation. Although the literature data 011 the chloride, bromide, and iodide are not extensive a,nd no transitions were reported, these three compounds nonetheless were studied to provide data for correlation with that of pentaerythrityl fluoride and pentaerythritol. This paper therefore reports the results of low temperature investigation of the heat capacities of pentaerythrityl chloride, bmmide, and iodide. Experimental Preparation of the Pentaerythrityl Halides .-The pentaerythrityl chloride used for the heat capacity measurements was prepared by the reaction C(CHzOH)( 4SOC12 +C(CH2Cl)b 4502 4HCI in the manner described by Mooradian and C l ~ k e . Fol~ lowing the addition of water to the pyridine solution to separate the product, the crude product was filtered and washed throughly with warm water. It then was recrystallized twice from 95% ethanol, the first time with Norit charcoal suspended in tihe ethanol. Following the recrystallizations the product was sublimed twice at about 5 mm. a t steam-bath temperatures. Gravimetric chloride analysis indicated 67.47% (theoretical: 67.56%); the melting point was 97”. The pentaerythrityl bromide used for the heat capacity measurements was prepared by the reation 3C( CHZOII)~ 4PBra --+3C( CHzBr), 4HsP01 according to the method outlined in “Organic Syntheses.”’o Purification of the crude product was accomplished b two successive continuous extractions with 95(% ethano1 followed by one recrystallization from 95% ethanol usin Norit charcoal, and five recrystallizations usin 95% ethano? alone. Gravimetric analysis showed 8 2 . 6 5 8 bromide by weight (theoretical: 82.43%); m.p. 163”. The pentaerythrityl iodide was prepared by the reaction C( CH2Br)* 4KI --+C( CH2I)d 4-4KBr according to the method outlined in “Organic Syntheses ,”lo For purification the sample was submitted to continuous extraction with 95% ethanol until the melting point reached 233”. The sample then was removed from the thimble of the extractor and recrystallized from benzene. The solvent was removed by high vacuum. Just before use of the sample, i t was recrystallized from benzene (to remove decomposition products produced during storage) and then kept in a darkened cold room (-10”) until the sample actually was loaded into the calorimeter. Volhard iodide determination indicated 88.39% iodide (theoretical: 88.17%). The sample melted with Borne decomposition a t 233”. The Calorimetric Apparatus.-The Mark I adiabatic calorimetric cryostat for use over the range 4 to 350°K. was an improved version of one constructed by Westrum, Ilatcher, and Qsborne,ll with the essential similarity that helium was used as the lowest temperature refrigerant
+
+
+
+
+
-+
(7) J. G. Aston and C:. H. Messerly, J. A m . Chem. SOC.,68, 2354 (1936). (8) I. Nitta and T. Watanabe, Bull. Chem. SOC.Japan, 13, 28 (1938); I. Nitta, 8. Seki, and M. Momotani, Proc. Japan Acad., 26, 25 (1950); I. Nitta, S. Seki, M. Momotani, R. Suzuki, and S. Nakagawa, zbid., 26, 11 (1950); I. Nitta, T. Watanabe, S. Seki, and M. Momotani, ibzd., 26, 19 (1950). (9) A. Mooradian and J. B. Cloke, J. Am. Chem. SOC.,67, 942 (1945). (10) H. B. Sohurink, “Organic Syntheses, Coll. Vol. 11,” John Wiley and Sons, New York, N. Y., 1943, p. 476. (11) E. E‘. Westrum, Jr., J. B. Hatcher, and D. mi. Osborne, J . Chem. Phus., 21, 419 (1953).
TEMPERATURE, IO0 200
0
‘K.
300
50
r; 40 W
-1
0
I
$
n
30
W
\
i
5
20
al V
IO
0 0
Fig. 1.-Heat
10 20 TEMPERATURE,
30 OK.
capacities of three pentaerythrityl halides.
instead of hydrogen, and that a heat exchanger (called the “economizer”) was provided to utilize the enthalpy of the effluent helium gas. The gold-plated copper calorimeter (laboratory designation W-6) waa 3.8 cm. in diameter and 7.7 cm. long. The thickness of the shell was 0.4 mm. Eight vanes of 0.1mm. copper foil aided in establishing thermal equilibrium. An entrant gold-plated copper heater well contained a platinum capsule-type resistance thermometer (laboratory designation A-3, calibrated by the National Bureau of Standards) within a cylindrical copper heater sleeve carrying 160 ohms of B. and S. No. 40-gage Advance (Constantan) Fiberglaa insulated wire, which was wound bifilarly in double threaded grooves and cemented in place with Formvar enamel. Calorimeter W-6 differed from typical calorimeters in that a Monel neck of 15 mm. dia. partially isolated the actual cover from the copper calorimeter, and thus allowed the cover to be soldered in place with Cerroseal solder without appreciably heating the calorimeter or the samples which were placed therein. The heat capacity of the calorimeter-heater-thermometer assembl was determined in a separate set of experiments. 8orrections were applied for the small differences in the amount of gaseous helium used in the loaded and empty calorimeter to facilitate thermal contact between sample and calorimeter. After a sample waa loaded into the calorimeter, the calorimeter was sealed and the ambient air evacuated by placing the calorimeter within a bulb attached to a high vacuum line. An atmosphere of helium gas at room temperature then was introduced through a small hole which subsequently was soldered shut. The contribution of the calorimeter-heater-thermometer assembly to the heat capacity w a ~less than 10% of the total at 1O0K., and increased gradually to 33% a t 80°K. and remained approximately constant at higher temperatures; in the case of the bromide and the iodide i t was less than 10% a t 10’K. and increased to about 55% at 100°K. and higher temperatures. A weighed quantity of Lubriseal stopcock grease was used to obtain thermal contact between the heater, thermometer, and calorimeter; the amount of Cerroseal solder employed to seal the calorimeter was adjusted carefully by weight to equal the quantity used on the empty calorimeter. The masses of the samples (in
DOXALD H. PAYXE AND EDGAR .!I WESTRUM,JR.
VOl. 66
TABLE I1 THERMODYNAMIC PROPERTIES OF PENTAERYTHRITYL HALIDES T , OK.
10 15 20 25 30 35 40 45 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300 273.15 298.15
31.92.159.37 32.84 168.36 33.79 177.57 34.74 36.60 205.19 37.58 214.38 38.50 223.51 39.47 41.44 250.80 42.41 260.03 43.44 269.34 44.31 48.44 Series 111. 255.18 42.91 264.16 44.18 47.77 284.41 48.69 288.99 49.53 293.63 50.12 214.84 38.48 51.83 307.24 52.86 Series IV. 40.56 242.56 41.50 249.99 42.30 255.96 42.96 44.73 272.31 45.97 277.59 47.12 282.66 48.22 50.29 297.80 51.00 1302.75 51.86 1307.65 52.741
186.78 232.55 278.54 273.16 298.22 224.47 261.39 287.71
35.68 40.44 45.93 45.87 51.12 39.55 43.72 49.32
195.99 241.64 287.62 279.90 302.76 233.96 266.84 292.78
10 15 20 25 30 35 40
45 50 60 70 80 90 100
110 120 130 140 150 1GO
CP
so
Pentaerythrityl chloride 1.09 0.84 2.85 1.59 4.91 2.69 4.01 7.01 8.92 5.46 10.62 6.97 12.12 8.49 13.46 9.99 14.67 11.47 16.85 14.35 18.76 17.09 19.71 20.52 22.11 22.22 24.63 23.60 24.99 26.94 26,29 29.17 27.55 31.33 33.41 28.75 29.92 35.44 31.04 37.40 32.14 39.32 33.24 41.19 34.34 43.01 44.81 35.51 46.57 37.05 39.33 48.35 41.54 50.15 51.94 42.01 42.45 53.66 55.34 43.34 56.99 44.35 45.43 58.63 46.54 60.24 61.84 47.64 57.51 44.68 61.54 47.44
HO
- HOo
- ( F o -T H o d /
[C(CH~CI)I] 2.72 0.57 0.77 12.32 1.11 31.70 1.55 61.54 2.08 101.4 2.67 150.4 3.31 207.3 3.96 271.3 4.64 341.6 6.03 499.5 7.41 677,6 8.78 874.1 10.14 1087.4 11.47 1316 12.77 1559 14.04 1816 15.29 2085 16.51 2366 17.71 2660 18,87 2965 20.02 3280 21.15 3607 22.25 3945 23.34 4294 24.39 4657 25 45 5038 26.48 5443 27.51 5864 28.52 6285 29.52 6714 30.50 7152 31.48 7601 32.44 8061 33.40 8532 30.81 7293 33.22 8444
Pentaerythrityl bromide [C(CHsBr)e] 5.18 0.69 2 07 22.81 2.08 5.01 55.23 3.92 7.91 101.2 5.97 10.42 158.6 8.05 12.46 225.2 10 10 14.15 299.6 12.09 35.56 380.5 13.99 16.79 467.3 15.82 17.92 656.8 19.27 19.94 865.3 22.48 21.72 1091 25.49 23.34 1332 28.32 24.81 1586 31.01 26.12 1854 33.56 27.36 2133 35.99 28.56 2425 38.32 29.71 2727 40.56 30.78 3040 42.72 31.84 3864 44.81 32.92 r
I
0.17 0.56 1.16 1.92 2.76 3.66 4.60 5.53 6.47 8.32 10.12 11.86 13.52 15.15 16.71 18.21 19.67 21 .OB 22.45 23.78
April, 1962 170 180 190 200 210 220 230 240 250 260 270 280 290 300 273.15 298.15
THERMODYNAMIC P R O P E R T I E S O F PENTdERYTHRXTYL
33.96 34.99 36.01 37.02 38.04 39.08 40.16 41.24 42.32 43,50 45.39 47.74 49.70 51.40 46.10 51.10
46.84 48.81 50.73 52.60 54.43 56.22 57.99 59.72 61.42 63.11 64.78 66.47 68.18 69.90 65.31 69.58
Pentaerythrityl iodide 10 2.’91 0.74 2.62 15 6.61 20 4.99 9.90 25 12.(52 7.50 30 14.61 9.97 35 16.29 12.35 40 17.69 14.62 45 18.139 16.78 50 20.00 18.82 60 21.96 22.65 70 23.67 26.17 80 25.20 29.43 90 26.60 32.48 100 27.90 35.35 110 29.1.2 38.07 120 30.28 40.65 130 31.41 43.12 140 32.51 45.49 150 33.GO 47.77 34.69 49.97 I60 170 35.77 52.11 180 36.84 54.18 190 37.E19 56.20 200 38.94 58.17 210 39.99 60.10 220 41.06 61.98 230 42.16 63.83 240 43.26 65.65 250 44.34 67.44 260 45.42 69.20 46.51 70.93 270 280 47.60 72 64 290 48.71 74.33 49.86 76.00 300 273.15 46 88 71.48 298.15 49.64 75.70 I
3699 4043 4398 4764 5139 5524 5921 6328 6745 7174 7618 8084 8572 9077 7763 8983 [C(ICH21)4] 7.26 31.07 72.63 128.9 297.0 274.3 359.4 450.9 548.2 758.2 986.6 1231 1490 1763 2048 2845 2654 2973 3304 3645 3998 4361 4734 5118 5513 5918 6334 6762 7200 7648 8108 8578 9060 9553 8255 9461
25.08 26.35 27.58 28.78 29.96 31.11 32.25 33.35 34.44 35.52 36.56 37.60 38.62 39.64 36.89 39.45 0.01 0.55 I .36 2.34 3.41 4.51 5.63 6.76 7.86 10.01 12.08 14.04 15.92 17.72 19.45 21.11 22.71 24.25 25.75 27.19 28.60 29.95 31.28 32.58 33.85 35.08 36.29 37.48 38.64 39.78 40.90 42,OO 43.09 44.16 41.26 43.97
observed in the chloride sample between 220 and
240°K. and in the bromide sample between ap-
HALIDES
751
proximately 260 and 290°K. Approximately 2 hr. mere required for the bromide to come to thermal equilibrium in the anomalous region, and about 4 hr. for the chloride. The bromide curve appeared to be more reproducible than the chloride one. Large variations in the rate of cooling these samples seemed to produce no effect on the rapidity of reaching equilibrium or upon the heat capacity itself in these regions. It is to be noted that failure to await complete thermal equilibrium may affect the heat capacity curve slightly, but it does not appear to affect total energy under the curve and will have only an extremely small effect on the entropy value. On the last series of measurements on the pentaerythrityl chloride the runs were made in 28 hr. of continuous operation, and the points obtained from these runs were used to determine the heat capacity curve. The entropy and enthalpy function were obtained by numerical quadrature of large scale plots of C, VS. log T. These functions, together with the smoothed values of the heat capacity read from these curves, are presented in Table I1 a t selected temperatures. Extrapolations below 6°K. were made by the Debye T-cubed approximations. The free energy function was obtained by difference. The thermodynamic functions are considered to have a probable error of less than 0.1% above 100°K. Suclear spin and isotope mixing contributions have not been included in the entropy and the free energy functions; hence, the values given are suitable for chemical thermodynamic purposes. As has been noted previously, the region over which slow equilibrium was obtained does not contribute significant uncertainty to the thermodynamic functions. Although these three halides have not revealed a “plastically crystalline” phase as originally designated by Timmermans,l2 there is still a possibility that they will undergo transition to this phase before melting. It is proposed to study the higher temperature thermal behavior to ascertain whether or not this is so and to study, where possible, the eiitropy of fusion to aid in the interpretation of the very interesting closely related molecules, pentaerythrityl fluoride and pentaerythritol. Acknowledgment.-The authors acknowledge with gratitude the partial support of the Division of Research of the U. 5. Atomic Energy Commission in this investigation. The cooperation of Dr. D. W. Osborne and the Argonne National Laboratory in making liquid helium available for these studies is appreciated. (12) J. Timmermans, Bull. soc. chim. Belg., 44, 17 (1835); I d chim. belge, 16, 178 (1951).
