July, 1959
THERMOCHEMISTRY AND VAPORPRESSURE OF ALIPHATICFLUOROCARBONS1133
THERMOCHEMISTRY AND VAPOR PRESSURE OF ALIPHATIC FLUOROCARBONS. ,4 COMPARISON OF THE C-F AND C-H THERMOCHEMICAL BOND ENERGIES1 BY W. D. GOOD,D. R. DOUSLIN, D. W. SCOTT, ANN GEORGE, J. L. LACINA, J. P. DAWSON AND GUYWADDINGTON Contribution No. 78 from the Thermodynamics Laboratory, Petroleum Experiment Station, Bureau of Mines, U . S. Department of the Interior, Bartlesville, Oklahoma Received December 8 , 1968
The heat of formation of four organic fluorine com ounds, including three completely fluorinated aliphatic compounds, was determined by a rotating-bomb method of coml!ustion calorimetry. An earlier technique that used sealed, fusedquartz ampoules for containing samples of volatile fluorine compounds was replaced by an improved technique that used sealed bags of a polyester film. For three of the compounds vapor pressure measurements were made by an improved method of comparative ebulliometry. The following values, in kcal. mole-”, are reported for the standard heat of formation, AHfoz9s.1a, of these compounds from graphite and gaseous fluorine and hydrogen: m-fluorobenzotrifluoride(g), - 184.17; perfluoromethylcyclohexane(g), -675.3; perfluoroethylcyclohexane( g), -769.7; and perfluoro-n-heptane( g), - 789.0. These values were used with other thermochemical data to obtain a direct comparison of the C-F and C-H thermochemical bond energies in aliphatic compounds. combustion products would lead to large and uncertain thermochemical corrections (even if it were possible to make satisfactory thin-walled ampoules with t,he 3-ml. capacity required for fluorocarbons). As originally disclosed a t the Calorimetry Conference of 1956, the bags of polyester film were devised to avoid the aforementioned difficulties and are expected to have numerous applications in combustion calorimetry. Therefore, their preparation and use will be . . described in detail. The polyester film used to make the bags has a low permeability for organic compounds, including highly fluorinated materials. Compounds of widely differing properties, such as fluorocarbons, m-fluorobenzotrifluoride, benzene, toluene, commercial isohexanes, tetramethyllead and water, were sealed successfully in these bags and held for long intervals without evidence of swelling of the film or of loss of sample by permeation through the film. Figure 1 illustrates preparation of a sample sealed in a polyester bag. The bag is made from a flat piece of film cut as shown in “a.” This flat piece is folded and heatsealed to make the empty bag shown in “b.” The heat sealing is done with a small soldering iron held near, but never touching, the edges of the piece, which is held (with edges extending approximately ‘/16 inch) between metal templates. Such a bag may be made without detectable change in weight of the film. The material that the bag is to contain is stored in a small, sealed glass vial and, immediately after the vial is opened, is transferred to the bag with a hypodermic syringe. Air is removed from the bag by applying slight pressure on the walls, and the neck of the bag is sealed with a special The basic procedures used in this investigation for com- clamp. The gas space is reduced as much as possible to bustion calorimetry2 and comparative ebulliometry3 have minimize errors in reduction of weights to in vacuo and been described. Only developments made in this investiga- errors caused by solution of gases in the liquid. Liquid is tion and procedures peculiar to it will be discussed here. removed from the neck (outside the clamp) by swabbing it with filter paper. After the neck is thoroughly dry, the Combustion Calorimetry tip is held with tweezers and sealed as in “c.” The maw Sample Containers Prepared from Polyester Film.of sample is determined by weighing the empty bag and Bags of polyester film were employed to introduce samples then the ba and its contents, after filling and sealing. Beof volatile organic fluorine compounds into the com- cause the fiym is prone to become electrostatically charged bustion bomb. The selection of containers for volatile during these operations, it is discharged before each weighSam les has been a particularly troublesome problem in the ing. As the film is slightly hygroscopic, the bag is allowed t o comtustion calorimetry of fluorine compounds. Pre- equilibrate with the atmospheric humidity prevailing in the viously, fused-quartz ampoules were used for compounds of balance case. After the final weighing the filled bag is relatively low fluorine contenta2 For compounds of high rolled and placed inside the platinum crucible used in the fluorine content, like those studied in this investigation, the combustion experiment. All of these operations of Sam le reaction of fused quartz with the hydrofluoric acid in the preparation are performed with the hands covered by ttin cotton gloves to protect the film from contamination. Use of the polyester sample containers simplified the (1) This research was supported b y the United States Air Force comparison experiments2 because it eliminated the “twin through the Air Force Office of Scientific Research of the Air Research and Development Command under Contract No. CSO-880-57-4. ampoules,” as well as the need for H2SiF6in the initial bomb solution. The reduction of the calorimetric data to standReproduction in whole or in part is permitted for any purpose of the ard states2 also was simplified because the bomb solutions United States Government. were not complicated by the presence of H2SiF6. (2) W. D. Good, D. W. Scott and G u y Waddington, THISJOURNAL, Polyester bags were compared with fused-quartz ampoules 60, 1080 (1956). as sample containers in experiments with m-fluorobenzo(3) G.* Waddington, J. W. Knowlton, D. W. Scott, G . D. Oliver, trifluoride. This compound, empirical formula C,H4F4,has S. S. Todd, W. N. Hubbard, J, C. Smith and H, M . Huffman, J. A m . as many fluorine atoms as hydrogen atoms in the molecule. Chem. Soc., 71, 797 (1949).
Systematic investigations of the thermodynamic properties of organic fluorine compounds are in progress in this Laboratory. One development of this program has been a precise method for determining the heat of combustion of fluorine-containing substances.2 This paper reports a modification of previous techniques to allow combustion calorimetry of completely fluorinated compounds, such as fluorocarbons. Values of heat of combustion and formation were determined for m-fluorobenzotrifluoride (m,cY,oc,a-tetrafluorotoluene),perfluoromethylcyclohexane [undecafluoro-(trifluoromethyl)-cyclohexane] , perfluoroethylcyclohexane [undecafluoro-(pentafluoroethy1)-cyclohexane] and perfluoro-n-heptane (n-hexadecafluoroheptane). Measurements of the vapor pressure of all except perfluoro-n-heptane were made by an improved method of comparative ebulliometry, and values of heat of vaporization were computed from the results. The values of heat of formation for the three fluorocarbons were used with other thermochemical data to obtain a direct comparison of the C-H and C-F thermochemical bond energies. Experimental
1134
GOODUOUSLIN,
SCOTT, GEORGE, LACINA,
DAWSON AND
Vol. G3
WADDINGTON
r
CFd varied from 70 to 79 for perfluoromethylcyclohexane, 73 to 80% for perfluoroet ylcyclohexane, and 62 to 64% for perfluoro-n-heptane. The proportion of fluorine that appeared as CF4 was a function of the amount of hydrogen available in the reactants in auxiliary oil and/or polyester film. Loss in weight of the platinum crucibles, up to 2 mg. per experiment, was observed. The loss probably was caused by formation of volatile latinum fluorides that later reacted with water to form hy&ous oxides. The thermochemical effect of such a reaction was assumed to be negligible. For instance, if the net reaction was Pt(c) 1/202(g) H20(liq) Pt(OH),(c) (I) then AH'288.16 = 18.9 kcal.,Sand reaction 2 mg. of platinum would evolve only 0.2 cal., or less than 0.003% of the total energy of the bomb process. Comparative Ebulliometry The comparative ebulliometer system described in Ref. 3 has been modified by addition of a third ebulliometer to the two for containing sample and water. The third ebulliometer contains benzene 88; a secondary reference substance and is used for comparative measurements in the range 70-145 mm. In this low pressure range, water does not boil steadily enough for precise results. Units of Measurement and Auxiliary Quantities All data re orted are based on the 1951 International Atomic Weigttsea and fundamental constantsab and the definitions: 0°C. = 273.16'K.; 1 cal. = 4.1840 abs. j . Measurements of temperature were made with platinum resistance thermometers calibrated in terms of the International Temperature Scale All electrical and mass measurements were referred to standard devices calibrated a t the National Bureau of Standards. For use in reducing weights in air to in vacuo, in correcting the energy of the actual bomb process to the isothermal bomb process and in correcting to standard states, the values in Table I were used for density p, specific heat cp, and (bE/ ~P)T of the various substances. These values were obtamed from the literature or, in parentheses, estimated by analogy with structurally similar substances.
+
a
Fig. 1.-Preparation
b
C
of polyester sample containers.
I t was selected for the comparison as being intermediate in its properties between hydrocarbons and fluorocarbons. As shown in a later section on Calorimetric Results (Table 111). concordant results were obtained with both sample containers, but improved precision was obtained with the polyester bags. Calorimetric Procedures.-The calorimeter (laboratory designation BMR-2) and bomb (laboratory designation Pt-5) have been described.a Bomb Pt-5, internal volume 0.353 l., was used in all combustion experiments with mfluorobenzotrifluoride, perfluoromethylcyclohexane and perfluoro-n-heptane. Bomb Pt-5 failed during the experiments with perfluoroethylcyclohexane and was replaced by a similar bomb, Pt-ab, internal volume, 0.349 1. The ignition circuit differed from that used previously. A short piece of platinum wire, 0.002 inch diameter, was used to ignite a fuse of cotton thread. The energy evolved when the wire was connected momentarily across a 20-volt transformer was sufficient to fuse the wire. For the combustion experiments with m-fluorobenzotrifluoride, the electrical ignition energy was assumed to be small and constant and was included in the energy equivalent of the calorimetric system. For the combustion experiments with the fluorocarbons, an electronic device was used to measure directly the input of electrical energy. The circuit was patterned after that described by Pilcher and Sutton.4 Observation of significant variations in values of electrical ignition energy suggests that the accuracy was improved by the direct measurement of electrical ignition energy. The experimental procedures were essentially those described for combustion experiments with Teflon.2 Samples of the discharge gases, freed from HF and sometimes reduced in oxygen content by scrubbing with sodium pyrogallate, were retained after representative combustion experiments for mass spectrometer scanning. No fluorinecontaining gases other than CFd were detected. The bomb gas was tested occasionally for carbon monoxide, but none was detected. For the combustion experiments with m-fluorobenzotrifluoride, perfluoromethylcyclohexane and perfluoro-n-heptana, the bomb was charged with oxygen to an initial pressure of 30 atm. An initial oxygen pressure of 40 atm. was necessary in the experiments with perfluoroethylcyclohexane to prevent the formation of a "sooty" deposit in the bomb. Brown resinous deposits remained on the crucible in preliminary ex eriments with perfluoroethylcyclohexanone and perfluoro-n-teptane. However, the formation of resinous deposits in experiments with the latter compound was revented in most experiments by using enough hydrocarfon oil as promoter to produce about 30% of the total energy of the bomb process. Formation of trace amounts (never more than 0.5 mg.) could not be entirely eliminated in the experiments with perfluoroethylcyclohexane. Thermochemical corrections for incomplete combustion were applied on the assumption that the heat of combustion of the deposit was 5 f 5 cal. mg.-l. Under the conditions of the combustion experiments, the fluorine in m-fluorobenzotrifluoride was converted quantitatively to HF. I n the experiments with the fluorocarbons, more than half of the fluorine appeared in the combustion products as CF,. The per cent. of fluorine appearing as 14) G. Pilcher and L. E. Button, 23 (1955).
Trans. Roy. SOC.(London),A148,
-
+
=i
TABLE I AUXILIARY QUANTITIES AT 25' g. ml.-1
Polyester film Perfluoromethylcyclohexane Perfluoroethylcyclohexane Perfluoro-n-heptane m-Fluorobenzotrifluoride
1.38 1,788 1.826 1.718 1.289
g.-l
g. -1
0.315 ( ,235) ( .242) ,258 ( .289)
-0.00069 - ,0066
-
.0062
-
.0069 ,0066
-
Materials m-Fluorobenzotrifluoride .-m-Fluorobenzotrifluoride, which was supplied by the Illinois State Geological Survey Division through the courtesy of Dr. G. C. Finger, was distilled in an efficient fractionating column by c. J. Thompson and H. J. Coleman of the Chemistry and Relining Branch of this Station. The sam le was a composite of selected fractions judged to be of higtest purity on the basis of refractive index determinations and had a purity of 99.91 f 0.05 mole % ' as determined by the time-temperature freezing point method. The material was dried by passing the vapor over freshly activated Molecular Sieve. Perfluoromethylcyclohexane.-Crude perfluoromethylcyclohexane was supplied by the Minnesota Mining and Manufacturing Company through the courtesy of Dr. T. J. Brice. This material was first treated with a neutral permanganate solution to remove olefinic impurities by oxidation. The olefin-free material was purified by the Chemistry and Refining Branch of this Station by physical means. (5) F. D . Rossini, D. D. Wagman, W. H. Evans, 9. Levine and I. Jaffe, "Selected Values of Chemical Thermodynamic Properties,' N.B.S.Circular 600, tQ.52. (6) (a) E. Wiohers, J. Am. Chem. SOC.,74, 2447 (1953): (b) F. D Rossini, F. T. Gucker, Jr.. H. L. Johnston, L. Pauling add G . W. Vinal, ibid.,.I4,2699 (1952): (c) H. F. Stimson. J. Rsaearch Nall. Bur.
Standards,42, 209 (1949).
July, 1959
THERMOCHEMISTRS AND VAPORPRESSURE OF ALIPHATIC FLUOROCARBONS 1135 TABLE I1 SUMMARY OF TYPICAL CALORIMETRIC EXPERIMENTS m-Fluorobenzotrifluoride (fuacd quartz)
m(benzoic acid), g. m(auxi1iary oil), g. m(succinic acid), g. m( polyester), g. nI(H20),mole n‘(HF), mole n’(H2SiFe),mole At.,, deg. mAEcO/M(benzoic acid), cal. mAEco/M(oil), cal. m4Eco/M(succinic acid), cal. mAEc’/M(polyester), cal. mAEc”/M( fuse) , cal. -4Ef dec. (“Os), cal. - A E , cor. to st. states, cal. - AEign., cal. &(cont.)(At,), cal.
Perfluoromethylcyclohexane
Comparison experiments 1.05125 0.98471 1 .22632 0.01728 ,56136 0,25143 .4915 .5241 ,5255 ,04938 .03582 .03558 ,000741 1,97429 1.97936 2.00383 - 6216.4 -7741.7 -6636.5 -189.8 - 1695.2 - 1389.7 -5.6 -5.9 -6.3 -0.4 -0.5 -0.4 -12.0 -11.5 -11.0
Perfluoroethylcyclohexane
1,15101
0.52154
0.21750
.4924 ,04926
.5018 .06789
1,97923 - 6354.1
1.97789 -7266.2
- 1674.9
-656.8
-6.2 -1.0
26.9
27.2
24.8
-7921,9 4002.3
-8017.2 4001.0
- 7905.8
- 7923.7
4004,4
4003.4
Combustion experiments 1.64081 1.57512 5.75873 0,01807 0.12062(40) 0.10623(51) 0.5523 0.5523 0.5523 1.97217 2.00359 1,96523 -7893.1 -8016.3 -7869.5 -27.7 -28.1 -28.3 0.6 11.5 12.1 21.3 0.5 0.4 0.3 27.2 6.0 6.3 5.2 198.5 659.3 580.4 0.4 ~~
Perfl tioro-nheptane
1.00652
-5.3 -1.4 -13.6 -0.8 26.4
-0.9
Espp(calor.)(-At,), cal. &.,,(calor.), cal. deg.-l m(compound), g. m( auxiliary oil), g. m(polyester), g. (at % ’ rel. hum.) n‘(H20), moles Atol deg. Eapp(calor.)(-At,), cal. E(cont.)( -Atc), cal. AEigo., cal. A E , cor. to st. states, cal. AE‘dec (“Os), cal. AEfhydr(HISiFG), cal. -mAE,O/M(fuse), cal. -mAEc’/M(oil) , cal. -?nAEco/M(polyester), cal. Cor. to s t . CF4conversion, cal.” Cor. for incomplete combustion, cal.
m-Fluorobenzotrifluoride (polyester)
5.63880 0.12842(32) 0.5523 1.96897 - 7882,6 -29.9 0.7 24.8 0.1 4.7 702,2 72.8 -2.4
-11.7 -0.8 25.3 -7917 4 4003.0 4.51368 0.22478 O,OO992(27) 0.5523 1.97488 - 7905.4 -28.0 0.7 17.1 0.5 6.6 2468.9 546.5 -20.0
~-
-7109.6 -4913.1 m4Eco/M(compound), cal. -7677. 1 -7366.3 -7289.6 -1260 8 AEcO/M(compound) , cal. g. - 1265.8 - 1088,5 -4678.8 -4676.7 Correction to the % of fluorine appearing as CF4in the combustion products corresponding to eq. IV, V and VI based on t,he value of the heat of hydrolysis of CFI determined in this Laboratorya2 (I
The method (distillation in an efficient fractionating column followed by silica gel percolation of selected fractions of distilled material) was that developed by Blumkin, Orrick and Gibson,’ but the limited amount of starting material did not permit purificatibn to the degree achieved by those workers. Two samples were used for experimental studies-a “best” sample, hereinafter designated as “A” (n% 1.28091)and a ~ shown. by second sample, designated as “B” ( n 2 0 1.2808~), infrared spectra to be significantly less pure. No quantitative value was obtained for the purity of either sample, but freezing curves, gas-liquid chromatograms and infrared spectra all indicated that the purity of even the “best” sample was only of the order of 98 mole %. The samples were dried by passing the vapor over freshly activated Molecular Sieve. Perfluoroethylcyc1ohexane.-Crude perfluoroethylcyclo(7) S . Bliirnkin, N. C. Orrick and J. D. Gibson, “Purification and Some Physical Properties of Undecafluoro-(trifluoromethy1)-cyclohexane.” Report No. K-(123. Carbide and Carbon Chemicals Division. Union Carbide and Carbon Cornoration, K-25 Laboratory Division. June 30, 1950.
hexane was obtained from the same source and was purified by the same methods as perfluoromethylcyclohexane. Two samples were used for experimental studies-a “best” sample, hereinafter designated “A” (n% 1.2884& and a less pure sample, designated “B,” which was a composite of fractions having refractive indices n 2 0 ~ between 1.28835 and 1.22858. Gas-liquid partition chromatograms yielded purity values of 96+ and 93+ mole yo for samples “A” and “B,” respectively. The samples were dried by passing the vapor over freshly activated Molecular Sieve. Perfluoro-n-heptane.-The perfluoro-n-heptane used in this work was furnished by Carbide and Carbon Chemicals Company, under contract to the Atomic Energy Commission, through the courtesy of Karl E. Rapp. Two samples of significantly different but not definitely known purity, hereinafter designated as samples “A” and “B,” were used. Timetemperature freezing point determinations of purity gave values of 99.8 and 99.4 mole % for samples “A” and “B,” respectively, if the value used for the freezing point for zero impurity Tr”was calculated from the experimental curves. However, this value of Tt” differed significantly from the literature value of the tripla point temperature.8
GOOD,DOUSLIN, SCOTT,GEORGE,LACINA,DAWSON AND WADDINGTON
1136
These observations suggest that formation of solid solutions may have invalidated the freezing-point method of purity determination. Sample "A" showed no impurity band between 200 and 240 mp in the ultraviolet spectrum,g and sample "B" had been treated chemically to remove igpurities responsible for that band. The samples were dried with P206 and by passing the vapor over freshly activated Molecular Sieve. Benzoic Acid, Auxiliary Oil and Succinic Acid.-The benzoic acid was National Bureau of Standards Sample 39g. The auxiliary oil and succinic acid were those described in ref. 2 . More recent determinations of AEc"/M gave the values (with uncertainty interval): auxiliary oil, - 10983.7 f 0.6 cal. g.-l; succinic acid, -3019.8 f 0.4 cal. g.-l. Polyester Film.-The polyester film, 100 gauge Mylar Type A, was purchased from the Film Department of E. I. du Pont de Nemours & Company, Inc. This polyester film, empirical formula C10HS04,is the condensation product of ethylene glycol and terephthalic acid. The amount of carbon dioxide obtained by combustion of the film was about 99.6% of that expected from the idealized formula. Less than 100% is reasonable because the film contains hydroxyl end groups, ash and, perhaps, trapped water. The film is very slightly hygroscopic. The amount of water it absorbs is approximately a linear function of the relative humidity, and the total amount absorbed in the change from 0 to 100% relative humidity is 0.462% of the weight of the particular sample of film used. The heat of combustion was determined for samples equilibrated with air of known relative humidity. The following equation was used to represent AEc"/M of the film as a function of relative humidity AEc"/M = -5476.1
- 0.2524 (relative humidity expressed in ye)cal. g.-l (11)
Cotton Thread.-The sample of unmercerized cotton thread used as fuse material had the empirical formula CH1.7740O.ss7. For AEco/M of the cotton thread, the value -4050 cal. g.-l was determined.
Calorimetric Results Calorimetric experiments selected as typical of each of the several series are summarized in Table 11. The symbols of this table are the same as those of Ref. 2. m-Fluorobenzo trifluoride.-Five pairs of satisfactory combustion and comparison experiments were obtained by using fused-quartz ampoules as sample containers. Another five pairs of satisfactory experiments were obtained by using the new polyester bag technique. The values obtained for AEcO/M, summarized in Table 111, refer to the reaction
+
C~HaFdliq) 70z(g)
+7coz(g) 60HzO(liq) = + 4HF.15Hz0(liq)
+
+
+
+
+
+
+
(8) G . D. Oliver and J. W. Grisard, J. Am. Ckcm. SOC.,73, 1688 195 1). (9) D. Grafstein, Anal. Chem., 26,523 (1954): D. N. Glew and L. W. Reeves. THISJOURNAL, 60, 615 (1956).
+
+
+
+
TABLEI11 m-FLUOROBENZOTRIFLUORIDE: COMPARISON O F METHODS A E c o / M , eal. g.-1 A R c o / M , cal. & - I (fused-quartz ampoules) (polyester bags)
-4675.3 -4681.4 -4677.7 -4678.2 -4678.8 Mean -4678.3 Std. dev. of mean f1 . 0
-4677.1
- 4578.2 -4676.7 -4678.6 -4676.5 - 4677.4 3t0.4
The foregoing equations correspond closely to the stoichiometry of the actual bomb process. The "A" samples of the fluorocarbons were of the highest purity that was practical to achieve. However, the purity fell far short of that usually desired for precision combustion calorimetry. To test for any effect of impurity, combustion calorimetry also was done on the "B" samples, which were of significantly lower purity. If the impurity had been material with a significantly different heat of combustion, different values of heat of combustion should have been observed for the samples of different purity. Actually, for none of the fluorocarbons was there any significant difference in the TABLE IV COMPARISON OF VALUESOF AEc"/M FOR FLUOROCARBON SAMPLESOF DIFFERENT PURITY, I N CAL. G.-l AT 25"
"A" Sample
Perfluoromethylcyclohexane -1265.3 -1265.5 1265.4 1265.8 -1265.1
Perfluoroethylcyclohexane - 1260.8 -1261.6 1259.3 -1259.4 -1260.0
-1205.4 f 0 . 1
-1260.2 5 0 . 4 - 1 0 8 8 . 4
-1266.0 -1266.8 -1266.2 1265.2 1264.5
- 1259.9
-
Av. value and stand. dev. of mean
(111)
Perfluoromethylcyclohexane C7F14(liq) 3.50z(g) 31.5H20(Iiq) = 3HF.10HzO(liq) 2.75CF4(g) 4.25coz(g) (IV)
+
Perfluoroethylcyclohexane : CsFls(liq) 4oz(g) 31.5H20(liq) = 3HF.10Hz0(liq) 3.25CF4(g) 4.75C02(g) (V) Perfluoro-n-heptane : C7Fdliq) 30&) 63H,O(liq) = 6HF.10HzO(liq) 2.5CF4(g) 4.5coz(g) (VI)
"B" Sample
I n the series of experiments with fused quartz ampoules, the corrections for chemical reaction of silica were large, 17 to 29 cal. per experiment. Uncertainties in these large quantities probably account for the poorer precision in that series. The appropriately weighted average value of AEco/ M from both series is -4677.6 f 0.4 cal. g.-l. Fluorocarbons.-The results for all combustion experiments with the fluorocarbons are listed in Table IV. The values of AEco/M refer to the equations
Vol. 63
Av. value and stand. dev. of mean Av. \ d u e and stand. dev. of mean for all expt.
I
-
Perfluoro-71heptane
- 1089.1 -1087.5 - 1088.2 -1088.8 -1088.6
j=0 . 3
-1090.1 -1093.4 -1083.9 1086,8 1086.9 -1088.8
- 1259.8 -1260.5
-
-1259,2 -1258.5
-11205.5 f 0 . 3 -1259.6 f 0 . 3 -1088.3 h 1 . 3
-12115.5 zt 0 . 2 -1259.9
& 0.3
-1088.4 *0.7
observed values of heat of combustion for the two samples. This observation suggests that the impurity in the samples had a relatively small effect on the heat of combustion. Such would be the case if the impurities were isomeric or, a t least, were other fluorocarbons. Ebulliometric Results Vapor Pressure.-The vapor pressures of mfluorobenzotriffuoride, perfluoromethylcyclohex-
July, 1959
THERMOCHEMISTRY AND VAPORPRESSURE OF ALIPHATIC FLUOROCARBONS 1137
ane and perfluoroethylcyclohexane were determined by comparative ebulliometry. The “best” or “A” samples of perfluoromethylcyclohexane and perfluoroethylcyclohexane were used. The differences in the boiling and condensation temperatures of the samples a t 1 atm. pressure were: m-fluorobenzotrifluoride, 0.001 O ; perfluoromethylcyclohexane, 0.004O; perfluoroethylcyclohexane, 0.005’. These small differences show that whatever impurity was in the samples had nearly the same volatility as the major component and, therefore, had little effect on the observed vapor pressure. The results are listed in Table V. The Antoinel” and Coxll equations selected to represent these results are nt-Fluorobeneotrifluoride : logp = 7.00795 - 1305.692/(t 215.766) log (p/760) = A(1 - 373.759/2‘)
+
where
(VII) (VIII)
+
log A = 0.864641 - 7.31027 X 10-4T 6.9136-1 X 10-7T2 Perfluoromethylcyclohexane : log p = 6.82184 - 1132.493/(1 211.066) (IX) log (p/760) = A(1 - 349.456/T) (X)
+
where
+
log A = 0.907847 - 10.32816 X 10-4T 10.92653 X 10-’T2 Perfluoroethylcyclohexane : log p = 6.85194 - 1217.729/(t 205.674) (XI) log (p/760) = A(1 - 374.135/T) (XII)
+
where log A = 0.921458
- 10.09293 X
10-4T
+ 10.0873 X 10-’T2
In these equations, p is in mm., 1 in ‘C. and T in ‘I C F group relative to the + C H group, b the difference for the >CFz group relative to the >CH2 group, c the difference for the -CFB group relative to the -CH3 group and d the difference for CF4 relative to CH4, then four equations may be written for the heats of the four reactions (13) W. H. Evans, T. R. hlunson and D. D. Wagman, J . Research Nall. Bur. Standards, 66, 147 (1955).
(14) F. D.Rossini. K. S. Pitzer. R. L. Amett, R. M. Braun and G . C. Pimentel, “Selected Values of Physical and Thermodynamic Properties of Hydrocarbon8 and Related Compounds.” Carnegie Press, Pittsburgh, P a . , 1953.
1138
GOOD,DOUSLIN, SCOTT,GEORGE,LACINA, DAWSON AND WADDINGTON
Vol. 63
TABLE V VAPORPREMURE ni MM. m-Fluorobenzotrifluoride p(obsd.1 Antoine eq.VII
%?
Ref. comp." B.P., OC. p(obsd.)a
71.87 81.64 92.52 104.63 118.06 132.95
40.448 43.210 45.984 48.764 51.560
+O.Ol
+ .02
+ +
35'174) 60.000 65 70 75 80
149.41
54.372
-
-
.oo
.05
'2?
eq. VIII
19.061 21.720 24.388 27.068 29.757 32.460
+ .01 + .01
Perfluoromethylcyclohexane
- p(calcd.1 cox
p(ob8d.) Antoine eq. IX
- p(calcd.) cox eq. x
-0.02 - .01 .00 .02 .01 .03
32.618
-0.02
-0.01
Perfluoroeth loyclohexane p(os6,d.) -*p(oalcd.) Antoine cox eq. XI ea.X11
$8;'
0.00 .02
38.098 40.834 43.570 46.326 49.086 51.864
.01 .02 -I-.03
+ + + +
54.667
-
-
-
-
.oo
+
.04
0.00 .01 .01 .01 .03 .04
-
+ + + + + +
+ +
-
-
+
+
.03
187.57 60.008 .06 .03 37.909 - .01 .01 60.275 .01 * 00 233.72 65.684 .05 .02 43.247 .01 .OO 65.941 .03 00 289.13 71.398 .OO .01 48.634 .03 .OO 71.653 .06 .01 355.22 77.155 .03 .02 54.069 .04 .OO 77.418 .07 .01 85 433.56 82.954 .06 .03 59.551 .08 .02 83.233 .06 - .01 90 525.86 88.794 .07 .02 65.084 .06 .OO 89.099 .01 .04 95 633.99 94.675 .OO .03 70.665 .03 .oo 95.011 .00 .00 100 760.00 100.598 .08 .02 76.296 - .03 .OO 100.975 .07 * 00 105 906.06 106.563 3- .04 - .01 81.975 - 1 . 0 -01 106.990 .20 .03 110 1074.6 112.569 .o .O 87.703 .1 .I. 113.055 - .3 .o 115 1268.0 118.615 .1 - .1 93.481 - .3 .O 119.166 - .4 .o 120 1489.1 124.703 - .2 .1 99.305 .2 .O 125.326 - .2 .o 125 1740.8 130.830 .o .O 105.176 .1 .O 131.532 .2 .1 130 2026.0 136.996 .1 .O 111,094 .O .5 137.786 .8 .o a The reference compound from 71.87 to 149.41 mm. was ure benzene and that from 149.41 to 2026.0 mm. was pure water. From vapor pressure data for benzene ["Selected galues of Physical and Thermodynamic Properties of Hydrocarbons and Related Substances" by American Petroleum Institute Research Project 44, Carnegie Press, Pittsburgh, Pa., Table 21kI and for water [N. S. Osborne, H. F. Stimson and D. C. Ginnings, J . Research Natl. Bur. Xtavdards, 23, 261 (1939)].
+ + + + + -
+ + + + + + -
+
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+
+ +
+ +
+
.
-
TABLEVI DERIWD DATAAT 298.16OK., KCAL.MOLE-' m-Fluorobenzotrifluoride HF.15HzOa
State
Liquid Liquid Liquid
Perfluoromethylc clohexane
? F . ~ OI H,O~
-767.61 f 0.14 -443.01 f 0.11 AHc' -767.61 f 0.14 -440.94 f 0.11 AHj" - 193.24 -683.4 AHVO 9.07 & 0.05 8.11 & 0.05 AHf" Gas -184.17 -675.3 a Concentration of H F to which AEcO and AHc' apply, eq. 111-VI. AECO
4 d = 67.7 kcal. 106 6c = 213.1 kcal.
a
+
a
+ 10b + 3c = 173.7 kcal. + 12b + 3c 197.7 kcal. =:
The solution of these equations is: a = 7.2 kcal., b = 12.0 kcal., c = 15.5 kcal. and ,j,= 16.9 kcal. The increase in E(C-F) - E(C-H) is much greater than any structural variation in the carbonhydrogen bond energy, E(C-H), and therefore represents a real increase in the carbon-fluorine bond energy, E(C-F), as more fluorine atoms are bonded to the same carbon atom. Resonance with structures of the type FR-C=F+
I
R
Perfluoroethyl. cyclohexane HF-lOHzO"
-504.05 i 0.23 -501.68 f 0.23 -778.9 9.20 f 0.05 -769.7
Perfluoro-nheptane
HF-lOHzO"
-422.38 f 0.21 -420.01 f 0.21 -797.7 8.686 -789.0
which is associated with the decrease in the C-F bond distance when more than one fluorine atom is bonded to the same carbon atom,16is probably also associated with the increase in the C-F bond energy. The values given for E(C-F) - E(C-H) may be used t o estimate the heat of formation of any aliphatic fluorocarbon if the heat of formation of the corresponding hydrocarbon is known. (15) L. Fading, "The Nature of the Chemical Bond," Cornell University Press, Ithaca, N. Y., 1945, p . 235.
