Dec., 1946
HEAT CAPACITY OF CYCLOPENTANE AND hkTHYLCYCLOHEXANE
253i
[CONTRIBUTION FROM THE CHEMISTRY LABORATORY, UNIVERSITY O F CALIFORNIA1
The Heat Capacity of Gaseous Cyclopentane, Cyclohexane and Methylcyclohexane B Y RALPH SPITZER' AND
The cycloparaffins are of interest because their geometr:y restricts internal rotation about the carbon-carbon single bonds. Therefore, a studyof the thermodynamics and molecular structure of these hydrocarbons should lead to some increased knowledge about the form and nature of the potential hindering rotation in hydrocarbons. Cyclopentane is of particular interest because it has been shown2that the classic planar model for this molecule is incorrect and the interaction between angle strain and restricted rotation leads to a puckered ring. In cyclohexane, the isomerism between chair and boat forms should lead to a contribution to the heat capacity of the gas from which scme information about the restricting potential should be derived. Althoiigh reliable measurements have previously been macle on cyclohexane and methylcyclohexane3 the tenperature range used was too small to be useful in deciding questions of molecular structure. Our temperature range was from about 25" abo1.e the boiling point to 230'. Experimental Apparatus.-The apparatus used €or measuring the heat capacities mas esijentially that described by P i t ~ e r . ~ One major chnnge in design was the substitution of a floating heater for the fixed heater which had previously been used in the vaporization unit. The use of this heater led t o much steadier boiling rates resulting in steadier temperatures and more reproducible results. The heater was a Chrome1 helix fastened to a thin-walled glass cylinder by glass rods. T'ery flexible leads made of four strands of 36 gage Formex coated copper wire were used. Materials.-Two samples of cyclopentane were used, both originally from the Tidewater Associated Oil Company. One contained 1.3% neohexane (analyzed by mass spectrometer) for which correction was made. The second sample, which w a s received only after some measurements were completed, was very pure, having been finally fractionated ai- the Sational Bureau of Standards. The cyclohexane was prepared from Eastman technical cyclohexaiie by treating with fuming sulfuric acid for a week t o remove benzene, Tvashitig, fractionally crystallizing three times and finally distilling from sodium in a 25-plate column. The Sational Bureau of Standards supplied a sample of methylcyclohexane Tvhose purity they had determined by freezing point depressioii to be 99 mole per cent. Corrections for Gas Imperfection.-In order t o check the validity of the Berthelot correction for gas imperfection, the heat capacities of cyclopentane and cyclohexane were mcasuretl a t reduced pressure.6 Even at the lowest temperaturcs used, extrapolation t o zero pressure gave results that Twre intlistiiiguishable from the Borthelot corrections within csperitnerii.al error; therefore, the Berthelot correction has been used in reducing the results t o the ideal gas state. The critical temperatures and pressures used were : ( 1 ) National R e s e i r c h Fellow i n Chemistry, 1943-1946. Present address: Deuartment of Chemistry, Oregon S t a t e College, Corvallis, Oregon. ( 2 ) Aston, S c h u m m n , F i n k and Doty, THISJOURNAL, 63, 2029 (1941); Pitzer, Scieizcc, 101, 67%(1945). ( 3 ) h I < > n t g o m e r yand D e Vries. THISJ O U R N A L . 64, 2375 ( 1 9 r Z ) . ( 1 ) l'itzcr, i b i d , 63, 2413 (1941). (.7) I ' i t i w aiicl (:wiitn. ; h i < / ,63, 3313 ( 1 9 4 1 ) .
KENNETH s. PITZER
520" K. and 44.2 atrn. for cyclopentane, 554" K. and 40.4 atm. for cyclohexane anti 572" E;. and 3 . 6 atrn. for rnethylcyclohexane.
Experimental Heat Capacities.-Table I presents the results of the measurements, the Berthelot corrections and the heat capacities o f the ideal gases. The latter are presented to facilitate comparison with theory which we hope to publish later. Figure 1 presents these results graphically For the results of other investigators, see ref. ( 3 ) . The curves drawn in Fig. 1 are empirical. The size of the symbols is equal to the experimental error of approximately 1%. I
I
300
400 500 600 Temperature, OK. Fig 1.-Experimental heat capacities a t one atmosphere.
Heats of Vaporization.-The present vaporization unit was constructed too tall and was not completely immersed in the thermostat ; the resulting heat leak led t o heats of vaporization which varied with the rate of flow. The values obtained by extrapolating to infinite rate of flow, however, agree fairly well with the heats of vaporization measured with other apparatus. The best values obtained for the heats of vaporization a t their boiling points of cyclopentane, cyclohexane and methylcyclohexane, respectively, are . 6370 * 40, 7183 * 30 and 7G00 * 50 calories Rlathews6 obtained 720.5 and 7515 for the latter two. ( 6 ) ivfatilrt+>,z h d , 4s. .xi2 (192(i)
H. H. SISLER,C. A. VANDERWERF AND STBPIIEN STEPI-IANOU
253s
TABLE I ~ ~ S P E R I M B N T A IHEAT , CAP.4CITIES
OF
GASEOUSCYCLO-
Methylcyclohr\aiie
1
P E S T A S E , CBC1,OHEXANE AXD hfETHYLCYCLOHEXANE
Calories per degree per mole Press., attn. T ," W .
Substance
Cyclopcntaiie
1
0 6
Cyclohrsaiie
1
353 372 395 424 463 5113 589 363 384 428 4(iO 495 521 544 384
0.46 0 . 3 0 384
CP
21.74 * 26.16 * 28.29 * 30.40 * 33.32 + 36.14 =t 38.36 * 24.67 * 34.59 * 38.94 =t 41.9 * 45.5 =t 47.2 =t 49.4 * 34.31 * 34.35 *
c p -
c,o
0.36 .31 ,526 .21 .16 .4 .12 .4 .10 .3 .22 .3 .38 .4 .27 .4 .22 .5 .18 .5 .15 .5 .13 .3 .17 .3 .11
0.2 .2 .3 .3 .3
cno
34.38 25.85 28.03 30.19 33.16 36.02 38.26 24.45 34.21 38.67 41.7 45.3 47.0 49.3 34.14 34.24
11.'
44 49 ,j3 38
4 6
43
18 *
.5
24
06
ii
18
58 00
=t
*
41 48 53 ,57
3s
CORVALLIS, ORECOY
RECEIVED AUGUST29, 1946
LABORATORY, cNIVERSITP
O F KaNSAS]
The System Formamide-Ammonia
BY HARRY H . S I S L E RCALVIN , ~ ~ A. VXNDERIYERFXKD STEPHEN STEPHANOU The fact that amides of the carboxylic acids stand a t a point intermediate between ammonia and carboxylic acids in the Brbnsted scale of acidities has already been discussed. Likewise, experimental evidience for the basic properties of these amides in anhydrous acetic acid and for their acidic properties in liquid ammonia has been cited. Compound formation was shown to take place in the systems acetamide-ammonia and acetamide-acetic acid. The systems formamideacetic acid, formamide-formic acid, formamidew-butyric acid, and forinamide-water also have been studied. The system formamide-ammonia remained, however, to be investigated. The investigation of this system was therefore undertaken as the second step in the present series of experiments. Experimental Materials.--The formamide was obtained from the Eastrnan Kodak Co. I t was dried by allowing it to stand over drierite f l x one week, and was purified by distillation under vacuurr , a middle fraction boiling from 107 to 109" uiitler 15 mm . pressure being retaiL1ed. The substance thus obtained melted a t 2.2 + 0.2 , in good agreement n i t h the values of 2.0 to 2.55" given in the l i t e r a t ~ r e . ~ The ammonia used was synthetic anhydrous ammonia ( 1 ) The first paper i n this series: Sisler, Davidson, Stoenner a n d Lyon, THISJ O U R N A L , 66, 1888 (1944). ( l a ) Present addres4: T h e Ohio S t a t e University, Columbus, Ohio. (2) English a n d T u r n e r , J . Chem. Soc., 107,774 (1'315). (3) E . g English and T u r n e r , ref. 2 ; Lowry a n d C u t t e r , J . Chem. S u c . . 136, 1.46'3 (1033); Taylor a n d Davis. J . P h y s . Chem.. 39, 1469 I, 192R 1.
13 62 24 82
Acknowledgments.-\Ye are grateful t o Dr. F. D. Rossini of the Bureau of Standards and Dr. H. U. Hyde of the Tidewater Associated Oil Company for providing the samples. Professor IT.D. Gwinn assisted in the design and construction of the new vaporizing unit with floating heater. Summary The heat capacities of gaseous cyclopentane, cyclohexane and methylcyclohexane have been measured from slightly above the boiling point to about 250'. h few measurements made a t reduced pressures indicated that the heat capacities of the ideal gas are given to good approximation by the Berthelot correction.
lCONTRIBUTION FROM T H E BAILEY CHEMICAL
Amphiprotic Substances.
398 439 480 . 1 -75.5 67.3 -76.0 -77.6 68.5 -77.6 fi8.0
-36.3 -44.0 -50.9 -55.2 -62.5 -7,5.1* -67.7 -75.4* -72.7 -74.0 -83.0 -990.5* -85.4 -86.6 -88.5
(c) Solid phasc, 2NH3.1-ICOSH2?
71.1 71.0
73.5 75.4 80.2 82.2 83.4 86.9
-91.9 -91.5 -94. o* -92.3 -93.0
-.. ii.T, rr
I I .9 79.2
(d) Solid phase, SHs -91.0 91.9 -91.4 94.3 100.0 -90.6 -85.8
-97.5* -95.0 -94.8 -96.0
-83.2 -80.9 -77.5
