324
LESTER GUTTMAN AND KENNETH S. PITZER
less weight to the high moment structures in both cases, a step which is quite reasonable. 1%is concluded therefore that the planar configurations appear to be more acceptable in the*casesof resorcinol and hydroquinone than is the concept of free or partially restricted rotation. summary
1. The dipole moments of catechol, resorchol and hydroquinone have been determined in benzene and are equal to 2.62, 2.07 and 1.4 D,respectively.
[CONTRIBUTION FROM THE
VOl. 87
2. The agreement between the experimental moment of catechol and the calculated value obtained by using vector addition of link moments and by assuming a cis planar structure involving a weak hydrogen bond is excellent. 3. Planar configurations for resorcinol and hydroquinone are more acceptable on comparison of experiment and calculation than are the structures assumigg either free or partially restricted rotation of the hydroxyl groups. COLLEGE PARK,
CHEMICAL LABORATORY OF THE
MARYLAND RECEIVEDNOVEMBER 27,1944
UNIVERSITY OF CALIFORNIA]
trans-%Butene. The Heat Capacity, Heats of Fusion and Vaporization, and Vapor Pressure. The Entropy and Barrier to Internal Rotation BY LESTER GUTTMAN AND KENNETH S. PITZER
Introduction and Discussion The potential b d e r to the rotation of a methyl group attached to an olefinic structure has been the subject of several principally concerned with the propylene molecule. In 1940 one of the present authorss pointed out that while an entropy value for propylene obtained on the basis of the Third Law of Thermodynamics was uncertain because of the possibility of end for end random orientation of the molecules in the crystal, the corresponding value for trans-%butene would not be uncertain because both ends of the molecule are the same. The study described below was undertaken to establish definitely the potential barrier, which would be expected to be the same for propylene and trans-%butene. The results as presented below show definitely that the potential barriers in trans-2-butene and propylene are in the vicinity of 2000 cal. per mole and that propylme molecules are oriented regularly in their crystals. This conclusion is in agreement with the later papers of Kistiakowsky and his collaborators'~~and the work of Telfair.6 Apparently the hydrogenation equilibrium data of Frey and Huppke* are in error hy factors near two (in the equilibrium constant), which accounts for the lower potential barriers reported by one of the present authors in 1937. While the conclusion stated above is quite definite, minor uncertainties remain in the assignment of vibration frequencies. Therefore it seems (1) K. S. Piterr, J . C h m . Phys., 6 , 473 (1937). (2) 0.B. Kistiakowsky, J. R. Lacher and W. W. Ransom, r b r d , 6, 900 (1938). (8) T . M. Powell and W. F. Giauque, TAISJOURNAL, 61, 2366 (1939). (4) B. L. Crawford, Jr., 0.B. Kistiakowsky, W. W. Kice, A. J. Wells and E. B. Wilson, Jr., ibrd., 62, 2980 (1Y39). (5) K . S. Pitzer. Chcm. Rev., 47, 39 (1940). (6) D. Telfair, J . Chem. Phys., 10, 167 (1842). (7) G. B. Kistiakowsky and A. G. Nickle, ibid., 10,78,146 (1942). (8) F. E. Frey and W. P. Huppke, Ind. Eng. Chem., 15,54 (1933).
best to postpone final statistical mechanical calculations until a later date. Experimental Material.-Through the kindness of Professor John G. Aston of Pennsylvania State College, there was made available t o us a sample of trans-2-butene, whose purity, as estimated from premelting behavior, was about 99.3%. Visual examination of the just-melted compound revealed the presence of liquid-insoluble impurity which would not have appeared in the premelting. By a single distillation in a 25-plate column, the liquid-insoluble impurity was apparently removed almost completely from a middle fraction. This fraction was further purified by a single fractional crystallization in vacuo, with the result that the material used in the measurements contained 0.37% of impurity, assuming ideal liquid solutions and no solid solution. The weight of sample used was 52.79 g., or 0.9409 mole, the molecular weight being taken as 56.104. Apparatus.-The calorimeter used in all the measurements was of conventional design, although a few modifications are worthy of note. The calorimeter proper, of copper, was joined to the filling line by about 1 meter of thin-walled I/*" german silver tubing. The german silver tube was well soldered to the heavy radiation shield, and hence heat leak down it was kept at a low value. Pure (c. P.) platinum wire, 0.1 mm. in diameter, was threaded back and forth through a bundle of thin-walled glass capillaries t o give a compact strain-free resistance thermometer, which was placed in a case so situated as t o be completely surrounded by the sarnfle. Copper foil between and around the tubes afforded thermal contact wit,h the walls of the case. Electrical connections to the thermometer were made through heavy platinum lea& sealed vacuum-tight thwugh glass beads and projecting a short distance above the upper surface of the calorimeter. The case was filled with helium at one atmosphere. It was originally intended that the thermometer should conform to international temperature scale requirements. Unfortunately, the inclusion of some short leads of less pure platinum prevented this. However, the thermometer was compared at numerous temperatures with another thermometer which had been calibrated in the desired manner.' Deviations from a lineur relation between their resistances were computed and plotted on a large scale, and a temperature scale drawn up by interpolation and careful Smoothing. Small changes which occurred sub(9) K . S. Pitzer and D. W. Scott, THISJOURNAL, 66, 803 (1943).
sequently, evidently due t o incomplete annealing, were detected and corrected by calibration against the triplepoint and vapor pressure of hydrogen, the upper transition, triple-point and vapor pressure of oxygen, and by comparison of the vapor pressurc of I)utenc in the calorimeter with its value in a carefully prcpard ice-bath. On the lateral surface of the calori~netcrwas wound a copper resistance thermoiiicter t o nicasui-e the tcriiperature of the radiating surface. Accurate corrcction could then be made for heat interchange during the detcrniiiiation of heats of fusion and vaporization. The vapor pressures were measured with a rn:inonieter connected directly t o the calorimeter. One arm was continually evacuated, and the difference iii height was observed with a Gaertner cathetorneter, the vernier of which was graduated t o 0.05 rnrn., estimation of half this distance being possible. The cathetonietcr scale had been cornpared with a standard meter bar a t frequent intervals, and the deviations had been found never to exceed 0.1 mm.
Vapor Pressures.-Corrections for capillary depression were made by means of the tables of Blaisdell.lo The pressures were corrected to OOC., and to standard gravitational acceleration. Table I contains the observed pressures, and the deviations from the equation log,, Porn. -1757,2K?/T - 0.0171290 T f 1.85411 X Tz
+ 11.57265
(1)
I
0
T A ~ LI1B OF I W ~ ~ - Z - B U T B N I Z
MELTING Purw
I
T,OK.
zhba., rm.
- Pcnlecl.
Ptrbi.
1.492 3.276 6.064 9.883 13.451 21.437 30.358 41.047
0.006 ,015 ,016
-
-
49,286
63.038 76.297
.017 .002 .002 .004
.006 .007 ,009 ,013
Melting Point.--The meltiiig point was observed as a fuiictioii (,I the fraction iiidtcc! by the usual procedure of introduciiig i n quantities of eiicrgy and n!lonhg tiine temperatiire to I)econie csscnti:i!lj. con>,tant. Table I1 contains the rcsults a t thc four highest points, comparatively 1itt.k premcltitig having occurred below the first teii1per;iture given. If Raoult's l a w is olxyed, tlic tcmpcrzlture plotted against the reciprocd of thc. frLtctionmelted should give a straight line \vhosc. intcrccpt a t zc ro (infinite dilution) is the triii- nic'lting poin.t, itnd whuse slope is proportional to t h e 11101c f r x t i o t - i oi iinpurity. Figure I shows such a Illot. Tlic t i ~ ciiic'ltiiig point is founcl to bc Iii';.OI 5: O.O.j"K.; the , which slope leach to tlic \ ~ i l u c .O . : Z % ~ ~iinpiirity, was usctl to corrcct t h r tic:!t caiwcities for prcmelting. T x h l c I I1 cc~iu;~irc.s the rtsults oC this research w i t h other \.;iiucds oi tliv iiicI!ing and (10) I i l : i i 4 t : l l , J . I/,&'/'. ['/I)>
,
3.
IN\
T,OK.
Fraction melted
0,030 .178 ,394 ,847 True meltirig point
trans-2-I1uTmE 0°C. = 273.113"K.
VAPOR PRESSURE OF
201.703 212.617 222.042 230.261 235. SO1 244.857 252.159 258.901 263.202 269.233 274.142
l.4lfJl.
ii
boiling points to be found in the literature. The discrepancy between the melting point of Todd ancl Parks and the present value is inexplicable on the basis of any reasonable limits of error in our temperature scale. Since they give no account of their method of treatment of premelting heat and extrapolatiori to the melting point, the difference could be ascribed to this source.
The boiling point calculated from this equation is 274.04OK. (OOC. = 273.16OK.). TABLE
I
2 4 . (Fraction melted)-'. Fig. I .
164.759 167.126 167.373 167.498 167.61 += 0.05"K.
TABLE I11 MELTING AND M . p.
T ,'IC.
R O I L I X C P O I N T S OF B. p.
7', "R.
107.3 167.3
274.09 274.07 167.61 * 0.05 274.04 167.72 ==0.10
....
f
0.05
It'U?LS-2-BUTENE Observer
Todd and Parks" Kistiakowsky, et al.12 Lamb and Roper13 This research Glasgow and Rossiiii14
Heat Capacities.-The heat capacities were nieasured froiri about 15°K. to the boiling point in several series. Except at the extreme temperatures ancl in the premrlting regidn, the error should not escecd ().af%. In all measurements on the liquid, the calorimeter was always kept colder than its surrounciiiigs, to prevent distillation. In consequence, the radiation corrections were larger than at lower temperatures, aiid the accuracy less. The liquid heat capncifies were corrected for vaporizalioii into the 1;lling line. This correction :tiriouritcci to about Y',; a t the highest point, and decrczlscd rapidly a t lower temperatures. The exp&meiital values, in terms of a defined calorie cclual to 4.1Y33 Iiit. jonles, are contained in Table ( 1 11 5 . 'A 1'4r!.s, ' J I i l : , J . J I I t S A L , 58, 131 (1Ll;Iu). 12) 0 . 13. Ristinkowsky, J. R. Kuhoff, 11. A. Smith a n d W. E. VaiiCli.in, i 6 i i , 57, 876 (1933). i , l . I ) .\. I?. 1.ariib a n d li. 12. Roper, ibid., 62, 896 (!940). ( 1 4 ) A. I
