MCCULLOUGH, FINKE, SCOTT,PENNINGTON, GROSS,MESSEIUYAND WADDINGTON Vol. 80
4i86
it would seem that n is slightly larger for the deuterides. The present values of Gfor LiH and LiD may be compared with those of Kapustinsky, Shamovsky and Bayushkina17 who measured the ultraviolet (17) A. F. Kapustinsky, L. M. Sharnovsky and K. S. Bayushkina, A c t a Physicochim. U.R.S.S., 7 , 799 (1937).
[ COSrRIBUTIOS
KO.
74
FROM THE
absorption spectra of LiH and LiD and, attributing the band to the process LiH(c)
+ Li(g) + Hig)
derived 219.2 and 220.8 kcal./mole for U L ~ Drespectively. ,
L-I.iH
and
LIVERMORE. CALIF.
THERMODYNAMICS LABORATORY, PETROLEUM EXPERIMEST STATION, BUREAU OF u. s. DEPARTMEST OF THE ISTERIOR]
MIXES,
2-Butanethiol : Chemical Thermodynamic Properties between 0 and 1000'K. ; Rotational Conformations' U Y J. P. YICCULLOUGH, H. L. FINKE,D. W. SCOTT,R. E. PENNINGTON, M. E. GROSS,J. F. MESSERLY AND GUYWADDINGTON RECEIVED APRIL24, 1958 The chemical thermodynamic properties of 2-butanethiol in the ideal gaseous state (0 to 1000°K.) were calculated by use of calorimetric, spectroscopic and molecular structure information. The thermodynamic and spectroscopic results show that 2-butatiethiol exists as a mixture of molecules in three distinct rotational conformations-two with about equal energies and a third with about 1.0 kcal. mole-' higher energy. Experimental studies provided the following information: values of heat capacity for the solid (12'K. to the triple point), the liquid (triple point to 307'K.) and the vapor (346 to 453'K.); the triple point temperature; the heat of fusion; thermodynamic functions for the solid and liquid (0 to 310'K.); heat of vaporization (318 to ?58'1(.); parameters of the equation of state; vapor pressure (38 to 121'); and standard heat of formation a t 298.16"K.
Thermodynamic investigations of organic sulfur compounds are made in this Laboratory as part of American Petroleum Institute Research Project 48. Comprehensive studies have been made of all seven isomeric C ~ H ~thiols O S and sulfides to determine accurately the effect of molecular structure on thermodynamic properties as well as t o provide useful data for each isomer. Results obtained for six of the isomers have been published.2 The present paper reports results for the racemic mixture of 2butanethiol (sec-butylmercaptan), the only one of the C,HloS isomers for which optical isomerism is possible. The experimental part of this investigation included studies by low temperature calorimetry. vapor flow calorimetry, comparative ebulliometry and combustion calorimetry. The detailed results are given later in the Experimental section. The results needed for calculating thermodynamic properties, as discussed in the next section, are collected in Table I. Calculation of Thermodynamic Properties Thermodynamic functions were calculated by standard methods of statistical mechanics and thermodynamics. Most of the parameters needed were obtained from spectroscopic and molecular structure information ; the few remaining were chosen so as t o obtain agreement with the observed values of entropy and heat capacity in Table I. 'The calculated thermodynamic functions and the (1) This investigation was part of American Petroleum Institute Research Project 48A on t h e "Production, Isolation and Purification of Sulfur Compounds and Measurement of Their Properties,'' which t h e Bureau of Mines conducts a t Hartlesvillc, Okla., and l.aramie, u'.Scott, J . P.SicCullough, J. I?, SIesserly, R ,1s. Penuington, I. A . Hossenlopp, H. L. Finke and Guy Waddington, THIS L).
80, 55 (195S), and earlier publications cited therein; (b) W.N. Hubbard, W. D. Good and Giiy Waddington, J. Phys. Chon., 62, t i l 1 i l 9 S S ) . JOURNAL,
TABLE I OBSERVED ASD CALCULATED MOLAL THERMODYNAMIC PROPERTIES O F 2-BUTANETHIOL I N THE IDEAL GASEOCS STATE Entropy, .So, cal. deg. -1 Heat capacity, C,', cal. deg. - 1 1'. OK. Obsd. Calcd. T ,O K . Obsd. Calcd.
298.16 318.0 328.6 336.7 358.1
87.67 89.57 90.54 91.27 93.22
8i.G 89.52 90.51 91.20 93 23
346.2 368.2 403.2 453.2
31.83 33.29 35.56 38.68
31.82 33.30 35.58 38.68
+
+ 5H?(g) l/gSq(g) = CaHloS(g). Heat of formation, A.HfoZs.le(obsd.)= -38.39 i 0.19 kcal.
4C(c, graphite)
Vibrational Assignment.-Available spectroscopic data for 2-butanethiol are collected in Table II.3-5 The data of ref. 4 and 5 were obtained with samples of the same highly purified material used in this research. Three spectroscopically distinguishable rotational conformations of "butanethiol are possible, as shown in Fig. I . The conformation with the ethyl group gauche t o the thiol group and trans to the methyl group is labeled A ; that with the ethyl group trans t o the thiol group and gauche t o thc methyl group is labeled B ; and that with the ethyl group gauche to both the thiol and methyl groups is labeled C. From considerations of the energy relationships among the rotational conformations of 1(:i) 1;.W.
Wyo.
( 2 ) (a)
observed value of the heat of formation in Table I were used in calculating the standard heat, standard free energy and equilibrium constant of Eormation a t selected temperatures between 0 and 1000°K.
V. Kohlrilu-ch and 1'. Khppl, .Woitafxh., 65, 25,5 ( I 9 X 3 ) .
(4) American Petroleum I n s t i t u t e Kesearch Project 44 a t the Car-
negie Institute of Technology, Catalog of R a m a n Spectral Dat.,. Serial No. 224. (n) Rcf i, Cnt:ilog < > f Infrared Spectral D n t n , Serin1 Sos. 15-15 ; i n d 162?.
Sept. 20, 1958
CHEMICAL
THERMODYNAMIC PROPERTIES OF 2-BUTANETHIOL
4787
TABLE I1 VIBRATIONAL SPECTRA OF 2-BUTANETHIOL, CM.-l Raman. liquid" API d
K. & K.
228 ( 1 / d 286 (1) 375 (4) 456 (2) 482 (0) 532 (3) 617 (6)
659 ( I / * ) G 8 3 (2) 788 (1) 821 (f/2) 842 (2) 954 (1/d 986 ('/z)
Infrared, liquidb API
Interpretatione
90 (vw) 143 (2) 167 (vw) 228 (2)
623 (19) 634 (6) 659 (5) 683 (12) 790 (6) 839 (11) 863 (3) 953 (6)
ca. 521
620
{
664 w 683 w ca. 780 w 797 s ca. 829 w 843 m 873 m 961 s 978 w 1000 s 1017 w 1074 m 1098 w
+
1151 (1) 1228 (1)
996 (4) 1017 (4) 1070 (6) 1095 (3) 1107 (4) 1123 (4) 1151 (8) 1228 (6)
1294 (1)
1295 (10)
1346 (0) 1388 (0)
1343 (5) 1383 (4)
1447 (6)
1447 (32)
2568 (8)
[1500-2500 cm.-l region omitted] 2574 (92) 2545 vs S H str., A
1018 (1) 1064 ('/z) 1107 (1)
H'
-
324 (4) 377 (17) 412 (3) 453 (7) 517 (1)
324 - 228 96 377 - 228 = 149 453 = 167 620 CCS bend., A Impurity CCS bend., A CCC bend., A CCC bend., C CCC bend., A Impurity CCC bend., C C-S str., A, and 228 + 377 = 605 2 X 324 = 648 C-S str., C C-S str., B 324 453 = 777 CHs rock., A CHz rock., C CH2 rock., B CSH bend., A CHa rock., A C-C str., A CHSrock., A CHI rock., A CHa rock., A 228 868 = 1096 794 = 1118 324 C-C str., A C-C str., A CH2 wag., A CH2 twist., A CH wag., A 324 998 = 1322 CH wag., A CHa bend., sym., A CH2 bend., and CHs bend,, unsym., A
+ +
1124 w 1156 s 1230 s 1285 w 1300 m ca. 1310 vw 1348 w 1377 s 1451 vs
[2500-5000 cm.+ region omitted] * Intena In parentheses are listed relative intensities. sity designated by: vs, very strong; s, strong; m, medium; w, weak; vw, very weak. The rotational conformation to which a fundamental frequency is assigned is denoted by the Not all of the lines insymbols A, B or C defined in text. cluded in ref. 4 are listed because some not reported by other workers can be explained logically as theoresult of excitation by mercury lines other than that at 4358 A.
propanetliio1,G n-butane' and 2 - m e t h y l b ~ t a n ethe ,~ A conformation of 2-butanethiol may be predicted to have the lowest energy, the B conformation only slightly greater energy and the C conformation appreciably greater energy than either of the other two. As will be shown later, satisfactory agreement with the calorimetric data was obtained by use of the simplifying assumption that the A and (6) R. E. Pennington, D. W. Scott, H. L. Finke, J. P. McCullough, J. F. Messerly, I. A. Hossenlopp a n d Guy Waddington, THISJOURNAL,78, 3266 (1956). (7) K. S. Pitzer, I n d . Eng. Chem., a6, 829 (1944). (8) D. W. Scott, J. P. McCullough, K. D. Williamson and G u y Waddington, THISJ O U R N A L 73, , 1707 (1951).
rotational conformations of 2-butanethiol.
B conformations have equal energies and that the C conformation has 1000 cal. mole-' higher energy. Thus, a t room temperature, the calculated concentration of molecules with either the A or the B conformation is 91 mole % and that of molecules with the C conformation is 9 mole yo. The large number of frequencies from 200 to 1000 cm.-' in the spectra of 2-butanethiol confirms the obvious conclusion that all three conformations are present in high enough concentration to be detected spectroscopically. For calculating thermodynamic properties, a complete vibrational assignment was needed for only one conf~rmation.~The A conformation was chosen for complete analysis because it probably is present in highest concentration a t room temperature. T o aid in the identification of skeletal bending frequencies, approximate normal coordinate analyses were made as described for the structurally similar molecule, Z-methylb~tane,~ except that interaction force constants were neglected in the present calculations. The results are in Table 111. Although the agreement between calculated and observed frequencies is poor, the results support the interpretation given in Table 111. TABLE I11 SKELETAL BENDING FREQUENCIES
+
{
C
6
A Fig. 1.-The
V,
A"
v , obsd.
calcd.
B"
167(vw, bd.)b 228(2, bd.) 324(4) 377( 17)
C"
199 254 310 360
218 324 360 360 442
412(3, d ) 405 453(7) 477 517( 1) 517 Rotational conformation. See alternative assignment in Table 11. (I
Frequencies of all three conformations also are apparent in the region of C-S stretching frequencies, 600-700 em.-*. I n fact, the observation of five frequencies in this region may indicate that spectroscopically distinct rotational conformations are produced by rotation of the thiol group about the C-S bond. However, the Ranian doublet at 609623 cm.-l probably is due t o Fermi resonance involving a fundamental a t 620 cm.-l and the sum combination, 228 377 = 605 cm.-'. The frequency a t 634 em.-' can be explained as shown in Table 11. The most intense frequency in this region, 620 cm.-l, was assigned to the A conformation. It is possible that C-C stretching frequencies of
+
(9) K. S. Pitzer,
J. Chem. P h y s . , 14, 239 (1946).
4785
MCCULLOUGH, FINKE, SCOTT, PENNINGTON, GROSS,MESSERLY AND WADDINGTON VOl. 80
the three conformations might differ enough to be principal moments of inertia is 19.28 X g.3 resolved, but only two frequencies not assigned as ems6. The reduced moments of inertia, taken 3s fundamentals of the A conformation appear in the the diagonal elements of the internal rotational 2.823 X expected regions of the spectra. These frequencies, kinetic energy matrix, are 5.179 X a t 1096 and 1107 ern.-', are satisfactorily interand 31.38 X g. C I ~ for . ~ torsion of preted as sum combinations of A fundamentals. either methyl group and of the thiol and ethyl Of the carbon-hydrogen deformation vibrations, groups, respectively. Interaction of internal roonly the CH, rocking mode is likely to depend much tations makes the neglect of off -diagonal matrix upon the rotational conformation of the molecule. elements a poor approximation. However, unccrThe assignments of the CH2 rocking frequencies a t tainties in some of the other paraxiietcrs used in tlic. 7‘34, 821 and 841 tin.-' to particular conformations calculations of therniodynamic propertics are proh cannot be made unambiguously, except that the ably more serious. weak frequency a t 821 cm.-l is logically assigned to TABLE IV the high energy C conformation. However, the CORRELATION OF VIURATIOSAL FREQUENCIES (IN C M . - ~FOR ) CH2 rocking frequencies of both 2 - m e t h y l b ~ t a n e ~ r ~ ~ MOLECULES OF THE TYPE CH8CHICH(X)CH3 and the gauche conformation of 1-propanethioP are X-group near 794 cm.-’, the frequency assigned to the A conIn CHaa OH%b SH8-S C1a.C Drn.4 Descriptive mode formation of 2-butanethiol. 3% 225 231 213 C C S bcnd., A 1% Assignment of the remaining frequencies in the CCX bend., A 367 347 324 335 292 267 spectra of 2-butanethiol was made by analogy with CCX bcnd., U 317 - 303 -. 2-niethylbutane8J0and by use of regularities noted CCC bcnd., A 415 331 377 382 352 :132 in the spectra of organic sulfur compounds.’l Be- CCC bend., I3 412 _cause the CH2 and CH3 bending frequencies near CCC bend., A 4G.l 467 453 4ti5 457 4j3 1450 cm.-‘ and the C-H stretching frequencies near C-S str., A 705 750 020 609 533 490 2950 cni.-l were not all resolved, average values C--X str., U 683 072 G11 579 _ _ _ _ _ _ ~ were used. The frequencies actually used in calcu634 629 -~ 581 54s C-S str., C lating thermodynamic functions are, for the ,4 C-C str., A 978 ‘392 978 ( S u t obsd. in Raconformation: CCS bending, 228 and 324; CCC nian) bending, 377 and 453; C-S stretching, 620; C-C C-C str., A 1150 1147 1124 1109 ll0ti stretching, 978, 1124, 11.56; CH, rocking, 794; C-C str., A 1175 1160 1155 1149 1148 1141 CSH bending, 86s; CH3 rocking, 957, 998, 1017 CHI rock., A 79.5 794 791 793 789 781 and 1072; CH2 wagging, 1230; CH2 twisting, 841 845 841 ___ 836,.. __ ....1283; CH wagging, 1300 and 1350; CH2 and CH3 CH2 rock., 13 -~ 816 817 82 1 CH2 rock., C bending, 1385(2) and 1450(.5); S-H stretching, CH2 wag. 1273 1250 1 2 z 1230 1204 11x7 2560; and C-H stretching, 29.50(9) mi.-’. 1300 1290 1285 (Not obsd. iii R:iBecause of the complex spectra that result from CH2 twist. J11311) the coexistelice of three conforniations of 2-butaneCH wag. 1335 1314 1300 1291 1270 12G7 thiol, the assignment in Table I1 is schematic a t 1350 1325 1318 1354 best. However, this vibrational assignment and CH wag. CHI bcnd., syrn. 1387 1376 1385 1380 1388 137‘3 reasonable values of other molecular structure paCHI bend. and CH3 rameters, discussed later, give excellent agreement bend., unsyrn. 1465 1455 1430 1450 1445 l-NO between calculated and observed thermodynamic CHI rock. 950 912 9.57 051 915 953 properties. Also, the correlation of fundamental 1020 1012 998 wo ‘38s frequencies of six secondary derivatives of butane, CH3rock. 1030 1031 1017 101G in Table IV, supports the assignment for 2-butane- CH3 rock. 1101 1100 1072 1063 1047 111:10 thiol. Complete assignments could not be made CH3 rock. 910 Only with X = CIia for the halogen clerivatives because only Ranian CH3 rock. 920 Only with X = CI13 spectra are available. Like 2-butanethiol, the 2- CH, rock. a Raman spectral data from “Rarnanspektren” by K . halobutanes appear t o exist as mixtures of two or \V. F. Kohlrausch, Akademische Verlagsgesellshaft Becker & three rotational conformations. Erler, Leipzig, 1943, and references cited therein. * Ref. 5, Moments of Inertia, Internal Rotation and An- Serial h-os. 431 and 750. The assignment of C-C1 strctchharmmicity.-The product of principal moments of ing frequencies for 2-chlorobutane is the same as that of J. inertia and the reduced moments of inertia for K. Brown and N. Sheppard, Trans. Faraday Soc., 50, 1164 (1954). Ranian data: XY.G. Braun, D. F. Spooiier anti internal rotation were calculated for the -4 con- M R. Fenske, Anal. Chem., 22, 1074 (1950). formation by the general method of Kilpatrick and Simple, threefold cosine-type barriers to internal Pitzer.I2 Conventional values of bond distances and angles were used: namely, C-C, C-H, C-S aFd rotation were assumed for the methyl groups. ValS-H bond distances, 1..54.1.09, 1.515 and 1.34 A., ues of barrier height selected are: 3100 and 4000 respectively; C-S-H bond angle 100’; and all cal. mole-‘ for the methyl rotations, as in 1-proother bond angles tctrahedral. The product of panethi016 and 2-propanethi01’~; and 1500 cal. mole-’ for the thiol rotation, an average value.6 (IO) T o conform with a correlation of t h e spectra of secondary deA potential function of the following form was asrivatives of butane, discussed later, the assignments given in ref. 8 for the CHz rocking and C-C stretching frequencies of 2-methylbutane sumed for rotation about the central carboil-carbon ( a t 7F5 and 7B5 c m . - l , respectively) have been interchanged. bond (11) D . W. Scott ami J . 1’. AlcCullough, THISJ O U R N A I , 80, 3361 (1‘358). (12) J. 1.;. K i l ~ m l r i c k.rnrl ;1 (10 ID).
5
I’itzcr. .I.
Cht’W
1’/1;5.,
17, IOlii
(13) J . 1’. hlccullough, H. L. Pitike, D . W. Scott, A I . E. Gross. 1. 1’. LIesserly, R. E. Peniiingtun and Guy Waddinfituii, Tms Juc~xar.. 76, 479tj (1054).
.
Sept. 20, 1958
CHEMICAL THERMODYNAMIC PROPERTIES OF 2-BUTANETHIOL
V(+)= (V0/2)(1- C o s 3 + ) , 0 < + < r / 3 a n d r < + < 2 r V(+) = (V0/2)(1 - COS36) f A E , r / 3 < 9 < r
where VOis the barrier height, 9 is the angle of rotation measured from the A conformation and AE is the energy difference between the C conformation and the A and B conformations, the last two assumed of equal energy. The contributions of internal rotation about the central bond to the thermodynamic properties were calculated as those for a simple %fold barrier of height 6500 cal. mole-' @Justhose due to "conversion" of molecules in the A and B conformations to those in the C conformation. The latter contributions were calculated by considering the "equilibrium" between rotational conformations.14 The entropy change for the formation of C from an equimolar mixture of A and B was assumed to be -R In 2, and the energy change, AE = 1000 cal. mole-', was selected simultaneously with the barrier height (6500 cal. mole-') to give best agreement between calculated and observed values of Cp'. The parameters, Y = 1000 cm.-' and Z = -0.50 cal. deg.-' mole-l, of an empirical anharmonicity function16 were evaluated from the heat capacity data a t higher temperatures. This function is insignificant a t 300OK. and increases to -0.58 and -0.26 cal. deg.-l mole-l in Cpo and S o a t 1000'K. Although this function has the temperature dependence of an anharmonicity contribution, i t also is in part an empirical correction for inadequate treatment of all internal degrees of freedom. The negative values in this instance do not necessarily indicate that negative anharmonicity coefficients are preponderant, because other factors could outweigh the expected positive anharmonicity contribution. Molal Thermodynamic Properties.-Calculated values of the thermodynamic functions are in columns 2-6 of Table V.16 Comparison with observed values is made in Table I. Agreement within 0.05r0 was obtained throughout the range of temperature in which So and Cpo were measured. Calculated values of AHf AFf and log Kf are in columns 7-9 of Table V. These values are based on values of the thermodynamic functions in columns 2-6, the experimental value of mfo2M.16 in Table I and values of the thermodynamic functions of C(c, graphite)," H2(g)l7andSz(g).'* Conclusion.-The method of calculation just described is adequate for calculation of thermodynamic properties a t temperatures inaccessible to calorimetric study, but the quality of the molecular structure information is disappointingly low. Only (14) K . S. Pitzer, J . Chem. P h y s . , 6 , 473 (1937) (15) J. P. McCullough, H . L. Finke, W. N . Hubbard, W. D . Good, R. E. Pennington, J. F. Messerly and Guy Waddington, THISJOURNAL, 76, 2661 (1954). (16) The vibrational contributions were computed by use of the tables of H . L. Johnston, L. Savedoff and J. Belzer, "Contributions to the Thermodynamic Functions by a Planck-Einstein Oscillator in One Degree of Freedom," O 5 c e of Naval Research, Department of the Navy, Washington, D . C., July, 1949; restricted internal rotation contributions were calculated from the tables of K . S. Pitzer and W. D. Gwinn, J. Chem. Phys., 10, 428 (1942) : anharmonicity contributions were computed from the tables of R. E. Pennington and K. A. Kobe, ibid., 22, 1442 (1954). (17) D. D . Wagman, J. E. Kilpatrick, W. J. Taylor, K S Pitzer and F. D. Rossini, J . Research Natl. Bur. Standards, 34, 143 (19453. (18) W. H . Evans and D . D . Wagman, ibid., 49, 141 (1952).
4789
qualitative significance can be attached to the molecular structure parameters that were determined. For example, the conclusion is valid enough that two of the rotational conformations have about the same energy and that the third has significantly higher energy. Both the calorimetric and the spectroscopic data are consistent with this conclusion. However, a large uncertainty must be assigned t o the values of the barrier height t o internal rotation about the central C-C bond, 6500 cal. mole-', and the energy differences between the three conformations, 0 between A and B and 1000 cal. mole-'between C and A or B. The most important difficulty is in making a complete vibrational assignment for a t least one conformation. Low temperature spectroscopic studies of 2-butanethiol or 2-halobutanes would be helpful in making a more reliable vibrational assignment. However, because these substances exist as racemic mixtures of optical isomers and are difficult t o crystallize, studies of the crystalline solids might be impossible. Experimental The reported thermodynamic property values are based on a molecular weight of 90.186 for 2-butanethiol (1951 International Atomic Weights) ,18 the 1951 values of fundamental physical constantsm and the relations: 0' = 273.16"K. and 1 cal. = 4.1840 abs.j. = 4.1833 int.j. Measurements of temperature were made with platinum resistance thermometers calibrated in terms of the International Temperature Scalez1between 90 and 500°K. and the provisional scalezz of the National Bureau of Standards between 11 and 90°K. All electrical and mass measurements were referred to standard devices calibrated at the National Bureau of Standards. The apparatus and methods used in the continuing program of this Laboratory evolve as improvements suggested by experience or modifications required for studies of different substances are made. Not all of the improvements have been described in the literature, but the basic experimental techniques used for 2-butanethiol are presented in published descriptions of apparatus and methods for low temperature calorimetry,za vapor-flow calorimetryz4 and comparative ebulliometry.~5 The Material.-Three samples of 2-butanethiol, prepared a t the Laramie, Wyo., Station of the Bureau of Mines, were used. Sample I was used for low temperature calorimetry, and in a calorimetric melting point study the purity was found to be 99.66 mole %. Because this sample did not meet the purity requirements for an API-USBM Standard Sample, the material was repurified. Sample I1 was part of the repurified material and was designated as API-USBM Standard Sample No. 19.z6 This sample was used for combustion calorimetry and comparative ebulliometry. As discussed in the next paragraph, sample I1 could not be crystallized, and attempts to make a calorimetric purity determination were unsuccessful. From mass and infrared spectral analyses and the known purity value for sample I, the purity of sample I1 was estimated conservatively to be 99.9 f 0.1 (19) E. Wichers, THISJOURNAL, 74, 2447 (1952). (20) F. D . Rossini, F . T. Gucker, Jr., H . L. Johnston, L. Pauling and G. W. Vinal, ibid., 74, 2699 (1952). (21) H . F . Stimson, J . Research Natl. Bur. Standards, 42, 209 (1949). (22) € I . J. Hoge and F. G. Brickwedde, ibid., !22, 351 (1939). (23) H. M. Huffman, Chem. Revs., 40, 1 (1947); H . M. Huffman. S. S. Todd and G. D . Oliver, THIS JOURNAL, 71, 584 (1949); D . W. Scott, D. R. Douslin, M. E. Gross, G. D. Oliver and H . M. Huffman, ibid., 74, 883 (1952). (24) Guy Waddington, S . S. Todd and H . M. Huffman, ibid., 69, 22 (1947); J. P. McCullough, D. W. Scott, R . E. Pennington, I. A. Hossenlopp and Guy Waddington, ibid., 76, 4751 (1954). (25) Guy Waddington, J. W. Knowlton, D . W. Scott, G. D. Oliver, S. S. Todd, W. N . Hubbard, J . C. Smith and H . M. Huffman, ibid., 71, 757 (1945). (26) W. E. Haines, R . V. Helm, G . L. Cook and J . S. Ball, J . P k y s . Chem., 60, 549 (1956).
4790
T ,OK.
h%CCULLOUGIl, FINKE, SCOTT, PENNINGTON,
GROSS,ivESSERLY
TABLE V THEMOLALTHERMODYNAMIC PROPERTIES so, ( F o - H o o l ( T , ( H a - H o o ) / T , H o - HOo, cal. deg.
cal. dep. - 1
kcal.
OF
AND WADDINGTON
Vol.
so
2-BUTANETHIOLa
G O ,
Aflf
b
AFf",b
kcal. log Kf" 0 0 0 0 n 0 -31.51 -31.51 Infinite 273.16 - 68.28 16.94 4.627 85.22 26.81 -10.75 -37.94 8.60 - 69.81 17.84 5,319 S7.65 28.51 - 8.25 298.16 -38.39 G.05 - 69.91 17.91 300.00 5.373 87.52 28.64 -38.42 - 8.00 5.87 - 75.56 21.43 96.99 35.38 -40.08 4-2.32 400 8 ,572 - 1.27 24.85 - 80.69 12.425 105.54 41.37 -41.41 13.09 500 - 5.72 000 - 85.5% 28.02 16.81 113.54 46.42 - 42.46 24. 08 - s.77 - 90.07 50.68 35.23 21.67 121.03 700 30.90 .- 43.23 -11.00 - 94.37 33.67 54.29 --4 3 . 77 26.94 128.04 so0 46.49 -12.70 57.37 - 14 , 1-1 134.61 - 98.49 36.12 57.79 32.51 900 - 14.03 - 102.40 38.39 00.02 -44.31 38.31) 140.79 1000 -15.11 69.14 T o retain internal consistency, some values are given t o one more decimal place than is justified by the absolute accuracy. * For the reaction 4C(c, graphite) 5Hl(g) l/gS*(g) = CdH&(g).
+
cat. deg. - 1
cal. deg. - 1
kcal.
+
mole y'. Sample I11 was used for vapor flow calorimetry. The purity of this sample also was estimated to be 99.9 3= 0.1 mole 70, but sample I11 may have been slightly less pure than Sample 11. Crystallization of 2-Butanethiol.-At the Laramie Station samples of 2-butanethiol could not be crystallized in a timetemperature freezing-point cell, even if seeded by a wire cooled t o liquid air temperatures. The tendency of singly branched hydrocarbons and sulfur compounds t o form organic glasses has been noted often.17 The fact that the racemic mixture of 2-butanethiol probably has a lower melting point than a pure isomer may accentuate the glass-forming tendency. However, sample I (99.66 mole Yo purity) was crystallized in the low temperature calorimeter. To initiate crystallization a protracted series of cooling and warming cycles in the range 55°K. t o just below the melting point (133O K . ) was necessary. Once crystallization began, the sample was maintained a t a temperature from 5 to 30" below the melting point. The rate of crystallization was extremely slow, and all efforts to accelerate the process were unsuecessful. I n all, initiation of crystallization required nearly 2 weeks, and completion of the crystallization process required another 3 weeks. After sample I1 was received, an attempt was made to crystallize it in the low temperature calorimeter. Numerous techniques tried over an interval of 6 weeks failed t o initiate crystallization. The common notion that pure samples of organic compounds crystallize more readily than do impure samples was not correct in this case. Apparently, a small amount of impurity was needed for nucleation. Heat Capacity in the Solid and Liquid States.-Low teniperature thermal studies were made with 47.150 g. of sample I. The sample was sealed in a platinum calorimeter with helium (3 cm. pressure at room temperature) added to promote thermal equilibration. The observed values of heat capacity CBRtdare in Table VI. Above 30"K., the accuracy uncertainty is estimated to be no greater than 0.2y0, except for possible uncertainty due to the 0.34 mole 7,impurity in the sample. Measurements of the heat capacity of the liquid were made with sample 11, and the results are included in Table VI. There is no systematic difference between the results obtained with the two samples, aud the average deviation of all values from a smooth curve is 0.03Yo. The heat capacity curve for the solid ( Caatdas. T ) has the iiormal sigmoid shape. The heat capacity of the liquid between 185 and 310'K. may be represented by tlie empirical equatioii Cezit,l(liq.) = 52.825 - 0.182332' ti.GOiC, X 10-'1'2 6.1419 X 10-7T3, cal. deg.-' molc-1 (1) In the indicated temperature range, eq. 1 represents the values listed in Table VI with average and maximum deviations of 0.03 and 0.05y0, respectively. The Heat of Fusion, Triple Point Temperature, Cryoscopic Constants and Purity of Sample 1.-Only one satisfactory determination of the heat of fusion was made, and this exIn a second periment yielded the value 1548 cal. mole-'. experiment, the crystallization time was reduced from about
+
(27) D. R. Douslin a n d €i, hI. H u f f m a n , TICISJ O I J R K A L , 6 8 , 170.1 (1946) ; unpublished results, this Laboralory
5 t o about 3 weeks, and the value obtained for the heat of fusion was 470 lower than the h s t value, obviously as a result of incomplete crystallization. Unfortunately, the exTABLE VI THE MOLALHEAT CAPACITYOF 2-BUTASETIIIOLIN CAL. DEG.-~ T,
OK."
C..tdb
Crystals
T,
S8.79 90.21 92.47 94.20 95.39 97.76 99.82 100.58 105.59 105.64 110.84 110.87 114.29 115.55 116.32 119.53 120.10 123.73 126.45
Casab
15.793 15.983 16.274 16.493 16.658 16.947 17.201 17.314 17.917 17.913 18.540 18.546 18.953 19.120 19.210 19.627 19.684 20.122 20.564
