Jan. 5, 1955
CHEMICAL THERMODYNA4MICPROPERTIES
tion between these two groups and explains why this compound lies on curve A. One is left with the conclusion that since mesityl oxide has a lower I P and YC=O than, for example, acetone (21)) the (CH&C=CH group must be classed with, but stronger than, CH3 in its capacity as an electron releasing group. I n conclusion, i t should be noted that only compounds with the same functional group, namely, the carbonyl group, can be considered in the present correlations. It would be impossible to include ethers or amines, etc., because the lone pair electrons are in a different configuration. Among themselves, however, ethers or amines, etc., should give the same sort of correlation of donor strength with IP. Lack of experimental data prohibits such a test a t the moment. Experimental All the spectra were recorded on a double beam spectrometer employing XaC1 optics.12 Immediately after each run the spectrum was calibrated with the 2851 cm.-’ and the 1603 cm.-l bands of a thin film of polystyrene. Spectral hands were reproducible to about = t 5 cm.-l a t 3,000 cm.-l and to about & 2 cm.-l a t 1700 cm?. Some runs where a (12) L. W. Herscher, H. D. Ruhl and N. Wright, J . Opt. SOL.Amev., in press.
[ COXTRIBUTION
SO.
66
FROM THE
OF
55
2-METHYL-1-PROPANETHIOL
strong solvent hand interfered with the measurement were performed differentially using a variable thickness cell. The great majority of the compounds were Eastman Kodak white label products which were used without further purification. Other reagents were prepared and distilled here. Hydrochloric acid solutions were prepared by bubbling anhydrous HC1 through the appropriate mixture for about 5 minutes. ;io attempt was made to analyze the mixtures for HC1 since its concentration was knovin to be small. Solutions of CBHSCCHin the carbonyl compound were prepared so that the concentration of the former was about 0.5 ilf. The carbonyl stretching frequencies were measured by making up a solution of the carbonyl compound in CClr to a strength of about 0.01 31, and using a cell 0.5 mm. in thickness. Gas phase values of the carbonyl stretching frequencies were measured using a 10 cm. cell at low pressures.
Acknowledgments.-Thanks are due to Professor K. Watanabe for permitting me to see his paper on ionization potentials before publication and for several other unpublished values of IP, to Dr. K. U. Ingold for his unpublished value for benzophenone, and to Dr. W. C. Steele for the measurements he kindly made in these laboratories. The encouragement of Dr. h’orman Wright is gratefully acknowledged. MIDLAND, MICHIGAN
THERMODYNAMICS LABORATORY, PETROLEUM EXPERIMENT STATIOX, BUREAUO F O F THE ISTERIOR] UNITED STATES DEPARTMEST
LfINES,
2-Methyl-l-propanethiol : Chemical Thermodynamic Properties and Rotational Isomerism1 BY D. LV. SCOTT, J. P. MCCULLOUGH, J. F. MESSERLY, R. E. PENNINGTON, I. -4. HOSSENLOPP, H. L. FINKE AND GUYWADDINGTON RECEIVED JULY 18, 1957 The chemical thermodynamic properties of 2-methyl-1-propanethiol in the vapor state (0 to 1000°K.) were calculated by methods of statistical mechanics utilizing observed values of the standard entropy, heat capacity and heat of formation. The thermodynamic results are consistent with spectroscopic evidence for a small energy difference between rotational isomers. Experimental studies provided the folloTving information: Values of heat capacity for the solid (12°K. to the triple point), the liquid (triple point to 350’K.) and the vapor (350 to 500’K.); the triple point temperature; the heat of fusion; thermodynamic functions for the solid and liquid (0 to 350°K.); heat of vaporization (321 to 362OK.I; second virial coefficient, B , in the equation of state, P V = RT(1 B / V ) ; vapor pressure; and standard heat of formation.
+
Comprehensive thermodynamic studies of 2methyl-1-propanethiol (isobutyl mercaptan) were made as part of studies of all seven isomeric thiols and sulfides, C4HloS.? The experimental part of this investigation consisted of studies by low temperature calorimetry, vapor-flow calorimetry, comparative ebulliometry and combustion calorimetry. The detailed results are given later in the Experimental section. However, the more pertinent results that are needed for the calculation of 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, Most (1) This investigation was part of American Petroleum Institute Research Project 48A on “The Production, Isolation and Purification of Sulfur Compounds and Measurement of their Properties,” which the Bureauof AMinesconducts a t Bartlesville, Okla., and Laramie, wyo. (2) D. W. Scott, H. L. Finke, J. P. McCullough, J. F. Messerly, R. E. Pennington, I. A. Hossenlopp and Guy Waddington, THIS J O U R N A L , 79, 10G2 (1957), and earlier publications cited therein.
of the parameters needed were obtained from available spectroscopic and molecular-structure information; the few remaining were selected to fit the observed values of entropy and heat capacity in Table I. Finally, the calculated thermodynamic TABLE I OBSERVED AND CALCULATED THERLIODYSAMIC PROPERTIES OF
T,
OK.
321.31 310.07 361.65
~-METHYL-~-PROPASETHIOL VAPOR
Entropy, S o , cal. deg. - 1 mole - 1 Obsd. Calcd.
88.91 90.64 92.61
T ,O K .
Heat capacity, CpO, cal. deg. -1 mole -1 Obsd. Calcd.
88.91 90.65 92.61
351.20 32.04 32.02 373.20 33.49 33.52 413.20 36.12 36.16 453.20 38.64 38.63 500.20 41.33 41.31 4C(c, graphite) 5Hdg) l/&(g) = C&oS(g) AHf02ss.,6(obsd.) = -38.48 i 0.22 kcal. mole-’
+
+
functions and the observed value of the heat of formation in Table I were used to calculate the standard heat, standard free energy and common loga-
56
SCOTT, -vCCULLOUGH, LfESSERLY, PENNINGTON,
rithm of the equilibrium constant of formation a t selected temperatures between 0 and 1000°K. Vibrational Assignment.-The harmonic oscillator contributions to the thermodynamic functions were computed from a vibrational assignment based on the spectroscopic data in Table TI."-" The TABLE I1 \'II%RATIOYAL
SPECTRA
OF
Chl
Infrared, liquid 11 APT^
Raman, liquids 6.82 K.8 APIi
224( 1) 339(4) 402(2) 427(3) 522(0) 668(3) 708(5 j
339(4) 402( 1) 429(4) 518(1/2?) 671(4) 711(ij
770(2 j
772(3) 805(2) 827(2 j 927(3) 9 5 i ( 4)
801(2) 824( 2 ) 024( 1) 955( 3 ) 1108(?) 1216(2j 1250(1) 1319(1) 1336(1) 1370(1/2)
{
1105(3) 1116(3) 1169(2j 1226(2) 1255(1) 1319(3) 1336(3)
2-,\ILETHYL-l-PROPAVETHIOL, -1
Interpretation
ca. 433 vw ca. 5 1 7
671 w ill m 743 vw 779 vw 809 w 828 rn 922 m 953 w ca. 1050 sh 1100 rn CQ. 1115 sh 1169 rn 1228 m 1259 s 1326 m
1
Skeletal bending Skeletal bending Skeletal bending Skeletal bending ~924~ - 402 =522 C-S str., C, isomer C-S str., Cl isomer 339 402 = 741 C-S-H bend CH2 rock CHarock CHarock CHI rock C-C str. CH3 rock C-C str. C-C str. CH2 twist. CH2wag CH wag CH wag
+
1371 CH3 sym. bend 1388 s 1423(2b) 1429(3) 1428 rn CHs bend 1452(4b) 1154(6) 1468 s CHI unsym. bend(4j [1500-2500 cm --I region omitted] S-H str. 2573(8b) 2577(8) 2564 w [2600-5000 cm.-' region omitted] In parentheses are listed the relative intensities on a scale of 10 for the most intense band. b Intensity designated by: s, strong; m, medium; w, weak; vw, very weak; sh, shoulder.
data of ref. 4 and 5 were obtained from samples of the same highly purified material used in this research. Two spectroscopically distinguishable rotational isomers are possible for 2-methyl-1-propanethiol, as shown in Fig. I . The appearance of
G/
Fig. 1.-Rotational
Gs
isomers of 2-methyl-1-propanethiol.
two moderately intense frequencies (671 and i l l cm.-l) in the region of C-S stretching frequencies (3) L. Kahovec and K. W. F. Kohlrausch, Z . p h y s i k . Chem., B48, 7 (1940). (4) American Petroleum Institute Research Project 44 a t t h e Carnegie Institute of Technology, Catalog of Raman Spectral D a t a , Serial N o . 227. ( 5 ) Ref. 4, Catalog of Infrared Spectral Data, Serial Nos. 1678 and 1079.
HOSSENLOPP, FINKE AND
\~7.\DDINGT(IN
1701. 80
is evidence that both isomers are present in significant concentration in the liquid a t room temperature. Arguments for assigning the higher of the two frequencies to the C1 isomer have been given by Brown and Sheppard for the analogous isobutyl chloride.6 Surprisingly, no other features of thc spectrum show the presence of a second isomer. 111 fact, all the observed frequencies below 500 ciii:~' and between 750 and 1100 cm.-] are needed t o account for the frequencies expected in these regions from a single isomer. The designatioris for the vibrational modes in Table I1 are somewhat schematic and are intended merely to show that the expected number of frequencies are assigned in the several regions of the spectrum. For the unsymmetrical CH3 bending and C-H stretching frequencies, which are not all resolved in the observed spectra, average values of 1460 and 2950 cm.-l were selected. The complete set of frequencies selected for the C1 rotational isomer is: 224, 339, 402, 429, 711, 772, 80'7, 827, 924, 9,jt5, 10-of melting temperature, To, function of fraction of total sample melted, F , are 1 Table V. Also listed in Table 5' are the values obtained for tlle triple point tcrripcrature, TT E , . , t h e iiiole fraction of im-
purity in the sample, -Y2'+,atid the cryoscopic constants,** A = A H r u s i o n / R T ~ . ~and . Z B = ~ / T T . P . - ACruS 2AH~,,i,,, calculated from the observed \-;dues of 7'. Afffuaion and ACfu8iu,l(15.38 cal. rkg.-l inole-'). Thermodynamic Properties in the Solid and Liquid States: -1'alues of tl~crn~oclynainic properties for the coiitictiset1 phases were ccmputed from the c;ilorimetric data for selected temperatures between 10 and 35OOK. The results are given in Table 1.1. The vnlucs a t 10°K. were cvinpntetl froin a Ilebye function for 4.5 degrees of freedom with 0 = 100.9'; these parameters were evaluated f r ( m the he;it THE
T.4BLE
Iy
TABLE VI MOLAL THERMODPSAMIC PROPERTIES O F 2-XIETHYL- 1PROPANETHIOL I S THE SOLID AND LIQUIDSTATES'
THE>IOLAL HEATCAPACITY O F 2 - - 5 1 E T H r L - l - P R O P A S E T I ~ I O I , IN CAI,. D E G . - - ~ ?',
Csatt,,Q
06.0
Crystals 12.41 1.185 12.% 1.294 13.4i 1 ,459 14.19 1,643 14.81 1.816 13.5(i 2.011 16.48 2.283 17.09 2.452 18.34 2.824 18.83 2.960 20.24 3.383 20 68 3,511 22.X 3.978 2 8 8 4.157 24.42 4.616 2.5.37 4.900 26.68 3.268 28.19 5.695 31.14 6.579 34.81 7.390 38.39 8.13 42.24 8.921 46.48 9 687 31 OR 10,482 53.67 10.915
