August, 1961
DIPOLEMOMEXTS OF DISUBSTITUTED CYCLOHEXANE DERIVATIVES
will give quite good agreement with all the experimental points; this displacement would call for :L Gilkerson14term E, = - kT in the above equation. The tendency of the log KA vs. 1/D curve to become concave-down in the range of low diPlvctric constants again suggests that ( 2 ) is a hetter approximation than ( 3 ) for the activity roefficient. The various modifications discussed above, which reduce f2, would give larger numerical values of K.k from a given set of data, leaving the C J term essentially unchanged, with the result that Libetter”values of KA would be obtained The limiting conductance of cesium iodide in water is 154.16 which is 0.06 unit higher, as experted, than the value 154.10 obtained by adding Voisinet’s value15 of 77.& for resiiim to Zeldes’ of 76.B4 for iodide. (The literature values were obtained by a Shedlovsky extrapolation, lvhich gives a value which is slightly l r s ~than the correct limit; see ref. 5, Fig. 15.1). The Walden products for cesium iodide give 8 monotone function of composition (ie., no maximum, as was found for potassium chloride). If the water point is omitted, the apparent Stokes radii are given within O.l-l.O% by the equations (14) W.R. Gilkerson, J. Chem. Phys.. 25, 1199 (1956). (15) W. E. Voisinet, Dissertation, Yale University, 1951; B. B. Owen, J. c h i n . phys., 49,C-72 (1952). (It)) €3. B. Owen and H. Zeldes, J. Chem. Phys., 18, 1083 (1950).
+
lo8 E(&+) = 0.9724 14.81/0 108 R(1‘)= 0.9774 14.89/D
+
1417 ( 6)
(7)
The corresponding graph for Cs+ is shown in Fig. 2, lower curve. ,4s was found for potassium chloride, tJhelimiting valucs for infinite D are unreasonably small. If the Sutherland coefficient” 4aq is used in place of the Stokes factor 6 ~ 7 in , order to allow for slippage of ions through a discontinuous medium of water molecules (which are of the same order of size as the ions involved), the corrected sum of the radii becomes 1.5 (1.950) = 2.93 = dA. This is still smaller than the sum of the crystallographic radii (3.85), but nearer the value UJ = 2.76 obtained from the conductance data in water, using the program for unassociated electrolytes. The value of U A for potassium chloride was found to be 2.53. The sequence for the two salts is in agreement with the observation that the association of KCl is greater than that for CsI a t a given value of dielectric constant; for example, at D = 15.37, K~(lcC1)= 500 while for CsI at D = 15.29, KA = 310. This is also the sequence which would be predicted from the crystallographic radii; the discrepancy in absolute values between the sizes of the lattice ions and those of the models from conductance, however, remains a t present iinexplained. Further work on other alkali halides is in progress. (17) W.Sutherland, Phzl. Mag., 9,751 (1008).
ELECTRIC DIPOLE MOMESTS OF SOME DISUBSTITUTED CYCLOHEXANE DERIVATIVES IX THE VAPOR STATE’ BYMAXT, ROGERS AND JAMES XI. CANON Department of Chemistry, Michigan State Uniwrsity, East Lansing, Michigan Received March 88,1961
The electric moments of trans-1,2-dibromocyclohexane, t~ans-1,2-dichlorocyclohexane, trans-1-chloro-2-bromocyclohexane and 1,Pcyclohexanedione have been measured in the vapor phase. Ratios of conformational isomers and energy differences between them have been estimated from the data and compared with values in the literature derived from measurements in solution. The comparison has revealed an anomalous solvent effect.
Introduction The t~uns-l,2-dihalogenocyclohexaneshave two conformational isomers, I (la-2u) and I1 (le-2e), which differ in electric moment, and under any
1
Br
Br
I
I1
I11
given conditions these substances exist as a mixture of the two forms. The electric moment of I, in which the substituents are both axial, is zero while that of 11, in which the substituents are both equatorial, should have essentially the same value as that observed for the corresponding cis-isomer. The two conformations of the cis-isomer (111), la2e and le-2a, have identical electric moments. It is possible, therefore, to compute the ratio of isomers from the observed electric moment and, assuming the (1) Supported by a grant from the National Science Foundation.
Boltzmann distribution, to derive the difference in energy between the isomers. Such measurements 2-4 have been made for truns-1,2-dichlorocyclohexane, trans-l,2-dibromocyclohexane, 3,4 and truns-l-chloro2-bromocy~lohexane~ in solution. The only vapor measurement reported is for trans-1,2-dichlorocyclohexane a t a single temperature. Since energy differences between conformational isomers would be expected to depend on solvent we have measured the electric moments of the above series of cyclohexane derivatives in the vapor state over as wide a temperature range as possible. The data have been used to compute ratios of conformational isomers and energy differences and our results have been compared with related values reported in the literature. (2) A. Tulinskie, A. DiGiacomo and C. P. Smyth, J. Am. Chem.Soe.,
75,3552 (1953). (3) K. Koaima, K. Sakashita and 8. Maeda, dbid., 76, 1985 (1954). (4) P. Bender. D. L. Flowers and €1. L. Goering, ibid., 17, 3463 (1955). (5) W. Kwestroo. F. A. Meyer and E. Havinga, Rcc. trav. chim., 73, 3563 (1955).
MAXT. K O G E RASD ~ JAMES AI. CANON
1418
Vol. 63
TABLE I The electric moment observed for 1,4-cyclohexanedione in : s ~ l u t i o nhas ~ , ~ been attributed to an DIELECTRIC CONSTANTS, MOLAR POLARIZhTIONS AND equilibrium mixture of chair and boat forms (IV ELECTRICMOMENTSOF SOMECYCLOHEXANE DERIVATIVES and V or VI, respectively) of the molecule. We IN THE VAPORSTATE have measured the moment of this substance in the T,OK. ( E - 1) x 106 P , ccJrnole P, D vapor phase to provide an estimate of the ratio of trans-1,2-Dichlorocyclohexane these forms and the energy difference between them 441.8 9379 110.7 2.30 in the vapor state. 468.2 8356 97.7 2.28 448.5 467.5
V
VI
Materials.--trans-l,2-Dichlorocyclohexane and trans-l,2dibromooyclohexane were synthesized by addition of the respective halogen to cyclohexene .e trans-l-Chloro-2 bromocyclohexane was prepared by bubbling hydrogen chloride gas into a slurry of N-bromosuccinimide, chloroform and cyclohexene .b These materials were purified by fractional distillation in z LCCUO. The 1,4-cyclohexanedione was a commercial :,ample (B and H Organic Chemicals Co.) purified by recrystallization from water and benzene. Apparatus.- The apparatus for measurement of electric momentb in tbe vapor phase has been described.9 The pressure in the measuring cell was balanced against an inert gas in a mercurv manometer by a diaphragm-type pressure transmitter and the pressure read on the manometer. The sample rontainw, measuring cell and connecting tubing were electrically healed to the desired temperature. Because of slow decomposiiion of the samples a t the higher temperature a fresh ssmple was used for each capacitance measurement. The sample was degassed zn vacuo then distilled into the measuring cell Calculations. -Values of the molar polarization P of the vapor a t each temperature were obtained from the pressure and capacitance readings in the usual manner.9 Van der Waals’ equation was used to compute the gas densities and the van der Waals constants a and b estimated from data for related mbstances. The temperature range covered was limited so that 1t was necessary to use the refractivity method and compute the electric moment from the equation
truns-l-Chloro-2-bromocyclohexane 8925 117.1 8103 101.6
2.21
2.17
448.5 467.5
trans-1,P-Dibromocyclohexane 8144 97.2 7566 94.9
2.00 1.99
468.2 490.3
1,4-Cyclohexanedione 4205 52.8 4071 53.6
1.39 1.44
dichlorocyclohexane. in the vapor state.2 The difference between the C-C1 and C-Br bond moments is small in these compounds and has here been neglected. Since the change in dipole moment over the small temperature range employed was less than the errors of measurement the mean value of p at the mean temperature has been used in computing xee. The energy difference AE=Eaa- E e e has been obtained from the Boltzmann equation assuming that the partition functions of the two forms are equal nao= ncsexp(E,,
- Eee)/RT
The values of xee and A E obtained in this way are shown in Table I1 along with the various values which have been reported in the literature. The only substance previously studied in the vapor state is trans-1,2-dichlorocyclohexaneand our value for the electric moment agrees well with that re= o o md(PTinn)T ported by Tulinskie, et aL2 where the molcz refraction M R D was computed from obThe diequatorial form I is more stable than the served refractive indices and densities of the liquids. diaxial form I1 for trans-1,2-dichlorocyclohexanein Estimated errors in the electric moments were computed from the standard deviation for e and average about f0.1 D. the vapor and in all solvents so far studied whereas The temperature range was limited by the vapor pressure the diaxial form is always the more stable for transon the lower end and by decomposition on the higher end of I ,2-dibromocyclohexane. It has been suggested4 the temperature range. The total range was only about 25“ which made it impossible t o obtain the distortion polariza- that this is a consequence of the greater crowding in tion experimentally. Our assumption that the atomic the le-2e form with the larger bromine atom. In polarization is negligible therefore introduces some uncer- the case of trans-l-chloro-2-bromocyclohexane the tainty into the I esults, especially for 1,4-cyclohexanedione. two forms are of very nearly equal stability in the vapor phase while benzene favors the le-2e form Results and Discussion The electric moments found for the four sub- and carbon tetrachloride the la-2a form. The change in AE in going from solvent to vapor stances studied are shown in Table I along with the observed diehctric constants, e, and molar polari- is fairly constant for the three substances studied. zations, P, of the vapors. The mole fraction of Using mean values we find AE(benzene) -AE(g) molecules in the le-2e configuration, Xee, may be = 400 cal./mole, and AE(CC14) - AE(g) = -170 computed, assuming an equilibrium between the cal./mole; also E(n-heptane) - AE(g) = -170 cal./mole from data for trans-1,2-dibromochlorola-2a and le-2e forms, from the equation hexane only.3 It would be expected that the energy p 2 = (nscm2 -t n,,maa6)l(nce nao) = Z c a m a 2 Taomaa7 of the polar le-2e form would be lowered in a solFor the three frans-1,2-dihalogenocyclohexanes we vent relative to its energy in the vapor state whereas have ut>ed niaa = 0 and taken for mee the value the non-polar la-2a form would have unchanged 3.12 D, which is the moment found for cis-1,2- energy. The value AE(so1vent) -AE(g) = 300 cal./mole was computed theoretically by Tulinskie, (6) 0. Hassel and E. Naeshagen, Tzdsskr. Kjemzog. Bergueaen, 10, 81 (1930). et al.,for trans-lJ2-dichlorocyclohexane in a solvent (7) C. G.LeFkvre and R. J W. L e F h r e , J Chem. Soc., 1696 (1935) of dielectric constant B E 2.2. Since the dielectric (8) H. OreengaId, “Organic Syntheses,” Vol. XII,John Witey and constants (at 25O) of benzene (2.27), carbon tetraSons, New York, I?. Y. chloride (2.24) and n-heptane (1.92) de not differ (9) M T Rugem and James M. Canon, unpublished results.
+
+
August, 1961
nECOMPOSITIOS ~ I X K L ’ I C SOF I J T H I U M 1’EHCHLOHA’l’L
TABLE I1 MOLEFRACTIONS OF CONFORMATIONAL ISOMERS, AND ENERQY DIFFERENCES BETWEEN THEY, FOR SOME CYCLOHEXANE DERIVATIVES This research Substance
trans-I,2Dichlorocyclohexane
zce
0.54
A I = Eso Ese
-
140b
Literature AE (g)
AE (bz) AE(CC1d
100
300’ - 503 4003 +1405 6504 8205 -SO3 -5003 -7 0 4 - 4004 -705 - 7005
trans-1,20.41 -330 Dibromocyclohexane -30 trans-1-Chloro- 0.49 2-bromocyclohexane 1,4Cyclo0.88 1900 hexanedione’ a z = fraction. b 811 energy values are in cal./mole.
3704
-2205
5605 13007
much the difference in AE would be expected to be about the same in all three cases. The reversal of the sign of AE(so1vent) - AE(g) in the case of carbon tetrachloride and n-heptane indicates that the above theory is not adequate. Specific solvent
1419
effects may exist which stabilize the non-polar la2a form in these solvents, contrary to the prediction of electrostatic theory, or As may not be aero. The apparent dipole moment of 1.4 D observed for 1,4-~yclohexanedionein the vapor state corresponds to about 12% of the boat form V if equilibrium between forms IV and V is assumed. The energy difference AE = E(boat) -,!?(chair) = 1900 cal./mole is computed on this latter assumption. However, these calculations have two serious defects. First, the atomic polarization, which we have neglected, may be except,ionally large for this molecule and a value P A = 10 cc./mole is not unreasonable by analogy with l14-benzoquinone.lo This would reduce the experimental moment to 1.1 D. Second, the boat forms of type VI may be present since they presumably do not differ much in energy from V. If the energy of forms V and VI is assumed equal the total contribution of these forms (neglecting atomic polarization) would be 22% and AE would become 2250 cal./mole. The value of A E does decrease on going from vapor to benzene solution as would be expected if the polar boat form is sta.bilized in solution relative to the non-polar chair form. (10) C. P. Smyth, “Dielectric Behavior and Structure,” McGrawHill Book Co., Ino., New Y Irk. N. Y., 1955.
THE DECO1CIPOSITIOK KINETICS OF LITHIUM PERCHLORATE BY MEYERM. MARKOWITZ AND DANIEL A. BORYTA Foote Mineral Co., Research and Engineering Center, Chemicals Division, P. 0. Box 615,West Chester, Pa. Received April 1, 1961
The thermal decomposition of pure lithium perchlorate and in admixtures with lithium chloride was studied over the temperature range of 392-415” by constant temperature thermogravimetry. It is shown by phase data that above 247’ any mixture of lithium perchlorate with its decomposition product, lithium chloride, always contains the perchlorate in solution. Over the temperature range covered, the kinetics follow the autocatalytic Prout-Tompkins rate law EA,^. = 52.2 f 4.1 kcal./mole) up to about 40% decomposition and then conform to first-order kinetics EA^^. = 62.0 =!= 4.1 kcal./ mole). The point of transition between the two rate laws occurs when the decomposing melt is saturated with lithium chloride. From the kinetic data, the solubility of lithium chloride in the melts was computed and a kinetically derived value for the heat of fusion of lithium chloride was obtained. The relationship of these studies to the thermal decompositions of the other alkali metal perchlorates is discussed.
Introduction The participation of a thermally decomposing material in the phase change from the solid to t.he liquid state usually complicates the observed kinetics.l Thus, potassium perchlorate has been reported t,o decompose according to two first-order rate laws2.3; one characteristic of the solid-phase decomposition and the other of the liquid-phase. Differential thermal analysis ~ t u d i e s ~of- rubidium ~ and cesium perchlorates have demonstrated the concomitant occurrence of fusion and decomposition phenomena, thereby indicating complex kinetics for these compounds. Lithium perchlo(1) C. E. H. Baivn in “Chemistry of the Solid State,” edited by W, E. Garner, Academic Press, New York, N. Y., 1955, pp. 254807. (2) A. E. Harvey, Jr., hl. T. Edrnison, E. D. Jones, R. -4. Seybert a n d K. A. Catto. J . Am. Chem. Soc., 76, 3270 (1954). (3) A. E. Harvey, C. J. Wassink. T. A. Rodgers a n d K. H. Stern, Ann. N . Y . dead. Sci., 79, 971 (1960). (4) hl. M. h‘larkowitz, J . P h w Chem., 62, 827 (1958). ( 6 ) M. Xf. h‘larkowitz and D. A. Boryta, ibid., 64, 1711 (1960). 16) M. Jf. Jlarkoaita, D. A. Boryta and R. F. Harris, ibid., 66, 2G1 (19til).
rate,4-6 on the other hand, shows a considerable temperature interval between fusion (247’) and measurable rates of reaction (392415O).’ Accordingly, it was felt that a study of the thermal breakdown of lithium perchlorate would be of interest inasmuch as the salt probably would be in the liquid phase during the entire period of decomposition. Thus, the system LiC104-LiCl was studied and the quantitative kinetic behavior of lithium perchlorate and of mixtures of lithium perchlorate with lithium chloride was investigated. Experimental Anhydrous lithium perchlorate, prepared as previously reported,* was analyzed for perchlorate content by precipitation as nitron perchlorate.8 Chloride was determined gravimetrically as silver chloride and chlorate was computed as the additional chloride produced after reduction by sulfurous acid. Analysis of product: c104-, 93.4 (calcd., (7) M. M. Markowitz and D. A. Boryta, Anal. Chem., Sa, 1588 (1960). (8) F. P. Treadwell a n d W. T. Hall, “Analytical Chemistry,” Vol. 11, John Wiley and Sons, Inc., New York. N. Y . , Ninth English Edition, 1942, pp. 383-385.
