2050
NOTES
1.6
acyclic compounds. The asymmetry of certain Narylamides3 provides convenient opportunities for measuring the changes in thermodynamic properties for an equilibrium between diastereoisomers (in the case to be reported here-epimers), which include a second site of asymmetry in the same molecule. Epimerization in these amides is slow enough > sec) for the expected proton magnetic resonance (pmr) signals to be obtained (see Figure 1)) but is rapid enough to allow attainment of equilibrium in a reasonable time. The most important condition that must be met before such measurements can be made is that a compound must be found whose epimers each give at least one pmr signal that is not obscured by other signals of its own or those of its epimer.
I .O
-
0.5
G w
a
3 v) v)
0.2
W
2
5
0.1
CJ
2 3
0.05
0.02 0.015
0
THREE-PHASE EQUILIBRIA SCHAFER AND HEITLANO
o
PRESENT INVESTIGATORS
*
I
I
I
I
I
1
1
Results and Discussion
I
0.72 0.74 0.76 0.78 0.00 0.02 0.04 0.06 0.08 IPK x io3 Figure 1. Comparison of data on the oxygen dissociation pressure of IrOz(s).
Fortunately, we were able to satisfy the necessary conditions for observing epimer equilibrium, as can be seen in Figure 1, with uC
certainties in these values are estimated to be h2.0 kcal/mole and f 2 . 0 eu, respectively. Our heat of formation values may be compared with = -58.1 kcal/mole and AH0298 = -64.0 kcal/mole reported by Cordfunke and h!teyer2b and = -52.4 kcal/mole reported by Schafer and Heitl~ind.~The previous investigators used Wohler and Jochum’s value for the heat capacity of Ir02(s). Other reported heat of formation values are -50 kcal/ mole by Wohler and Witzman2&and -40 kcal/mole by Wohler and J ~ c h u m . ~ Combining with S 0 2 g 8 [Ir(s)] = 8.48 0.04 eu and S O 2 9 8 [Oz(g)]= 49.01 f 0.10 eu, given by Kelley and King,8yields 8’298 [IrO,(s) ] = 13.7 eu, which appears reasonable on comparison with S O 2 9 8 values given by Kelley and King for similar compounds. Acknowledgments. The authors thank Dr. J. H. Norman for his helpful discussions and Mr. J. N. Dixon for performing the X-ray analyses.
*
(8) K. K. Kelley and E. G. King, Bureau of Mines Bulletin 592, U. S. Government Printing Office, Washington, D. C., 1962.
Thermodynamics of an Epimer Equilibrium’
n
1 CG-c-c-N ’(
,CH~-CH:
CI
1 Methyl group a gives a doublet for each epimer. The low-field component of the doublet for the minor epimer and the high-field component for the major epimer stand sufficiently clear of other signals to permit quantitative measurement of relative abundance of epimers from the areas under these signals. I n some solvents and with very good resolution, a very rough estimate can also be obtained from the methyl signals for group b. For this group, the signals from the minor isomer come at the higher field-the reverse of the positions of the signals for methyl group a. The remaining signals are not useful for our present purpose. There is high multiplicity for the methylene signals because of methylene proton noneq~ivalence~ that is still further complicated by overlap of signals for proton c. Inspection of the peaks from group a shows that one epimer is somewhat more abundant than the other. Because of the aromatic substituent on the amide nitrogen, the multiplicity of signals for I cannot be ascribed to cis-trans isomerism resulting
by T. H. Siddall, I11 Savannah River Laboratory, E. I . d u Pont de Nemoure and Co., A i k e n , South Carolina (Received September 69,1966)
Apparently, little information is available concerning the relative stability of diastereoisomers2 that are T h e Journal of Physical Chemistry
(1) Work performed under Contract No. AT(07-2)-1 with the U. S. Atomic Energy Commission. (2) E. L. Eliel, “Stereochemistry of Carbon Compounds,” McGrawHill Book Co., Inc., New York, N. Y., 1962, p 138. (3) T. H. Siddall, 111, and C. A. Prohaska, Kature, 208, 582 (1965); also, T. H. Siddall, 111, Tetrahedron Letters, 50, 4515 (1965).
NOTES
2051
7.0
8.0
6.0
I
I
I
1
5.0
4.0
3.0
2.0
I.o
PPm Figure 1. Pmr spectrogram of compound I, 25% by volume in CHClzCHClz at 100'.
from slow rotation around the carbonyl-to-nitrogen bond.4 Table I lists the abundance ratio of epimers for 25 vol % concentrations of compound I in various solvents at about 40". Very roughly, the ratio decreases with increasing polarity of the solvent ; however, there must be other factors that influence the equilibrium. For example, the abundance ratio for the Table I: Effect of Solvent
Solvent"
Carbon disulfide Benzene Carbon tetrachloride Nitrobenzene sym-Tetrachloroethane Deuteriochloroform Methanol I-Bromonaphthalene Acetone Dimethyl sulfoxide With 25 vol % of compound I.
Epimer ratio (at about 4 0 9
1.8 1.8 1.6 1.6 1.6 1.6 1.5
1.4
epimers of I in methanol solution is high on the basis of solvent polarity alone. Table I1 : Effect of Temperature Temp, Solvent"
OC
1-Bromonaphthalene (A) Nitrobenzene (B) sym-Tetrachloroethane ( C )
50
(A)
75
(A 1
100
1.33
1.49,1 . 3 1 ~
(B 1 (C )
(B) (C ) (A) (B ) (C ) (B)
Epimer ratio
125
140
1.56 1.28, 1 . 5 ~ ~ 1.51, 1.46b 1.57 1.53, 1.63b 1.32 1.56 1.45 1.78 1.54 1.6"
With 25 vol % of compound I. From CHa signals of ethyl group. Close to collapse temperature, peaks not sharponly an approximate value.
1.4 1.0 (4) H. S. Gutowsky and C. H. Holm, J. Chem. Phys., 25, 1228 (1956).
Volume 70, Number 6 June 1986
2052
NOTES
entropy difference. A comparison of epimers, one of which is a meso form, would be interesting.
Experimental Section
4, 1
Pmr spectra were obtained with the Varian A-60 spectrometer and the V-6057 variable-temperature accessory. Compound I was prepared by the addition of N-ethyl-a-naphthylamine, with triethylamine as an HC1 scavenger, to a-chloropropionyl chloride in slight excess in methylene chloride in an ice bath. After filtering off the triethylamine hydrochloride, compound I was purified by molecular distillation and then analyzed. Anal. Calcd: C, 68.8; H, 6.1; N, 5.4. Found: C, 69.0; H, 6.2; N, 5.3.
I25'C
1
I
I .o
2.0
0
PPm
On the Radiolytic Isotopic Exchange
Figure 2. Pmr spectrogram of methyl signals a t different temperatures ; solvent, CHC12CHC12.
of Gaseous Nitrogen
Tables I1 and I11 give the results of studies with three solvents over the temperature range 50 to 125". At about 140" the lifetime of epimerization becomes so short that signal resolution is lost (Figure 2). At 160" merging of the separate signals for the epimers is nearly complete in 1-bromonaphthalene. Within experimental error the enthalpy difference between epimers is zero. Apparently most, if not all, of the free energy difference between epimers is in the entropy term (see Table 11).
The Weizmann Inatitute of Science and the Soreq Nuclear Research Center, Rehovoth, Israel (Received November 15, 1966)
Some time ago' we investigated the isotopic exchange of nitrogen between N214?14 and N2l5~l5 in the gas phase under radiolytic conditions. We have found a value for Gexch namely 9.5 0.25 in the pressure range 33-440 mm. A major part of this exchange has been attributed to the dissociation of excited nitrogen molecules formed by processes other than the recombination of Nz+ e-. I n a recent study,2 another value for Gexchwas re0.5, and a different interpretaported, namely, 7.3 tion of the mechanism of radiolysis of nitrogen was offered, based on the intramolecular isotopic exchange of N4+, which is presumably the predominant ionic species under the experimental conditions. These results prompted us to reinvestigate the system experimentally and to try to reconsider critically the mechanism of this exchange process. I n several independent series of experiments, over a dose rate range between 0.2 and 60 Mrads/hr, using X-rays, y-rays, reactor mixed radiations, and K1.85 p-particles as an internal source of radiation, over a total dose range from 10 to 800 Mrads, Gexoh = 9.5 f 0.5 was obtained. Nitrous oxide was used as gaseous dosimeter (G(-NzO) = 12) as well as Fricke's dosim-
+
Table I11 : Thermodynamic Data Equi-
AF,
librium
cal/mole
Solvent
ratio"
at 50°
AS, cal/(deg mole)
1-Bromonaphthalene Nitrobenzene sym-Tetrachloroethane
1.40 f 0.Ogb
216 i 40
0 . 6 7 =t0.12
1 . 5 5 ?C 0.12 281 += 50 1 . 5 6 i 0 .0 l b 286 f 5
0.87 f 0.14 0.89 += 0.02
a Averaged over four temperatures: 50, 75, 100, and 125' Typical deviations for dupli(average deviation from mean). cate determinations a t same temperature were 0.10.
On the basis of such limited data the origin of the entropy difference between these epimers is highly speculative. However, it seems likely that the difference arises largely from differences in internal rotational states of the epimers. Since both epimers have onefold symmetry axes and exist as d,l pairs, there appears to be no other physical reason for the The Journal of Physical Chemiatry
by M. Anbar and P. Perlstein
*
(1) M. Anbar and P. Perlstein, J . Phys. Chem., 6 8 , 1234 (1964). (2) D . H. Dawes and R. A. Back, ibid., 69, 2385 (1965). (3) M. Anbar and P. Perlstein, Israel AEC Report IA-1048, 1965.
