NOTES
2705
Olson and Mulford6 have studied the equilibrium PuN(s) = Pu(1, saturated with N) 0.5 N2(g) in the temperature range 2290-2770’K. It was not possible to obtain the standard thermodynamic properties from these results because the activity of saturated liquid plutonium is not known. They have estimated AH’298 I -64 kcal/mole, favoring a value of -70 kcal/mole which is equal to that reported for UN.? Rand and Kubaschewski have indicated? that for nitrides having the NaCl structure, generally S(MN) - S(M) = 1 f 1. This leads to ASozg8 = -22 f 1 for the standard reaction. Using 11.39 cal/mole deg for ST,,,, s Z 9 d for P u , ~ 3.02 cal/mole deg for STo0- Szgsfor 0.5N2,9and an average heat capacity of 12 cal/mole deg, one obtains AHo2g9= -76 kcal/mole from the emf results. The standard free energy of formation of PUN from these emf data indicates that AG’700 = -60 kcal/mole. If the suggested6 value of AHo2g8 = -70 kcal/mole is accepted, this leads to A S o z ~ 8= -13 cal/mole deg, which is not in agreement with the value of 1 1 for S(PuN) - S(Pu). Although it is not possible to reconcile these differences, the correct value for AH’298 is probably more negative than -70 kcal/mole.
+
*
Acknowledgment. This work was sponsored by the
U. S. Atomic Energy Commission. (5) L. Brewer, et ai., “The Transuranium Elements,” McGraw-Hill Book Co., Inc., New York, N.Y., 1949,p 863. (6) W. M. Olson and R. N. R. Mulford, J. Phys. Chem., 68, 1048 (1964). (7) M. H. Rand and 0. Kubaschewski, “The Thermochemical Properties of Uranium Compounds,” John Wiley and Sons, Inc., New York, N. Y., 1963,p 41. (8) R. Hultgren, et al., “Selected Values of Thermodynamic Properties of Metals and Alloys,” John Wiley and Sons, Inc., New York, N. Y., 1963,p 226. (9) K. K. Kelley, Li. S. Bureau of Mines Bulletin No. 584, U. S a Government Printing Office, Washington, D. C., 1960, p 132.
Deuterium and Tritium Isotope Effects in the Methoxide-Promoted Elimination Reaction of 2,2-Diphenylethyl Benzenesulfonatel
by A. V. Willi2 Chemistry Department, Brookhaven National Laboratory, Upton, New York, and Columbia University, College of Pharmacy, New York, New York 100BSa (Received April 19, 1966)
A kinetic study of primary deuterium and tritium isotope effects at different temperatures was under-
taken with special interest in the problem of tunneling. The reaction chosen for this purpose was the elimination of 2,2-diphenylethyl benzenesulfonate under the action of sodium methoxide in Methyl Cellosolve solution (H” = H, D, or T) (CeH6)zCHzCHzO02SCeHs f CH30- + PhZC=CHz CH30HZ
+
+ CsH6S03-
Since previous work by Shiner and co-workers4 was concerned with the ethoxide-promoted elimination reaction of l-bromo-2-phenylpropane, it was decided to study the more acidic 2,2-diphenylethyl system. In this work, the reactions of the compounds with H” = H or D could easily be followed (in separate experiments) by observing the increase in the ultraviolet absorption at 255 and 260 mp. The rate of the tritiumlabeled compound was measured by following the decrease of radioactivity in the toluene-soluble fraction of the reacting solution.
Experimental Section Methods of preparation of starting materials are analogous to those reported by Shiner and Smith4”: 2,2-diphenylethanol was made from ethyl diphenylacetate by reduction with LiA1H4 and was allowed to react with benzenesulfonyl chloride in ether solution in the presence of pyridine. The crude sulfonic ester was purified by repeated crystallizations from ethyl acetate-n-hexane. For the preparation of the isotope-substituted materials, ethyl diphenylacetate had been exchanged with ethanol-d (99.0% isotopic purity, purchased from Merck Sharp and Dohme of Canada Ltd., Montreal) or with ethanol-t (prepared by hydrolysis of ethyl o-formate with tritium-labeled water) in the presence of catalytic amounts of sodium eth~xide.~”Three subsequent exchanges were carried out for the preparation of the D compound, and the isotopic purity at the 2 position (99.0% or better) was checked with the aid of nmr spectra. The activity of the tritium-labeled material was 0.5 mcurie/mmole. In a product analysis experiment, 1.3 g of 2,2diphenylethyl benzenesulfonate and 0.3 g of sodium methoxide in 50 ml of Methyl Cellosolve were heated to 71’ for 28 hr. The main product was 1,l-diphenylethylene as identified by ultraviolet (A, 250 mp) and nmr spectra of the ether-soluble fraction. There (1) Research performed under the auspices of the U. S. Atomic Energy Commission. (2) Visiting chemist to Brookhaven National Laboratory, 1964. (3) Where inquiries regarding this paper should be sent. (4) (a) V. J. Shiner and M. L. Smith, J . Am. Chem. Soc., 83, 593 (1961); (b) V. J. Shiner and B. Martin, Pure Appl. Chem., 8 , 371 (1964).
Volume 70, Number 8 August 1966
NOTES
2706
Table I: Isotope Effects in the E2 Reaction of (CgHg)2CHZCH~002SCgHg Temp,
kz (uncor), M-1 sec-1
7 -
40.15 49.85 60.05 71.10
H” = D
HZ = H
OC
1.21 x 3.38 x 7.90 x 2.23 X
‘ ksN2/k2 for the
10-3 10-3 10-3 lo-*
2.20 x 6.36 x 1.56 x 4.73 x
10-4 10-4 10-3 10-3
n= = T 9.47 x 2.90 x 7.52 x 2.21 x
10-6 10-4 10-4 10-8
aa
0.0535 0.055 0.060 0.0635
kdkD (cor)
h/kT
r
(cor)
(es 2)
5.61 5.42 5.18 4.84
13.4 12.2 11.1 10.7
1.50 1.48 1.47 1.50
tritium experiments,
order solvolysis as a possible side reaction, even for an was less than 0.1% stilbene (ultraviolet spectra, order of magnitude as low as 0.5%. (Stilbene had stilbene: A,, 296 mp) and less than 1% (C6Hs)ZCHCH20CH3 or ( C ~ H ~ ) Z C H C H ~ O C H ~ C H(nmr ~ O C H ~been found as the only product in the first-order acetolysis of 2,2-diphenylethyl to~ylate.~)In the kinetic spectra). experiments with the tritium-labeled compound, the For the kinetic experiments, the initial concentrations radioactivity of the toluene-soluble fraction did not were 7 X or M 2,2-diphenylethyl benzenesuldecrease to normal background. Even after more M sodium methoxide. A stream fonate and 2 X than 10 half-lives, it remained at a value about 300 times of nitrogen was bubbled through the solutions for 20 as much as the background or 6% of the zero-time min; then the solutions were filled into ampoules. activity. The final activity was slightly dependent on The sealed ampoules were heated in a thermostat for the temperature of the kinetic experiments (Table I). different time periods. The illethyl Cellosolve used The same final value was obtained, however, when the for the kinetic experiments was freshly distilled. (It measurements were repeated at the same temperature was distilled twice in a weak stream of nitrogen; sodium after two subsequent crystallizations of a mixture of metal had been added previous to the first distillation.) the original starting material (which was already In the H and D experiments, the ampoule solutions, purified previously) with the twofold amount of pure after having been brought to room temperature, were unlabeled 2,2-diphenylethyl benzenesulfonate. Therediluted 10- or 20-fold with illethyl Cellosolve and then fore, it must be concluded that the residual radioactivity measured at 255 and 260 mp in a Beckman DU spectrowas due to the product of an Sx2 side reaction of 2,2photometer. The reference cell was filled with Methyl diphenylethyl benzenesulfonate. With 2,2-diphenylCellosolve of the same origin as that used for the ethyl p-nitrobenzenesulfonate the corresponding S N ~ preparation and dilution of the solutions. I n the tritside reaction is much more important (40% for the ium experiments, 5 ml of ampoule solution was trans2-deuterio compound), and the products can easily ferred with a pipet h t o a separatory funnel with 50 ml be identified with nmr analysis.6 of water (containing enough HCI to neutralize the methI n these examples, the experimental second-order oxide in the sample) and 25 ml of toluene. The funnel rate constant is the sum of the rate constants for biwas strongly agitated for 2 min. (It had been estabmolecular elimination and nucleophilic substitution lished in previous experiments that under these conditions the extraction of the starting material is essenkz = k ~ 2-k h h . 2 (1) tially complete. Less than 0.02% of labeled diphenylethyl benzenesulfonate remains in the aqueous phase For the tritium experiments, the fraction CY = while less than 0.1% of hydroxylic tritium can be found ksNz/kz may be obtained from the ratio: (tritium in the toluene phase (dried with CaCl2). After sepaactivity in toluene solution from infinite-time sample)/ ration of the layers, the toluene solution was dried for (tritium activity in toluene solution from zero-time about 1 hr with a small amount of calcium chloride, and sample). I n order to evaluate the rate constants then it was filtered. The filtrate (5 ml) was added to k m ( T ) , k E z ( D ) , and k E z ( H ) (Or shorter: k T , k D , and k ~ ) 15 ml of scintillation solution (PPO POPOP in for the elimination reaction alone, k S N 2 must be subtoluene), and the p activity was measured in a Packard tracted from the k2 values of the tritium, deuterium, “tricarb” 314 X liquid scintillation counter.
+
Results and Discussion On the basis of the practically complete absence of stilbene in the product, it was possible to exclude firstThe Journal of Physical Chemistry
(5) S. Winstein, B. K. Morse, E. Grunwald, K. D. Schreiber, and J. Corse, J . Am. Chen. Soc., 74, 1113 (1952). J. G. Burr, ibid., 75,
5008 (1953). (6) A. V. Willi, unpublished work.
NOTES
2707
and protium experiments. This correction is relatively small in comparison to kz, and the secondary isotope effect on ksN2may be neglected here as it cannot exceed a few per cent of ksN2. Experimental results for the over-all second-order rate constants k 2 ( ~ ) , kz(D1, and k2(T) and the S N ratios ~ CY are collected in Table I. The sixth and seventh columns of Table I contain the deuterium and tritium isotope effects in the elimination reaction, as computed from the corrected rate constants. The isotope effects in this reaction are smaller than those in the ethoxide-promoted elimination of 2-phenylpropyl bromiden4 In this study, we obtain the following Arrhenius parameters
AD/AH = 0.905 (f18%), EaD
- EaH
=
1016 (*Ill) Cal
AT/AH = 1.03 (*33%), EaT
log A H = 11.01 (f3%),
- EaH
1617 (*214) Cal
E,H = 19,960 ( h l l l ) cal
The isotopic ratios of the preexponential factors AD/AH and AT/AH are unity within the experimental error. The corresponding results found by Shiner and nf artin4bfor the elimination of 2-phenylpropyl bromide are 2.53 and 3.01. Evidence for tunneling on the basis of &/AH’ may be considered as positive only if the experimental value is significantly larger than 2”/’ = 2.8. This is the high limit which refers to an unsymmetric transition state in which two bending vibrations and one (unsymmetric !) stretching vibration of the hydrogen to be transferred have become classical oscillators. Another criteria for tunneling is based on eq 2. The ratio r must be equal to 1.44 if the only cause of the log (kT/kH)/Iog (kD/kH) = ?’
(2)
isotope effect is the zero-point energy diff erence.**O It may be a little higher for primary isotope effects as the contribution of the YL* ratio may be as high as 1.57. If there is considerable tunneling, r must be smaller than 1.44 because the rate of proton transfer is increased much more than the rates of deuteron and triton t r a n ~ f e r . ~Values of r calculated from experimental data are given in the last column of Table I. They do not indicate tunneling. Almost the same values of r can be computed from Shiner and Martin’s data. It appears that there are large differences in the transition states and barrier dimensions for different proton-transfer reactions-as it may be expected. While tunneling is apparently unimportant in two elimination reactions, there is unambiguous evidence for tunneling in the fluoride-catalyzed enolization of 2carbethoxycyclopentanone’O and in the 2,4,6-collidinecatalyzed ionization of 2-nitropropane. I* Acknowledgment. The author wishes to thank Dr.
J. Bigeleisen and Dr. M. Wolfsberg for having suggested a tunneling study by comparison of D and T effects, for many valuable discussions concerning the whole isotopes field, and for their interest in this work. Furthermore, he thanks Dr. D. L. Christman (Brookhaven National Laboratory) for his practical advice on the experimental tritium work and the Smith Kline French Foundation for a grant to the College of Pharmacy for the purchase of a liquid scintillation counter. (7) R. P. Bell, “The Proton in Chemistry,” Cornel1 University Press, Ithaca, N. Y.,1959. (8) C. G. Swain, E. C. Stivers, J. F. Reuwer, and L. J. Schaad, J. Am. Chem. SOC.,80, 5888 (1958). (9) J. Bigeleisen, “Tritium in the Physical and Biological Sciences,” Vol. I , International Atomic Energy Agency, Vienna, 1962,p 161. (10) R. P. Bell, J. A. Fendley, and J. R. Hulett, Proc. Roy. SOC. (London), A23.5, 453 (1956). (11) L. Funderburk and E. S. Lewis, J . Am. Chem. Soc., 86, 2531 (1964).
Volume 70,Number 8 August 1966
