Mechanism of
Hydrogen Exchange
299
Reaction
Microwave! S iidy of the Mechanism of Hydrogen Exchange Reaction between Propylene and p-Toluenesulfonic Acid
Toshlhiko Kondo,* Masaru Ichikawa, Shuji Saito, Sagami Chemical Research Center, Nishiohnuma, Sagamihara-shi, Kanaga wa. Japan
and Kenzi Tamaru Wep,srtrnentof Chemistry. The University of Tokyo. Hongo. Bunkyo-ku, Tokyo. Japan
(Received June 79, 1972)
Publication costs assisted b y the Sagami Chemical Research Center
During the course of the hydrogen exchange reaction between propylene and deuterated p-toluenesulfonic acid, the distribution of the propylene-& and -4species was studied by microwave spectroscopic technique. The results demonstrated that the intermediate of the exchange reaction (or isomerization) is the isopropyl carbonium ion, and neither n-propyl carbonium ion, protonated cyclopropane, nor a push-pull mechanism is involved.
Many investagations have been carried out on carbonium ion intermediates.' In the hydrogen exchange reaction between projpylene and deuterium (deuteron), many possible mechanisms may be considered: n-propyl ( C H ~ C H ~ C H Z )isopropyl ,~ ( C H Z C H C H ~ ) , n-propenyl ~ (CH3CH=CM) ,2 isopropenyl (CHzC=CHz), a-allyl (CH2CH=CH2), x-allyl (CH2.2CHo2CH2)3,4 intermediate mechanisms, as well as concerted (push-pull) mechanism.5 Microwave spectroscopic analysis of the distributions of the propylene-& and -d2 species formed during the course of the reaction makes it feasible to elucidate the mechanism of hydrogen exchange reaction of propylene. The deuterated p-toluenesulfonic acid (PTS) on silica gel was prepared and treated as described by Sakurai, et al.,6 and propylene (23.3 cm pressure) was allowed to pass over the deuterated F'TS (16 mmol) at 20" in a closed circulating system (ea. 350 ml). The deuteron of the PTS slowly attacked propylene and about 5 and 34% of the propylene was transformed into the d l species in 100 and 845 min, respectively. The results of the microwave analyses are shown in Table 1. The propylene-2-dl (C&CD=CH2) was not detected throughout the exchange reaction, thus excluding the possibility that the n-propyl or isopropenyl species is the reaction intermediate, since these intermediates should yield propylene-2-dl alone. The absence of propylene-2-dl also excludes the possibility that intramolecular hydride shift, cation .center migration, or protonated cyclopropane formation1 Lakes place during the reaction. It is demonstrated that the distribution of the d l species remained constant during the reaction and was nearly equal to that expected from the isopropyl intermediate mechanism; the isopropyl intermediate, GH~CCHZD,formed by the addition of a deuterium atom (ion) to propylene, rearranges to the propylene-d1 species by dissociation of a hydrogen atom (ion) from one of the two methyl groups, The ratio of the dl species formed should be propylene-3-dl (CH2DCH=CH2) 60%, cis-propylene- I d l (em-CH,$H=CHD) 20%, and trans-propylene-l-di (trans-CW&H=CHD) 20%, according to an equal probability of the dissociation of each hydrogen atom of CH:%and GH2r), if no secondary kinetic isotope effect is taken into consideration. It is important for support
of this mechanism that the ratio mentioned above does not vary throughout the course of the reaction. It is in sharp contrast with the case of a concerted mechanism and a 7r-allyl intermediate mechanism, where the statistical distribution of dl species should be the same as the isopropyl intermediate, but the initial product should be propylene-3-dl a 1 0 n e . ~Thus ~ ~ the results for the d l species clearly dem, onstrate the isopropyl intermediate mechanism (isopropyl carbonium ion) and exclude the concerted mechanism, the x-allyl mechanism, and others. In this reaction, therefore, p-toluenesulfonic acid cannot be a neutral tautomeric ~ a t a l y s tThese .~ results are also in good agreement with those of ab initio molecular orbital calculations of nine C&l7+ cations by Radom, et a1.,8 who showed that the most stable cation among them is a isopropyl carbonium ion and that the n-propyl carbonium ion and the "nonclassical7' carbonium ions have energies higher by 17 kcal/ mol and more. The figures given in parentheses in Table I are those calculated9 for the d2 species on the assumption that the exchange of the second hydrogen atom follows the same mechanism, neglecting kinetic isotope effects. The calculated values are roughly realized by the observed results, though there are observed fairly large deviations. The amounts of propylene-1, I-d2 (CH3CH=CD2) and propyl(1) For recent reviews, see ( a ) C,'J. Collins, Accounts Chem. Res.. 4, 315 (1971); Chem. Rev., 69, 543 (1969); (b) C. C. Lee, Progr. Phys. Org. Chem., 7, 129 (1970); (c) J. L, Fry and G. J. Karabatsos in "Carbonium Ions," Vol. 2, G. A. Olah and P. v. R. Schleyer, Ed., Wiley-Interscience, New York, N. Y., 1970. (2) T. Ueda, J . Hara, K. Hirota, S. Teratani, and N. Yoshida, 2. Phys. Chem. (Frankfurt am Main), 64, 64 (1969) (3) T. Kondo, M. Ichikawa, S. Saito, and K. Tamaru, Buil. Chem. SOC. Jap., 45, 1580 (1972). (4) S. Naito, T. Kondo, M. Ichikawa, and K. Tamaru, J , Piiys. Chern., 76, 2184 (1972). (5) (a) D. M. Brouwer, J. Catai., 1, 22 (1962); (h) C. G . Swain and J. F. Brown, Jr.*J. Amer. Chem. SOC.,74, 2538 (1952). (6) Y. Sakurai, Y. Kaneda, S. Kondo, E. Hirota, T. Onishi, and K. l a m aru, Trans. Faraday Soc., 67, 3275 (1971). (7) P. R. Rony, J. Amer. Chem. SOC., 90, 2824 (1968); 91, 6090
(1969). (8) L. Radom, J. A. Pople, V. Buss, and P. v. R. Schleyer, J . Amer. Chem. SOC.,94, 311 (1972). (9) Y. Marino and E. Hirota, J. Chern. SOC. Jap., Pure Chem. Sect., 85,
535 (1964). The Journalof Physical Chemistry, Voi. 77, No. 2, 1973
Kondo, Ichikawa, Saito, and Tarnaru
300
TABLE I: Distribution of Propylene-& and -d2 Species during the Hydrogen Exchange Reaction between Propylene aad Deuterated p-Toluenesulfonic Acid at 20"
Reaction time, iniri cis-Proplyene- 7-d;, % trans-Proplyene- 7-dl Proplyene-3-dl Proplyene-2-dr
11 16.4 17.3
66.3 0
57 16.1 19.4 64.5 0
161 16.9
I 7.8 65.3 eo
Propylene- 7, l-P2, YO
b
0 11.0(7.4) a 20.2(28.0) 24.9(28.5) 43.9(36.1)
cis-Proplyene- 7 , 3 4 2 trans-Proplyene- 7,3-d2 Proplyene-3,3-(.'2 Proplyene-2,3-c'z,etc.
**%,
845 17.0 17.3 65.7
0.20
aThe figures in parentheses are calculated values.
0.70
1.43
0 14.4(7.8) 19.5(29.0) 24.9(28.2) 41.2(35.0)
0 (0) 8.6
10 days 17.7
I 8.4
63.9 C 8.4 ( 8.2) 25.0(28.3) 32.8 (29.4) 33.8(34,1)
0 (0) 27.1
Mean deuterium content of propylene is given by the equation P ' = 100(~6~1=i!di/6Z6i=~d~)
ene-3,3-d2 (CHD&N=CH2) were more than the calculated values, but the amount of propylene-1,3-d~ (CHzDCH==CHD) was a little less, which suggests that a hydrogen bound on the carbon atom to which a deuterium is attached is slightly more reactive for the exchange reaction due to the kinetic isotope effect. However, the results obtained aftei a long reaction time (10 days) were in reasonable agreement with the calculated values, since it is considered that the microwave analysis was performed more accurately for the case of a higher concentration of
The Journal of Physical Chemistry, Vol. 77, No. 2, 1973
0 (0) 6.9
1500 17.9 17.4 64.7
dz species. Therefore, the propylene-d2 species are also produced through the isopropyl intermediate. The isopropyl carbonium ion intermediate mechanism was consequently demonstrated by the new approach to arialyze not only propylene-& but also -d2 by the microwave spectroscopic technique during the hydrogen exchange reaction between propylene and deuterated PTS, which is in good agreement with the kinetic and microwave results obtained in the n-butene isomerization over deuterated PTS.6
