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SOLVENT ISOTOPEEFFECTON HYDRATION RATEOF OLFEINS
Nov. 20, 1030
ORGANIC AND BIOLOGICAL CHEMISTRY [CONTRIBUTION FROM THE COLLEGE O F CHEMISTRY AND PHYSICS,
THEPENNSYLVANIA STATE UNIVERSITY]
Evidence of r-Complex Formation from the Solvent Isotope Effect on the Rate of Hydration of Olefins1 BY E. LEE PURLEE~ AND ROBERT W. TAFT, JR. RECEIVED M A Y 22, 1950 The effect of deuterium in the solvent on the aqueous acid-catalyzed rates of hydration of 2-methyl-2-butene and l-niethyl1-cyclopentene has been determined. The results provide evidence that a facile proton transfer from hydronium ion to olefin Drecedes the rate-determining steD. Since it has been shown that the conjugate acid formed cannot be a carbonium ion, t6e existence of a *-complex isimplied.
Ciapetta and Kilpatrick using the chloroacetic sample of pure 2-methyl-2-butene, (B) is that for acids made a determined effort to detect general a sample removed after the first half-life of a hyacid catalysis in the hydration of isobutylene by dration experiment in 50% deuterated water; (C) aqueous acids. Only specific hydronium ion caial- is that for a sample of deuterated 2-methyl-2-buysis was found. This result suggests, though it does tene.’ not require, that the hydration mechanism involves a reversible proton transfer from hydronium ion to isobutylene preceding the rate-determining step. On the other hand, Levy, Taft and Hamniett have shown that olefin removed after 50% of the original has reacted contains no isomer in the hydration of either 2-methyl-2-butene or 2-methyl-1-butene.4 This result excludes a reaction sequence in which carbonium ion is formed in a preliminary proton transfer, since these two olefins lead to the same carbonium ion (with all a-hydrogen atoms equivalent), which equilibrates a t 25’ to give 89% 2methyl-2-butene and 11% 2-methyl-1-butene.5 We have obtained the equivalent result by carrying out the hydration of 2-methyl-Zbutene by one molar nitric acid in SOYo deuterated water. If a carbonium ion were formed preceding the rate-determining step it could be detected readily by the \ appearance of deuterium in the unreacted olefin B during the course of the hydration reaction. The v elimination of proton is substantially favored over that of the deuteron in the reverse step, both statistically (7 to 1) and kinetically (approximately 2 to 1 ) . 6 D H ___f
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H3C
D
The olefin removed after one-half time of the reaction does not contain a detectable amount of deuterium. In Fig. 1, (A) is the infrared spectrum of a
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W A V E LENGTH I N MICRONS,
Fig. 1.-Infrared spectra: (A) pure 2-methyl-2-butene; (B) olefin sample removed after the first half-life of the hydration of 2-methyl-2-butene in 50% deuterated water; (C) sample of deuterated 2-methyl-2-butene.
(1) The work reported herein was carried out on Project h-R055-295 between the office of Naval Research and The Pennsylvania State L‘niversity. Reproduction in whole or in part permitted fur any purpose of the United States Government. (2) Allied Chemical and Dye Corp. Fellow for 1953-1934; taken One is clearly faced with only two alternatives: in part from the P h . D . Thesis of E. Lee Purlee, The Pennsylvania State either the reaction involves a rate-determining proUniversity, June, 1954. ton transfer with a large Bronsted a (so that the (3) F. G . Ciapetta and R?. Kilpatrick, THIS JOURNAL, 70, 639 (1948). only detectable catalyst is hydronium ion), or there (4) J. B. Levy, R . W. T a f t , Jr., and L. P. Hammett, THISJOURNAL, is a preliminary proton transfer to give a conjugate 76, 1253 (1953). acid of the olefin which loses in the reverse step the (5) F . D. Rossini, K . S. Pitzer. e l al., “Selected Values of Physical and Thermodynamic Properties of Hydrocarbons,” Carnegie Press, (7) Prepared by shaking the olefin with acidified DzO for an ex1953, p. 475. tended period under conditions such that the hydration reaction is (0) J. V. Shiner, THISJOURNAL. 7 6 , 2927 (1053); cf. also ref. 13. appreciably reversible (al)oiit 50’); r f ref. 13.
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E.LEE PURLER AND ROBERT W. TAFT, JR.
X3OS
same proton added, i.e., not all of the a-hydrogen atoms are equivalent in this conjugate acid. In order to distinguish between these two alternatives we have determined the effect of D 2 0 content of the solvent on the rate of hydration of two olefins, 2-methyl-2-butene and l-methyl-l-cyclopentene. According to generally accepted theory of the isotope effect, a rate-determining proton transfer is retarded by replacing H30' by D30+.8 .In example is provided by the mutarotation of glucose for which the hydrogen-ion catalytic constant is 307, less in DsO than H20.9 On the other hand, the process involving the proton transfer preceding the rate-determining step is generally facilitated by 200 to 300% in DzO relative to H2O. This factor is attributed primarily to the greater acid strength of D 3 0+. following Gross, loC has suggested that a safer criterion for distinguishing between these two mechanisms is the following : The rate-determining proton transfer process should be retarded linearly with the mole fraction of deuterium, n, in the solvent, as Hamill and LaMer have observed for the mutarotation of g l ~ c o s e . ~The solvent isotope effect (either facilitating or retarding) for the preliminary proton transfer mechanism should follow a relationship (non-linear in n) derived by Butler and his students based essentially upon the relative acidities of H 3 0 + and D30+." That is, according to this approach, a rate greater in DzO than in H20 is not the necessary criterion of a preliminary protCJn transfer, but instead it is the correlation of the solvent isotope effect by Butler's equation. lXre have applied both criteria in an attempt t o elucidate the olefin hydration mechanism. Results The rates of hydration a t unit pressure of gaseous 2-methyl-2-butene by 0.973 ill nitric acid of varying D20-H20 content content were measured at 30.00°. The results are given in Table I. The experimental procedure used has been previously described.l1
The rates of hydration a t unit pressure of gaseous 1-methyl-1-cyclopenteneby 0.0909 M nitric acid of varying DzO-HzO content at 35.20' have been obtained from measurements on the rate of the reversible dehydration of I-methyl-1-cyclopentanol. The general experimental procedure and equations have been described earlier.I2 The modification i t i procedure necessary to prevent the results froni being affected by deuteration of the carbinol is described in a subsequent paper.13 The results arc given in Table 11. TABLE I1 HYDRATION OF GASEOUS 1METHYL-~-CYCLOPESTENE BY 0.0909 A i NITRIC ACID OF VARYISGIJz0-H20 COSTENTAT 35.20' IN UNITSOF 1 0 6 MOLES-L.-'-MIS. '
RATECOSSTANTS,kp,
FOR THE
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0.000 299 ,423 ,514 ,650 ,748 ,872 ,903 937 I 000
I 1.7(i 11.69 12.29 12.76 12.98 13.60 13.50 13.40 13.01 1 2 . 7 (extrap.)
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