3751
Orgunic and Biologicd Chemistry Chemical Reactions under High Pressure. XIII. The Activation Volumes of Some Solvolysis Reactions' W. J. le Noble, B. L. Yates, and A. W. Scaplehorn Contribution from the Department of Chemistry, S t a t e University of New York, S t o n y Brook, New York 11790. Received M a r c h 8,1967
Abstract: The activation volumes of exo- and endo-2-norbornyl brosylate have been measured; the former is less negative by several cubic centimeters/mole. This is interpreted in terms of the Drude-Nernst equation for electrostriction. The observation is consistent with the assumption that the transition state of the exo-brosylate is characterized by charge delocalization. The same conclusion applies t o endo-3-cis-bicyclo[3.1 .O]hexyl tosylate when this is compared to its exo isomer. The activation volumes of several monocyclic tosylates (C4-C8)were also measured; the pressure acceleration for C4is somewhat smaller than that for the others. The pressure effect o n the solvolysis rate of anti-7-norbornenyl tosylate is larger than that of any of the other substrates; it is suggested that this is a n indication of r-electron participation. This is further supported by the fact that among several benzenesulfonate esters differing only in their ability to form symmetrical transition states through x participation, those substrates able to d o so have the more negative activation volumes. t was shown in a recent communication2 that the solvolysis of e x o - 2 - n o r b o r n y l b r o s y l a t e is c h a r a c terized by a smaller a c t i v a t i o n volume than those of its endo e p i m e r and of cyclopentyl brosylate. An argument was made that this is consistent with the controversial idea that p a r t i c i p a t i o n by a n e i g h b o r i n g u bond is o c c u r r i n g in the transition state of the exo-brosylate. This participation would presumably disperse the incipient positive c h a r g e between t h e C1 and Cz carbon
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atoms.
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The diffuse charge is t h o u g h t to be less effective than a localized c h a r g e in a t t r a c t i n g the surrounding solvent, a n d hence volume d i m i n u t i o n is less p r o n o u n c e d than it w o u l d otherwise be. Since the effect is s m a l l and may conceivably be m a s k e d by such u n k n o w n , coincid e n t a l f a c t o r s as t h e p a c k i n g efficiency of solvent molecules a r o u n d t h e substrates, it w a s clearly desirable to look for f u r t h e r examples. We r e p o r t here in addition t h e effect of h y d r o s t a t i c pressure on t h e solvolysis rates of exo- a n d endo-3-ci.r-bicyclo[3.1 .O]hexyl tosylates, of a n u m b e r of cycloalkyl tosylates (C,-C,), of anti-7n o r b o r n e n y l tosylate, a n d of several p a i r s of s a t u r a t e d and u n s a t u r a t e d nitrosylates, and f u r t h e r discuss the p r e m i s e of this m e t h o d . Experimental Section Materials. Acetone was treated with potassium permanganate and Drierite at room temperature for 2 days and fractionated through a 50-cm Fenske column filled with glass he lice^.^ The exo-2-norborneol, cyclopentanol, cyclohexanol, cycloheptanol, cyclooctanol, cholesterol, and 5-a-cholestan-3P-01used were com(1) Presented in part a t the 150th National Meeting of the American Chemical Society, Atlantic City, N. J., Sept 1965. (2) W. J. le Noble and B. L. Yates, J . A m . Chem. SOC.,87, 3515 (1965). (3) J. I C1 > Br) to charge dispersal into empty d orbitals. Elsewhere, Brower observed a pressure-induced shift in the equilibrium constant for the hydrolysis of PtC14?to PtC13(H20)- in the direction of the former, and attributed this to charge concentration. Gonikberg, et invoked this factor to account for the solvent dependence of the activation volume of the exchange of radioiodide with n-propyl iodide, and Hamann used (29) (a) L. G. Hepler, J . Phj's. Chent., 61, 1426 (1957); (b) S . D. Hamann, work cited in ref 23; (c) P. Mukerjee, J. P ~ J sChem., . 65, 740, 744 (1961); (d) S . W. Benson and C. S . Copeland, ibid., 67, 1194 (1963). However, Laidler has proposed a linear dependence: A. M. Couture and I
