J. zyxwvutsrq Am. Chem. Soc. for multiple proton exchanges during an encounter. Multiple exchange during an encounter has been observed for several proton-exchange r e a c t i ~ n s , ~ ' ~ , ~ - ~ but our data do not permit a decision as to whether the observed diminution of selectivity arises from incomplete equilibration of the isomeric bases or from multiple proton exchanges during the encounter. We consider that the observation of positional selectivity in this exchange reaction is strong evidence for Scheme zyxwvut I1 and for the Swain-Grunwald mechanism. This mechanism is an attractive one, which must occur, and it has been widely accepted. However, the chief evidence for it is the inhibition by acid of the proton exchange of ammonium ion^,'^^^^ and this inhibition is seen only in nonideal solutions. Nonideality is inherent, since >0.1 M H+ is required in order that reprotonation of the intermediate be competitive with a process whose rate constant is above lo9 zyxwvuts s-l. Yet inhibition by acid is a thermodynamic necessity in such nonideal solutions. It is readily shown that the simplest mecha- nism, without kH, leads to an observed first-order rate constant for exchange given by where k2 is the second-order rate constant for encounter-controlled reprotonation of amine A by H', and ho is the acidity function governing protonation of A. Since ho increases faster than [H'] in strong acid, kOMmust decrease in acid, and since [H']/h, is a strong function of water activity,39a 500-fold decrease in kow is consistent with a 7.4-fold decrease in water activity, despite a denial.38a Moreover, there is an implicit assumption that k2 is independent of acidity, whereas proton mobility does decrease at high concentrations.22 Thus nonideality taints the evidence for (38) (a) Emerson, M. T.; Grunwald, E.; Kaplan, M. L.; Kromhout, R. A. J. Am. Chem. zyxwvutsrqpon Sot. 1960,82,6307. (b) Sheinblatt, M.; Gutowsky, H. S. Ibid. 1964,86,4814. (c) Grunwald, E.; Ralph, E. K., zyxwvutsrqp 111 Ibid. 1967,89, 4405. (d) Cocivera, M. J. Phys. Chem. 1968, 72, 2515. (e) Grunwald, E.; Lipnick, R. L.; Ralph, E. K. J. Am. Chem. Sot. 1969, 91, 4333. (39) Perrin, C. J. Am. Chem. Sot. 1964, 86, 256. Robertson, E. B.; Dunford, H. B. Ibid. 1964, 86, 5080. 1982, zyxwvu 104, 201-206 20 1 the Swain-Grunwald mechanism. Our results are also obtained in nonideal solutions, but all comparisons are intramolecular, and the only uncertainty arising from nonideality is in the second-order rate constants, whose values are immaterial so long as they are accepted as being in the range of encounter control. Thus these results are an independent confirmation of the existence of the Swain-Grunwald mechanism. Conclusions and Summary To the best of our knowledge, here is the first demonstration of positional selectivity in an encounter-controlled proton exchange. These proton exchanges are so favorable thermodynamically that they are expected to be encounter controlled, and the second-order rate constants support this. However, positional selectivity forces us to conclude that the reaction is not completely encounter controlled. Even though hydroxide is a sufficiently strong base to remove all amidinium protons upon encounter, so that their acidity should be immaterial, it is always the most acidic proton that exchanges fastest. We therefore conclude that the Swain- Grunwald mechanism is operative and that the rate-limiting step is in part the breaking of a hydrogen bond in the amidine hydrate. Simulation, using reasonable rate constants, shows that the ob- served selectivity is consistent with this mechanism. We therefore conclude that these results represent independent evidence for the Swain-Grunwald mechanism. Acknowledgment. This research was supported by National Science Foundation Grants CHE76-02408 and CHE78-12256. The NMR facilities had been supported by National Institutes of Health Grant RR-708. We are grateful to Dr. Eric R. Johnston for assistance with FT-NMR studies of exchange in D 2 0 and with homonuclear decoupling zyxwv . Registry No. 1 (R = H), 50676-76-1; 1 (R = CH,), 52018-42-5; 1 (R = Ph), 53356-58-4; 1 (R = CH,N+CC(CH,)2N=NC(CH,)2), 79246- 17-6; 2 (R = CH,), 79734-87-5; 2 (R = Ph), 79734-88-6; 3 zy (n = 5), 79734-91-1. 79734-89-7; 3 (n = 7), 79734-90-0; N,N-dimethylacetamidinium ion, Displacement Stereochemistry and Product-Formation Selectivities in the Solvolysis of Cyclooctyl p-Bromobenzenesulfonate J. Eric Nordlander,* Philip 0. Owuor, Donna J. Cabral, and Jerome E. Haky Contribution from the Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 441 06. Received May 18, 1981 Abstract: Configurational analysis by 2H NMR of the products of solvolysis of (E)-cyclooctyl-2-d and (E)-cyclooctyl-4-d brosylate in acetic acid and 80% acetone has established that substitution without rearrangement occurs with complete retention of configuration while substitution under 1 $hydride shift takes place with complete inversion at the migration origin. The reaction is concluded to proceed by direct initial formation of a 1,5-hydrogen-bridged cation. Solvolysis of cyclooctyl-I-dbrosylate in several solvents has shown elimination to be favored from the C-1 over the C-5 side, whereas selectivities for competitive substitutions are similar at the two positions. Elimination is thus indicated to take place largely from first-formed tight ion pairs while displacement proceeds through more dissociated intermediates. The cyclooctyl and other medium-ring systems are of distinctive importance to solvolysis theory by reason of their characteristic rearrangements under transannular hydride shift.' Cope and Gale2 determined that net 1,5-hydride migration occurs to the (1) Reviews: Sicher, J. Prog. Stereochem. 1962, 3, 202. Prelog, V.; Traynham, J. G . In 'Molecular Rearrangements", de Mayo, P., Ed.; Wiley- Interscience: New York, 1963; Part 1, Chapter 9. Cope, A. C.; Martin, M. M.; McKervey, M . A. Q. Rev., Chem. Sot. 1966, 20, 119. 0002-7863/82/1504-0201$01.25/0 extent of zyxwvut 53%, 60%, and 162% on solvolysis of cyclooctyl- 1 ,2,2,8,8-d5 brosylate in acetic, formic, and trifluoroacetic acid, respectively. Regioalternative rearrangements were found to be negligible.* Parker and Watt3 synthesized the cis- and trans- cyclooctyl-5-d brosylates and deduced a 1O:l preference for transposition of a trans- over a cis-5-hydrogen in acetolysis. (2) Cope, A. C.; Gale, D, M. J. Am. Chem. Sot. 1963, 85, 3741. (3) Parker, W.; Watt, C. I. F. J. Chem. Sot., Perkin Tram. 2 1975, 1647. 0 1982 American Chemical Society