Isotope effects in nucleophilic substitution reactions. VI. The effect of ion pairing on the transition state structure of S_N2 reactions

Abstract

Spectroscopic and conductivity studies of sodium thiophenoxide solutions in four different solvents and the secondary a-deuterium kinetic isotope effects found in the presence of 15-crown-5 ether demonstrate that the secondary a-deuterium kinetic isotope effect and transition state structure for the SN2 reaction between sodium thiophenoxide and n-butyl chloride are significantly different, depending on whether the ionic reactant is a solvent-separated ion-pair complex or a free ion. In all three solvents in which the form of the ionic reactant changes, a smaller isotope effect and tighter transition state are found for the reaction with the ion-pair complex.KENNETH CHARLES WESTAWAY et ZHU-GEN LAI. Can. J. Chem. 66, 1263 (1988).On a rkalisk des Ctudes basCes sur la spectroscopic ou sur la conductivitk de solutions de thiophknolate de sodium dans quatre solvants diffCrents et on a mesurC des effets isotopiques cinCtiques secondaires des deuterium en a en prksence d'kther 15-couronne-5; I'ensemble de ces rksultats dCmontre que l'effet isotopique cinCtique secondaire du deuterium en a ainsi que la structure de l'ktat de transition de la rkaction SN2 du thiophCnolate de sodium avec le chlorure de n-butyle sont assez diffkrentes suivant qu'il s'agit d'une paire ionique complexe skparke par un solvant ou d'un ion libre. Dans les trois solvants oh il se produit des changements dans la forme des rkactifs ioniques, la paire ionique complexe conduit i un plus petit effet isotopique et i un Ctat de transition plus compact.[Traduit par la revue]

Isotope effects in nucleophilic substitution reactions. VII. The effect of ion pairing on the substituent effects on SN2 transition state structure

“…The identical Hammett p values for the two types of reactions show that this technique is not useful in distinguishing between the lengths of the a-carbon-sulfur bonds in the free ion and solvent-separated ion pair complex transition states (1).…”

“…[2] (~a ' ) -s -@ Z + c~H~cL~&(cH,)~c~H, NO,-+ L = H , D Z = CH30, CH3, H, and Cl Obviously, the first requirement for this study of the effect of ion pairing on substituent effects was to demonstrate that the form of the reacting thiophenoxide ion could be changed from a free ion to a solvent-separated ion pair complex under the reaction conditions. Several results indicate that sodium thiophenoxide exists as ion pairs in methanol at high concentrations (1). First, the slope of the conductance versus concentration plot decreases sharply at a sodium thiophenoxide concentration of approximately 2 X M. In addition, there is a rapid change in the uv spectrum when the sodium thiophenoxide concentration is 1 X M. Finally, the addition of 15-crown-5 ether causes the same changes as dilution in the uv spectrum of sodium thiophenoxide solutions.…”

“…secondary a-deuterium kinetic isotope effects with concentration (1) indicate that the solvent-separated ion pair must exist as a complex consisting of several solvent-separated sodium thiophenoxide ion pairs, eq. [3].…”

“…[I], changes the secondary a-deuterium kinetic isotope effect from 1.034 t. 0.003 to 1.118 +. 0.009 (1). In fact, this change in the form of the reacting nucleophile seems to be a general phenomenon because the secondary a-deuterium kinetic isotope effect changes in at least three other SN2 reactions when the form of the nucleophile is altered ( I ) .~ These results clearly demonstrate that the magnitude of the secondary a-deuterium kinetic isotope effect and transition state structure can be strongly dependent on the form of the reacting anion in an SN2 reaction.…”

“…Recent work by Westaway and Lai (1) has shown that the form of the reacting anion has a significant effect on the magnitude of the secondary a-deuterium kinetic isotope effect and transition state structure of an SN2 reaction. For example, changing the form of the reacting anion from a solventseparated ion pair complex (ref.…”