Spectrophotometric study of ion pairing in diphenylmethyl alkali metals salts

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]

Section: Proof Of An Ion Pair and Afree Ion Reactionmentioning

confidence: 99%

“…One way of determining the type of ion pair is to determine how a change in the cation affects the uv spectrum of the ion pair (6,7). If a contact ion pair is present, a change in cation, e.g., from lithium to cesium, will cause a large shift in the A, , , of the ion pair.…”

Section: Type Of Ion Pairmentioning

confidence: 99%

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

Westaway¹,

Lai²

1988

“…Buncel et al (4,5) used a spectroscopic test to determine whether an ion pair exists in solution as a contact ion pair or as a solvent-separated ion pair. The test involves determining how changing the metal ion in an ion pair affects the absorption maximum in the uv spectrum.…”

Section: Introductionmentioning

confidence: 99%

Isotope effects in nucleophilic substitution reactions. VIII. The effect of the form of the reacting nucleophile on the transition state structure of an S_N2 reaction

Fang¹,

Westaway²

1991

YAO-REN FANG and KENNETH CHARLES WESTAWAY. Can. J. Chem. 69, 1017Chem. 69, (1991. A spectroscopic investigation indicated that lithium thiophenoxide exists as a contact ion pair complex in dry diglyme whereas the other alkali metal thiophenoxides exist as a solvent-separated ion pair complex in diglyme. The addition of small amounts of water converts the lithium thiophenoxide contact ion pair complex into a solvent-separated ion pair complex. A smaller secondary a-deuterium kinetic isotope effect and a larger Hammett p value are observed when the nucleophile is the contact ion pair complex in the SN2 reaction between n-butyl chloride and thiophenoxide ion in diglyme. This indicates that the transition state for the contact ion pair complex reaction is tighter with a shorter nucleophile-a-carbon bond than the transition state for the solvent-separated ion pair complex reaction. The secondary a-deuterium kinetic isotope effects for the free ion and the solvent-separated ion pair complex reactions between sodium thiophenoxide and n-butyl chloride in DMF suggest that the loosest transition state is found when the nucleophile is the free ion.

“…The cation independent A, , , of the uv spectra of lithium, sodium, and cesium thiophenoxide solutions, at concentrations where the salts exist as ion pairs in methanol, indicates that the nucleophilic reactant is a solvent-separated ion pair (7). Finally, the extremely sharp change in the uv spectrum and in the Can.…”

Section: Resultsmentioning

confidence: 96%

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

Lai¹,

Westaway²

1989

ZHU-GEN LAI and KENNETH CHARLES WESTAWAY. Can. J. Chem. 67, 21 (1989).The secondary a-deuterium kinetic isotope effects and substituent effect found in the SN2 reactions between a series of para-substituted sodium thiophenoxides and benzyldimethylphenylammonium ion are significantly larger when the reacting nucleophile is a free ion than when it is a solvent-separated ion pair complex. Tighter transition states are found when a poorer nucleophile is used in both the free ion and ion pair reactions. Also, the transition states for all but one substituent are tighter for the reactions with the solvent-separated ion pair complex than with the free ion. Hammett p values found by changing the substituent on the nucleophile do not appear to be useful for determining the length of the sulfur-a-carbon bond in the ion pair and free ion transition states. On a trouvt que, lorsque le nucltophile est un ion libre, les effets isotopiques cinttiques secondaires des atomes deuterium en a ainsi que I'effet de substituant qui ont Ctt observes lors de reactions SN2 d'une strie de thioph6nolates de sodium substituts en para avec l'ion benzyldimtthylphtnylammonium sont beaucoup plus importants que lorsque ce nuclCophile est une paire d'ions complexe stparte par le solvant. On a trouvt que les ttats de transition sont beaucoup plus compacts lorsqu'on utilise un mauvais nuclCophile tant dans des reactions d'ions libres que dans des rtactions de paires d'ions. Par comparaison avec les rtactions se produisant avec I'ion libre, les ttats de transition de tous les substituants, 2 I'exception d'un, sont plus compacts pour les reactions avec la paire d'ions complexe stparte par le solvant. I1 ne semble pas que les valeurs p de Hammett que l'on trouve en faisant varier la nature du substituant attach6 au nucleophile soient utiles pour determiner la longueur de la liaison entre le soufre et le carbone en a , tant dans la paire d'ions que dans les etats de transition de l'ion libre.Mots cl6s : Effets isotopiques, couplage ionique, substitution nucltophile, rtactions SN2, ttats de transition.[Traduit par la revue] Introduction 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. 1, vide infra) to a free ion in the SN2 reaction between n-butyl chloride and sodium thiophenoxide in DMSO at 20°C, eq. [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 struct...