Carbanion mechanisms. 9. Spectrophotometric study of triphenylmethyl alkali metal salts. Contact and solvent-separated ion pairs in ethereal solvents

. Can. J. Chem. 57,999 (1979). A spectrophotometric study of diphenylmethyllithium (DPM-Li+) and diphenylmethylpotassjum WPM-K+) in ethereal solvents has yielded information on ion pairing and solvation phenomena in these carbanion systems. Different spectral absorptions are observed, characteristic of two types of contact ion pairs (unsolvated and partially solvated) as well as the solvent separated ion pair species, on varying the cation and solvent. This contrasts with our previous observations with triphenylmethyl alkali metal salts where only contact and solvent separated ion pairs were observed. The effect of 18-crown-6 polyether and the effect of temperature changes on the ion pairing equilibria are evaluated. Thermodynamic parameters are obtained for equilibria pertaining to the DPM-Li+/THF and DPM-Li+/DME systems. The results are discussed in relation to literature reports on ion pairing in these systems as derived from nmr studies. Comparison with triphenylmethyl alkali metal salts yields information relating to delocalization and steric effects on ion pairing. E. BUNCEL, B. C. MENON et J. P. COLPA. Can. J. Chem. 57,999 (1979). Une etude spectrophotomktrique du diphenylmkthyllithium (DPM-Li+) et du diphknylmkthylpotassium (DPM-K+) dans des solvants kthkres a fournit des informations concernant les paires d'ions et la solvatation dans ces systemes carbanioniques. Lorsqu'on a fait varier le cation et le solvant, on a observe diverses absorbtions spectrales qui sont caracteristiques de deux types de paires d'ions de contact (non-solvatk et partiellement solvate) de m2me que des paires d'ions skparks par le solvant. Ces resultats different de ceux obtenus au cours de nos travaux antkrieurs avec les sels triphenylmkthyles de mktaux alcalins alors qu'on avait observe uniquement la prksence de paires d'ions de contact ou skparks par le solvant. On a Bvaluk I'effet de polykther-6 couronne-18 et de changements de temperature sur les tquilibres des paires d'ion. On a obtenu des paramktres thermodynamiques pour l'kquilibre relatif aux systemes DMP-Li+/THF et DMP-Li+/DME. On discute des resultats en fonction de rapports, parus dans la litterature, concernant les paires d'ions preseiltes dans ces systemes qui sont basks sur des etudes rmn. Une comparaison avec des sels triphknylmethyles de metaux alcalins fournit des informations concernant les effets stbiques et la dklocalisation sur le pairage des ions.[Traduit par le journal]In our study of triphenylmethyllithium (TPM-Li') parison of our results. The necessity of investigating and triphenylmethylpotassium (TPM-K') in ether-ion-pairing phenomena by different experimental eal solvents by uv-visible spectroscopy, it was found techniques has been emphasized (5). An earlier that these carbanion alkali metal salts showed spectrophotometric study (6) of 1,l-diphenylhexylspectral behaviour characteristic of contact or lithium in THF or di-n-propyl ether and benzene solvent-separated ion pairs, depending on the nature or hexane mixtures was interpreted in terms of of the...

Section: Discussionmentioning

confidence: 55%

“…This solvent replacement was performed at least twice in each case. The spectrophotometric measurements were performed as previously (1).…”

Section: Methodsmentioning

confidence: 99%

mentioning

confidence: 99%

See 1 more Smart Citation

Spectrophotometric study of ion pairing in diphenylmethyl alkali metals salts

Buncel¹,

Menon²,

Colpa³

1979

“…Conductivity measurements (3), nrnr spectroscopy (4,5), and uv spectroscopy (6,7) have been used to study the phenomenon of ion pairing. Two of these techniques, uv 'Author to whom correspondence may be addressed.…”

Section: Proof Of An Ion Pair and Afree Ion Reactionmentioning

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

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]

“…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.