Effects of deuterium substitution on the rates of organic reactions. XI. .alpha.- and .beta.-Deuterium effects on the solvolysis rates of a series of substituted 1-phenylethyl halides

Kinetic chlorine isotope effects attending the solvolysis of several ring-substituted 1-phenylethyl chlorides in alcohol-water solvent mixtures are reported. The k35/k37 values are insensitive to the identity of ring substituents and to solvent composition. Results are interpreted in terms of an SN1 heterolytic process incorporating a significant amount of internal return. Theoretical calculations suggest that the incipient chloride ion in the transition state may be strongly hydrogen-bonded. On a mesurt les effets isotopiques cinttiques du chlore qui accompagnent les solvolyses de plusieurs chlorures de phtnyl-1 Cthyle substituts sur le cycle. Les valeurs de k35/k37 sont insensibles h la nature des substituants sur le cycle ainsi qu'h la composition du solvant. On interprbte les rtsultats en fonction d'un processus htttrolytique SN1 incorporant une part importante de retour interne. Des calcultes thkoriques suggbrent que I'ion chlorure qui attaque dans l'ttat de transition pourrait Ctre fortement lit par des liaisons hydrogbnes.[Traduit par la revue]

Section: Isotope Effect Studiessupporting

confidence: 85%

Section: Transition State Variationmentioning

confidence: 99%

See 1 more Smart Citation

Chlorine isotope effects in the solvolysis of substituted 1-phenylethyl chlorides

McLennan¹,

Stein²,

Dobson³

1986

“…12 to 15% per deuterium at 105") when that carbon-nitrogen bond is breaking. A similar effect has been observed in solvolytic processes (2). Wolfsberg and Stern (3) suggest that the a-isotope effect arises from the difference in the initial and transition states of the force constants of the HCX bond, the major contribution arising from the bending force constant.…”

supporting

confidence: 62%

Secondary deuterium kinetic isotope effects on the thermolysis of 1-pyrazolines

Al-Sader¹,

Crawford²

1968

Secondary kinetic isotope effects on four deuterated 1-pyrazolines support the formation of a trimethylene intermediate during gas phase thermolysis. Calculation of cyclization relative rate constants for the trimethylene intermediates reveals that substitution of hydrogen by deuterium has no effect when the terminal methylenes are substituted, but the rateconstants are decreased when thecentral methylene's hydrogens are substituted. Explanations are advanced for all of the observed effects.

“…(3) Near room temperature the cr-deuterium isotope effect, kH/kD, of typical SN2 reactions is approximately 1 .O, while kH/kD for SN 1 reactions is about 1.15 (8).…”

Section: Introductionmentioning

confidence: 99%

Model Calculations of Kinetic Isotope Effects in Nucleophilic Substitution Reactions

Bron¹

1974

The results of calculations indicate that a previously proposed model for the transition state in "borderline" substitution reactions can be generalized and, as a result, the observed differences in the carbon-13 and deuterium isotope effects of S s l , Ss2, and "borderline" reactions rationalized. Although the conclusions may apply more generally, the standard reaction investigated is the solvolysis of benzyl bromide. The importance of resonance interaction with the phenyl ring, the significance of the product-or reactant-like character of the transition state, and the influence of the magnitude of force constants in determining isotope effects are examined. The temperamre dependence of kinetic isotope effects i n solvolysis is also investigated.Les rksultats des calculs indiquent que le mod2le proposk anttrieurement pour I'ktat de transition dans les rkactions de substitutions "limites" peut &tre gtnkralisk. En cons&-quence, les difftrences observkes dans les effets isotopiques du carbone-13 et du deutkrium dans les rkactions S r l , Ss2 et les rtactions "limites" peuvent &tre rationaliskes. La rtaction standard que I'on a Ctudike est la solvolyse du bromure de benzyle, bien que les conclusions peuvent &tre gknkralistes. On examine I'importance d e l'interaction de rksonance avec l e cycle du phtnyle, du caractkre de I'ttat de transition en fonction de sa ressemblance avec le produit initial ou final et l'influence de la grandeur des constantes d e force lors de l a dttermination des effets isotopiques. On examine d e plus la dipendance entre la tempkrature et les effets isotopiques sur la cinktique de solvolyse.[Traduit par le journal]Can. J. Chem.. 52, 903 (1974) Introduction as in ion pair formation (2), the overall isotope The mechanism of solvolytic reactions with effect is the product of an equilibrium and a characters between the two extremes of SN1 and kinetic isotope effect and unless one of the SN2 (I), the "borderline" cases, is still not isotope effects can be calculated o r measured clearly resolved. Reaction schemes of varying separately, the overall isotope effect becomes a complexity have been proposed (2-6).less useful parameter. The simplest mechanism involves a single T o explain experimental observations a model transition state with a high degree of charge for the transition state may be proposed. An separation (7). The other more complex mechan-example is Ingold's (1) model for second order isms for the borderline cases supposedly follow nucleophilic substitution reactions. a mixture of two competing reaction paths. The By use of well known computational methods first path leads via the traditional (1) SN2 (12-19) a suitable model of reasonable geometry transition state to products while the second may and bonding (force field) should then reproduce involve several carbonium ion-like species (2, 5). the observed isotope effects (12)(13)(14). For large The kinetic isotope effect method has been transition states it has been shown (12) that used extensively to obtain information about pa...