Model Calculations of Kinetic Isotope Effects in Nucleophilic Substitution Reactions

Abstract

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...

13C kinetic isotope effects and reaction coordinate motions in transition states for SN2 displacement reactions

“…bond order-force constant relationships we have employed for these basic models are given in Table 6. These models are very similar to the constant bond order models used previously (5,(13)(14)(15)40) but it should be noted that none of these transition states will fit all the kinetic isotope data for the methyl iodide-chloride ion system (17). The first calculations we will consider will be those for vL = 0 and for total bond order, n, = 1 = n, + n,, withF,, = +(FcclFc,)'12(see Table 5) where the force constants for the CI and the CCl bonds are given in Table 6.…”

“…In particular the value of 8 suggests that all these transition states are product-like in the RcB,-Roc plane, i.e., 8 is between -90" and -45". When n, is reduced to 0.45, the magnitude of v, decrea-ses as d o the isotope effects and the AX(C*)'s; however, we note that for n, = 0.4, 8 is positive, i.e., the reaction coordinate motion is symmetric stretch, 'As Bron has pointed out, such a model for an S,I reaction would only apply if no intermediates were involved in the reaction (15). It is now known that the S,1 category includes a rather large family of mechanistic possibilities (1 1, 12).…”

“…16) in an analysis of some 13C, 35C1/37C1, and deuterium kinetic isotope effects for the bimolecular displacement reaction, methyl iodide plus chloride ion in dimethylformamide (17). During this model study we have examined these earlier calculations (13)(14)(15) and the calculated 13C kinetic isotope effects for two other closely related S,2 displacement reactions, methyl iodide plus cyanide ion (18) and methyl iodide plus iodide ion (19). In this paper we report some results of this analysis and comment upon some questions concerning the properties of the transition states for S,2 displacement reactions as inferred from kinetic isotope effect studies.…”

Isotope effects in nucleophilic substitution reactions. IV. The effect of changing a substituent at the α carbon on the structure of SN2 transition states

“…Westaway and Ali have recently shown that the magnitude of the secondary adeuterium kinetic isotope effect can be related to the nucleophile -leaving group transition state distance in a closely related series of reactions (6). This is possible because the magnitude of secondary a-deuterium kinetic isotope effects in S,2 reactions is primarily determined by the differences in the energy of the C,-H(D) out-of-plane bending vibrations in the initial and transition states (31,34,35). Since the point of structural change is four bonds away from the C,-H(D) bond, the frequencies of the C,-H(D) out-ofplane bending vibrations are the same (1450 + 5 cm-I) for all theparrz-substituted benzyl chlorides used in this study.…”

Partition Function Ratios of the Methyl Halides Differing Only in Isotopic Substitution

“…Furthermore, an accurate knowledge of p.f.r. 's in isotope effect calculations eliminates at least one of the many uncertainties inherent in these computations (6,26).…”