Intrinsic barriers structure effects

Table I. Effect of Organometal Structure on the Coefficient and the Intrinsic Barrier. Transfer...

It seems clear that for reactions of carbocations with nucleophiles or bases in which the structure of the carbocation is varied, we can expect compensating changes in intrinsic barrier and thermodynamic driving force to lead to relationships between rate and equilibrium constants which have the form of extended linear plots of log k against log K. However, this will be strictly true only for structurally homogeneous groups of cations. There is ample evidence that for wider structural variations, for example, between benzyl, benzhydryl, and trityl cations, there are variations in intrinsic barrier particularly reflecting steric effects which lead to dispersion between families of cations. 

We can also consider cases in which the intrinsic barrier is altered. Two such effects are steric hindrance and contribution of charge-separated structures to the transition state. Steric hindrance raises the energy of the transition state compared to that of a similarly exothermic unhindered model. This can be accomodated by considering an increase in the intrinsic barrier, which therefore makes the isotope effect rise. In ref.11 this is alternatively interpreted in a quadratic representation of the surface as an increase in the interaction force constant, and thus also correlated with an increase in the tunnel correction. An example of such an enhancement is the large value of the isotope effect in the trityl radical mesitylenethiol reaction in Table 1. 

The contribution of polar structures reduces the barrier and also the intrinsic barrier. This results for non thermoneutral reaction in a reduction of isotope effect. This has been a controversial subject for several years it is extensively covered by Russell29. The variation with substituents in the low isotope effects for the reaction of aryl radical with arene thiols were explained using such an effect. We may possibly further account for the lower intrinsic barrier for the R-H-Cl system (3.8 Kcal) than for the R-H—S system (5 Kcal) in terms of the greater electronegativity of chlorine. 

A more reasonable hypothesis is that the transition state is imbalanced, as shown in equation (102), but that there is a structural feature characteristic of carbene complexes, absent from other carbon acids, that masks the imbalance by reducing a. The most likely candidate is the 7r-donor effect of the methoxy group. Inasmuch as the contribution of 151 leads to resonance stabilization of the carbene complexes, this resonance is expected to add to the intrinsic barrier of proton transfer. This is because, as is true for resonance effects in general, its loss at the transition state should be ahead of the proton transfer. As Z becomes more electron... 

A further characteristic of Fig. 3, and of McClelland s data, is that within structurally related reaction families the plots are quite linear, even where some of the rate constants closely approach their limiting values. This is contrary to a simplistic view that selectivity (represented by the slope of the plot) should depend on reactivity. The linearity of such plots has been analyzed in detail by Richard8 who attributes it to compensation between effects on reactivity of changes in thermodynamic driving force and changes in an intrinsic kinetic barrier to reaction. Much of this section will be devoted to explaining this proposal. 

According to the Equation (30) the experimental isotope effect depends not only on the intrinsic isotope effects a,-, but also on the rate constants k2 and k. The intrinsic isotope effects describe the structure of the transition states and the commitment reflects the relative heights of energetic barriers of competitive reactions. If k2i. k -[li(x l), the formation of intermediate B is the rate-limiting step and experimental isotope effect is equal to ai(aexp = i). When intermediate B returns to substrate much faster than forms the product k2, k it (x 1), the experimental isotope effect is aexp = (a1a2)/a 1. For more complex multistep reactions the analysis of isotope effects is analogous, however the commitment factor become a complex collection of kinetic terms.54... 

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