Intrinsic rate constants steric effects

Steric effects reduce rate and equilibrium constants of nucleophilic additions but the question how the intrinsic barrier is affected does not always have a clear answer. Comparisons of intrinsic rate constants for the addition of secondary alicyclic amines versus primary aliphatic amines suggest that k0 is reduced by the F-strain. This implies that the development of the F-strain at the transition state is quite far advanced relative to bond formation. The effects of other types of steric hindrance on k0 such as prevention of coplanarity of Y in the adduct or even prevention of jt-overlap between Y and the C=C double bond in the alkene have not been thoroughly examined and hence are less well understood. 

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. 

Steric hindrance and electronic (inductive and resonance) effects are involved in the intrinsic reactivity of R . The two same effects also determine the reactivity of the monomer (M). To evaluate the proper reactivity of M irrespective of that of R, it can be measured by what is called methyl affinity. By convention, this affinity (a) is taken equal to the ratio of the rate constant of addition (ki) onto the monomer double bond to the rate constant of a reference reaction, which is the transfer reaction (k -) of CH3 to isooctane (abstraction reaction) ... 

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