A REASSESSMENT OF THE ISOINVERSION RELATIONSHIP

Calculations have been carried out on model systems with two potentially rate determining steps on a single reaction path. Taking the ratio of the diastereoisomers as P, the maxima found on plots of In P us. 1/T are shown to derive from the intersection of a line characteristic of one rate-determining step and a second line characteristic of the transitional region in which no single step is rate-determining. On this interpretation, the quantities previously calculated from the difference between the slopes of these lines and termed delta Delta Delta H-double dagger and delta Delta Delta S-double dagger have no simple physical meaning. The isoinversion relationship involving these quantities is not therefore a conventional enthalpy-entropy relationship. For those sets of reactions that involve little change in the inversion temperature, the isoinversion relationship is shown to be a necessary consequence of the geometry of the plot and the definition of the terms.